Engineer's Relay Handbook

Introduction

1.1 The Problems of Standardization

The industries using relays are many and varied. They are also somewhat isolated from one another with respect to having a common relay language. Over the years, each company , by usage and practice, has tended to establish differing designations for identical or similar relay items. The desirability for a standardization of relay terms, definitions and symbols is thus apparent. There are several sources for definitions; the C37 subcommittee on protective relays, C83 committee on Components for Electronic Equipment, and IEEE Standards Coordinating Committee on Definitions (SCC10) which as published ANSI/IEEE Std 100-1984 Dictionary of Electrical and Electronics Terms. IEC has Technical Committee 41 on all types of relays and publication 50 (446) is a chapter of definitions on electrical relays. These sources have been used in the definitions given in this Chapter 1. [All chapters were carefully reviewed and modified as necessary to standardize and comply with the latest ANSI standards-e.g., alternating current abbreviated as "ac"-ANSI Y19-10.]

1.2 Scope of This Section

This section covers definitions, terminology, and symbols of relays for general switching purposes, military applications, electronic circuits, and airborne and space-age equipment. Certain specialized forms of relays have their own set of standards and may be designated by other terms. For a discussion of relays associated with electric power apparatus see Relays and Relay Systems Associated with Electric Power Apparatus, C37.90-1971 of the American National Standards Institute. Also, see NEMA Standard for Industrial Control, ICI-1954, as revised to May, 1958, or later.

1.3 Defining Relay Performance

There is a sequence of events in relay pick up (operate) and dropout(release) with respect to current rise and decay. The events are defined in terms of duration of coil current, armature motion, and contact actuation.

Fig. 1.1 Time traces typical of relay pickup.

Fig. 1.2 Time traces typical of relay dropout.

 

Figures 1.1 and 1.2 show contact performance-as a series of oscillograms-for relay with a normally open contact, a normally closed contact, and a transfer (break-make) contact. Table 1.3

In Table 1.3 preferred and non-preferred terms relating to relay performance are summarized. These terms are fully defined in alphabetical sequence in Paragraph 1.7Fig. 1.4 Relationship of relay performance to definitions.

Figure 1.4 is a graphical presentation of relay performance related definition.Fig. 1.5 Two typical relay forms with basic parts idenitied.

Figure 1.5 depicts basic relay parts.Pick Up (Figure 1.1) Upon coil energization, current begins to rise at a decreasing rate, but no armature movement occurs until the power develops sufficiently to operate the contact spring load. This period is sometimes referred to as waiting time. Contact actuation occurs during the armature movement. The final actuation time exceeds the initial actuation time by the amount of the contact bounce. For normally closed contacts, operate time and initial operate time are identical. On break-make contacts, the time interval between initial opening of the normally closed contact and closure of the normally open contact is called transfer time. Dropout (Figure 1.2) On de-energization of the coil, the magnetic flux does not die out immediately. The length of time it persists depends upon the release characteristics of the coil (fast-to-release, slow-to-release, and the like). The sequence of events described under pickup is essentially reversed under dropout. It will be seen that a normally open contact may be momentarily reclosed as a result of armature rebound off the backstop. This effect, which is not always present, depends on many factors, such as contact spacing, contact spring load, backstop design, and the like.

Relay Designations, Symbols, and Diagrams

1.4 Contact Assembly Designations

The contact switching combinations available on a relay are defined in terms of number of poles, number of throws(single or double), normal position(open or closed contacts),and the sequence to make and break. The various combinations have been given form letter symbols in Figure 1.6 to simplify overall identification. Abbreviations used to define the exact nature of the contacts are as follows:

Number of poles. The term single pole (SP) contact denotes that all contacts in the arrangement connect in one position or another to a common contact. A double pole (DP) contact consists of two single-pole contact arrangements actuated by the same mechanical system and operation concurrently. Likewise, a triple pole (TP) contact consists of three single pole contact systems. Larger numbers of poles in relays are generally indicated by the number of single pole contacts followed by a "P" (i.e., a four pole relay is 4P).

Number of throws. Single throw (ST) contact combinations have a pair of contacts open in one relay position and closed in the other. Double throw (DT) contact sets have three contacts. The common one is in contact with the second, but not with the third, in one position of the relay, and reverse this connection in the other relay, and reverses this connection in the other relay position. The basic double throw contact combination is the break-make (Form C).

Normal position of contacts. The combination in which the contacts are open in the normal or nonoperated condition of the relay is designated normally open (NO).
The combination in which the contacts are closed in the de-energized or unoperated position is designated normally closed (NC).

Double make and double break. These contact combination have two independent contacts both connected to a third contact in one position of the relay. They are designated double make (DM) when normally open and double break (DB) when normally closed.
Sequence of abbreviations. When abbreviations are used to designate a contact assembly, the following order is used: (1) Poles (2) Throws (3) Normal position (4) Double make or break (if applicable).
Example: SPST NO DM refers to single pole, single throw, normally open, double make contacts.

NO-Normally Open
B-Break
M-Make
DP-Double pole
DT-Double Throw
NC-Normally closed
DB-Double break
DM-Double make
TP-Triple pole

1.5 Relay Letter Symbols

In accordance with American National Standards Institute Y32.2-1975, the following letter symbols may be used with any relay symbols to show the special features a relay possesses.

The term "slow" and "fast" are relative, and the degree of rapidity is not to be implied by the use of the symbol on a relay. Relays that are dc operated are not marked.

1.6 Relay Coil and Contact Symbols

Commonly used symbols for relays of various kinds and applications are illustrated in Figure 1.6 and 1.8 through 1.12. (ANSI Y32.2)

Fig. 1.6 Symbols for Relay Contact Combinations established by the American National Standards Institute (ANSI).

The heavy arrow indicates the direction of operation. Contact chatter may cause some electrical discontinuity in forms D and E. Symbols taken from ANSI C83.16-1971 and Y32.2-1975.
The proper polarization for a polarized relay is shown by the use of plus (+) and (-) designations applied to the winding leads. The current in the direction indicated is to be interpreted to move, or tend to move, the armature toward the contact shown nearest the coil on the diagram. If the relay is equipped with numbered terminals proper numbers should be shown.
The following relay abbreviations are to be used on elementary wiring diagrams drawn according to the Joint Industry Conference Standard for Industrial Equipment (JIC):

 

Fig. 1.7 Preferred contact arrangement in a relay pileup.

Fig. 1.8 Symbol used in motor control relay circuits (JIC-NMTBA).

(a) Two wire control is generally thought of in relation to a pilot device such as a thermostat, pressure switch, etc., or to a simple maintained SPDT toggle or push-button switch. As the term implies, these devices require the use of only two wires between the control unit and the starter. The device is connected in series with the main contactor coil of the starter and the opening or closing of the pilot device directly controls the de-energizing or energizing of the starter. The major feature of a two wire control system is low voltage release. The starter drops out in the event of a power failure but picks up or recloses automatically when the power is restored, (b) In three wire control the main contactor coil of the starter is wired in series with its own NO auxiliary contacts. The "Start-Stop" push-button station, which requires the use of three wires between the control and the starter, is connected in parallel with the coil. In the event of a power failure, the starter will drop out and remain de-energized until the "Start" button is depressed. Since the starter drops out when there is a power failure and will not pick up or reclose again until the start button is depressed, this Control system provides low voltage protection.
Graphic Symbols is DoD mandatory. Section 22 covers letter designations for components and "K" is the letter for relay-e.g., K1,K2,etc. See Footnote to paragraph 1.1.

Fig. 1.9 Alternative symbols for relay contact combinations.
 

Sources of symbols: IEC-International Electrotechnical Commission; JIC-Joint Industry Conference, Electrical Standards for Industrial Equipment; NMTBA - National Machine Tool Builders Association/Electrical Standards. Mod. Tel-Modern telephone practice. Note: CT indicates continuity transfer, one asterisk denote Make-Before-Break, and two asterisks denote Break-Make-Before-Break.

Fig. 1.10 Combined symbol of relay coil and its contacts.

APPLICATION: RELAY WITH TRANSFER CONTACTS
 

Fig. 1.11 Comparison Relay, Electronic and Logic symbols.

 

Fig. 1.12 Symbols used in cam-operated timer control.
 

1.7 Relay Terms and Definitions-Glossary

The following definitions do not include terms peculiar to mathematical formula, statistical analysis, relay reliability studies, and the like. Such terms are defined in the appropriate chapter. When cross-references are made, the preferred terminology is the one under which the definition appears.

Actuating card. See card, armature.
Actuating system. See actuator.
Actuation time. See time, actuation.
Actuator. The parts of a relay that convert electrical energy into mechanical work.
Add-and-subtract relay. See relay, bidirectional.
Adjustment. The modification of any or all of the elements of tension, shape, or position of relay parts (to affect one or more of the operating characteristics or to meet mechanical requirements);e.g., adjustments of armature gap, restoring spring force, contact force or contact pressure.(See Figure 1.13.)

Air gap. See gap, armature; gap, contact; gap, heel.
Airline See gap, heel.
Ampere-turns. The product of the number of turns in an electromagnetic coil winding and the current in amperes passing through the winding. On ac, the rms current value is generally used in the product of current and turns and is referred to as rms ampereturns.
Antifreeze pin. See residual screw, pin, plate, stud, or shim.
Arm, armature. (1) on some types of armature, the protrusion or lever employed to actuate the associated contact spring pileup. (2) The moving part or pans of a reed switch.
Arm, contact. A current-carrying protrusion, resilient or nonresilient, onto which contacts may be fastened. Also see spring, contact.
Armature. The moving magnetic member of an electromagnetic relay structure.
Armature, balanced. A relay armature that rotates about its center of mass and is therefore approximately in balance with both gravitational (static) and accelerative (dynamic) forces.
Armature contact. See contact, movable.
Armature, end-on. A relay armature whose principal motion is parallel to the longitudinal axis of a core having a pole face at one end.
Armature, long-lever. An armature with its contact-actuating arm greater in length than the distance from the armature hinge, bearing, or fulcrum to the portion of the armature opposite the pole face. (See Figure 1.14.)
Armature, plunger or solenoid. A relay armature that moves within a tubular core in a direction parallel to its longitudinal axis.
Armature residual gap. See gap, residual.
Armature, short-lever. An armature with its contact-actuating arm length equal to or lesser in length than the distance from the armature hinge, bearing, or fulcrum to the portion of the armature opposite the pole face.(See Figure 1.14.)

Armature, side or armature, flat. A relay armature whose motion is perpendicular to the longitudinal axis of a core having a pole face at one side.
Armature stroke. See buffer, armature.
Armature stud. See contact, normally closed.
Back contact. See contact, normally closed.
Backstop, armature. That part of the relay which limits the movement of the armature away from the pole face or core. In some relays a normally closed contact may serve as the backstop.
Bank. One or more contact levels of a stepping switch.
Bar. See card, armature; relay bar.
Bearing, armature. The point at which the armature bears against the heelpiece (fulcrum) or the member securing the armature to the relay (See hinge, armature).
Bias, electrical. An electrically produced force tending to move the armature towards a given position.
Bias, mechanical A mechanical force tending to move the towards a given position.
Blade. (1) See spring, contact. (2) Sometimes used to define the centilever portion of the reed switch contained with the glass envelope.
Blocking. The minimum number of continuous hours that a relay will block rated rms off-stage voltage at maximum case temperature.
Bobbin. A spool of structure upon which a coil is wound.
Bounce, armature. See rebound, armature.
Bounce, contact. Internally caused intermittent and undesired opening of closed contacts, or closing of open contacts, of a relay, caused by one or more of the following: (1) Impingement of mating contacts; (2) Impact of the armature against the coil core on pickup or against the backstop on dropout; (3) Momentary hesitation or reversal of the armature motion during the pickup or dropout stroke (See Figures 1.1 and 1.2).
Bounce time. See time, contact bounce.
Break. The opening of closed contacts to interrupt an electric circuit.
Bridging
(1)Normal bridging: The normal make-before-break action of a make-break or D contact combination. In a stepping switch, the coming together momentarily of two adjacent contacts by a wiper shaped for that purpose in the process of moving from one contact to another.
(2)Abnormal bridging: The undesired closing of open contacts caused by a metallic bridge or protrusion developed by arcing.
Buffer, armature or bushing, armature of lifter, armature or pusher, or stud armature. A normally insulating member that transmits the motion of the armature from one movable contact spring to another in the same pileup.
Buffer, spring or buffer, contact spring, or bushing, spring or stud, spring. A normally insulating member that transmits the motion of the armature from one movable contact spring to another in the same pileup.
Bunching, contact. The undesired, simultaneous closure of make-and-break contacts during vibration, shock, or acceleration. Also, the simultaneous closure of the contacts of a continuity transfer or bridging contact combination.
Bushing, armature. See buffer, armature.
Capacitance CISO The maximum allowable capacitive coupling between two specified test points.
Card, armature. An insulating member used to link movable contact springs to the armature on some relay types (also called lifter). (See relay, bar.)
Change-over contact; Two-way contact (deprecated). A combination of two contact circuits including three contact members, one of them being common to the two contact circuits. When one of these contact circuits open, the other is closed and vice versa. (IEC) (See double throw contact.)
Characteristic quantity (of a measuring relay). An electrical quantity, or one of its parameters the name of which characterized the relay and the values of which are the subject of accuracy requirements. (IEC)
Characteristic, static. See load curve and pull curve.
Chatter, armature. The undesired vibration of the armature due to inadequate ac performance or external shock and vibration.
Chatter, contact. Externally caused, undesired vibration of mating contacts during which there may or may not be actual physical contact opening. If there is no actual opening but only a change in resistance, it is referred to as dynamic resistance. (See Figures 1.1 and 1.2)
Clapper. Sometimes used for an armature that is hinged or pivoted; see armature, end-on.
Close, to close. A latching relay closes when it changes from the unoperated to the operated condition. Usually refers to the completing of continuity of the main contact.
Close pulse. A short, high level pulse applied to the coil of a latching relay.
Closed value, measured As the current or voltage on an unoperated latching relay is step applied the lowest value at which all contact function.
Closed value, specified As the current or voltage on an unoperated latching relay is step applied the value at or below which all contact must function.
Coil An assembly consisting of one or more windings, usually wound over over an unsulated iron core on a bobbin or spool. May be self-supporting, with terminals and any other required parts such as a sleeve or slugs.
(1)Concentrically Wound-A coil with two or more insulated windings wound one over the other.
(2)Double Wound-A coil consisting of two windings wound on the same core.
(3)Parallel Wound-A coil having multiple windings wound simultaneously, with the turns of each winding being contiguous.(see winding, bifilar.)
(4)Sandwich Wound-A coil consisting of three concentric windings in which the first and third windings are connected series aiding to match the impedance of the second winding. The combination is used to maintain transmission balance.
(5)Tandem Wound-A coil having tow or more windings, one behind the other, along the longitudinal axis. Also referred to as a two, three, or four-section coil, etc.
Coil, blowoutSee magnet, blowout.
Coil, close.The coil of latching relay which, when power is applied, closes the relay.
Coil, trip. The coil of a latching relay which, when power is applied, trips the relay.
Comb. An insulating member used to position a group of contact springs as on wire-spring relays
Combination, contact or contact form. A single-pole or basic contact assembly.(See Figure 1.6.)
Contact.
(1)The portion of current-carrying members at which electrical circuits are opened or closed.
(2)The current carrying pans of a relay that engages or disengages to open or close electrical circuits.
(3)Used to denote a combination or set. (Contacts also used)
Contact armature.
(1) A contact mounted directly on the same armature.
(2) Sometimes used for a movable contact.
Contact assembly. An assembly of contact members, with their insulation, which close or open their contact circuit by their relative movement. (IEC)
Contact follow.The further specified movement of the contact tips (points) when making and after they have just touched and while they are traveling in the same direction as that of the moving contact member. (IEC)
Contact force. The force which two contact tips (points) exert against each other in the closed position under specified conditions. (IEC)
Contact gap.The gap between the contact tips (points) under specified conditions, when the contact circuit is open. (IEC)
Contact member. A conductive part of a contact assembly which is electrically isolated (points) after they have just touched.(IEC)
Contact roll. When a contact is making, the relative rolling movement of the contact tips (points) after they have just touched.(IEC)
Contact tip. That part of contact member at which the contact circuit closes or opens. (IEC)
Contact wipe. When a contact is making, the relative rubbing movement of the contact tips (points) after they have just touched. (IEC)
Contact, auxiliary. A contact combination used to operate a visual or audible signal to indicate the position of the main contacts, establish interlocking circuits, or hold a relay operated when the original operating circuit is opened.
Contact, back. See contact, normally closed.
Contact, bifurcated. A forked, or branched, contacting member so formed or arranged as to provide some degree of independent dual contacting.
Contact bounce. See bounce, contact.
Contact, break. See contact, normally closed.
Contact, break-before-make. A contact combination in which one contact opens its connection to another contact and then closes its connection to a third contact. (See "C" contact combination in Figure 1.6.)
Contact, break-make. See contact, break-before-make..
Contact, bridging. A contact combination designed to close one contact before opening another. (Usually applied to stepping switches: for relays, see contact, continuity transfer.)
Contact, chatter. See chatter, contact.
Contact, continuity transfer. A contact combination in which contact closes connection to another contact and then opens its prior connection to a third contact. (See "D" contact combination in Figure 1.6.)
Contact, double break. A contact combination in which contact on a single conductive support simultaneously open electrical circuits connected to two independent contacts. This provides two contact air gaps in series when the contact is open (see V or Y contact combination in Figure 1.6 ). Note: In B combination is terminal is brought out form the movable contact. In the Y combination , it is not.
Contact, double make. A contact combination in which contacts on a single conductive support simultaneously close electrical circuits connected to the contact of two independent contacts, and provides two contact air gaps in series when the contact is open. (Sometimes called normally open, double-make contact.) (See U or X contact combination in Figure 1.6) Note: In U combination a terminal is brought out from the movable arm. In the X combination it is not.
Contact, double throw. A contact combination having two positions as in break-make, make-break, and the like. (See Figure 1.6.)
Contact, dry circuit. A contact that carries current but neither opens nor closes while its load circuit is energized. Incorrectly used if referring to low level contacts.
Contact, dynamic resistance. See dynamic contact resistance.
Contact, early. A contact combination that is adjusted to functions before other contact combinations when the relay operates.
Contact final actuation time. Total time from beginning of coil energization or de-energization to the end of bounce; i.e., contacts mechanically in final resting position. (See Figures 1.1 and 1.2.)
Contact, fixed. See contact, stationary.
Contact, front. See contact, normally open.
Contact, initial actuation time. Time from beginning of coil energization or de-energization to first opening of closed contact; i.e., at beginning of bounce. (See Figures 1.1 and 1.2)
Contact interrupter. On a stepping relay or switch, a contact combination operated directly by the armature that opens and closes the winding circuit, permitting the device to step itself.
Contact late. A contact combination that is adjusted to function after other contact combinations when the relay operates.
Contact, low level. Contact that control only the flow of relatively small currents in relatively low-voltage circuits; e.g., alternating currents and voltages encountered in voice or tone circuits, direct currents in the order of microamperes, and voltages below the softening voltages of record for various contact materials (that is, 0.080 volt for gold, 0.25 volt for platinum, etc.) Also defined as contacts switching loads where there is no electrical arc transfer of detectable thermal effect and where only mechanical forces can change the conditions of the contact interface.
Contact, main. The primary set of contacts of a relay, usually defined as those having the highest current rating.
Contact, make. See contact, normally open.
Contact, make-before-break.See contact, continuity transfer.
Contact, make-break See contact, continuity transfer.
Contact, movable. The member of a contact combination that is moved directly by the actuating system. This member is also referred to as the armature contact or swinger contact.
Contact, nonbridging. A contact combination in which the opening contact opens before the closing contact closes. (Usually applied to stepping switches; for relay's, see contact, break-before-break)
Contact, normally closed. A contact combination which is closed when the armature is in its unoperated position. (See "B" contact combination in Figure 1.6.)
Contact, normally open. A contact combination that is open when the armature is in its unoperated position. (Generally applies to monostable relays.) (See "A" contact combination in Figure 1.6.)
Contact, off. normal-A form C contact combination on a stepping switch that is in one condition when the relay or stepping switch is in its normal position and in the opposite condition for any other position of the relay or stepping switch; i.e., when not in its reset or home position.
Contact, operate time. Time from initial energization to the first opening of closed contact or first closing of open contact, prior to bounce. (See Figure 1.1 and 1.2)
Contact, passing. The combination of a contact assembly designed for opening or closing in a passing fashion the corresponding contact circuit when the relay changes over. The passing may occur either when the relay picks up or when the relay drops out or both during picking-up and dropping out.
Contact, permissive make. A term applied to a contact combination in which the movable contact spring is pretensioned so that it will close of its own force when unrestrained. Also, defined as a contact that is mechanically driven open and permitted to make. (See Figure 1.15.)
Contact, preliminary. See contact, early.
Contact, reed.
(1)A glass-enclosed, magnetically operated contact using thin, flexible, magnetic conducting strips as the contacting members.
(2)Contact assembly, the contact members of which are blades either fully or partly of magnetic material and which are moved directly by a magnetic force. (IEC)
Contact release time. Time form initial de-energization of the relay coil to the first opening of a closed contact prior to bounce. (See Figures 1.1 and 1.2.)
Contact, sealed. A contact assembly sealed in a compartment separate from the rest of the relay.
Contact, snap action.A contact assembly having two or more equilibrium positions. In one, the contact maintain a substantially constant contact force during the initial motion of the actuating member until stored energy snaps the contacts to a new position of equilibrium.
Contact, stationary A member of a contact combination that is not moved directly by the actuating system.
Contact, transfer. Either a contact, break-make or contact, continuity transfer.
Contact transfer time. Time during which the moving contact first opens from a closed position and first makes with the opposite throw of the contact. It is floating in a non-contacting position prior to bounce and after energizing or de-energizing the coil. (See Figure 1.1 and 1.2.)
Contact weld. A contact failure due to fusing of contacting surfaces to the extent that the contacts fail to separate when intended.
Contactor. See relay, power.
Continuous on. The minimum number of hours a relay will continuously conduct rated rms current.
Core, coil. The portion of the magnetic structure of a relay about which the coil is positioned.
Crosstalk. The electrical coupling between a closed contact circuit and other open or closed contact on the same relay or switch, expressed in decibels down form the signal level.
Current, intermediate. The range of current (milliamperes) at which formation of carbonaceous material may significantly affect contact resistance.
Current, leakage. (Maximum off-state current)T_D (rms)-The parameter is an effective current and is specified at maximum-load voltage. In solid-state relays, it is the current that flows through the load when the relay is in the off-state. Does not apply to electromechanical relays where contacts are open and current cannot flow.
Current, minimum load.I_TMIN(rms)-The minimum current required to maintain the relay in the on-state (nominal load voltage applies). Applies mainly to solid-state relays.
Current, non-repetitive surge-I_TSM. The maximum allowable, non-repetitive, peak, sinusoidal current that may be applied to the output for one full cycle at nominal line frequency. Relay control may be lost during and following the surge until the junction temperature falls below the maximum rated temperature.
Current rated contact. The current which the contacts are designed to handle for their rated life. See rating, contact.
Current, maximum rate of rise on state (di/dt). The maximum non-repetitive rate of current rise the output can withstand without being damaged.
(1) With the relay output(s) turned on by the application or removal of the control voltage and/or current.
(2) With the relay output(s) driven into break-over with the input at non-operate level. Current, repetitive overload-I_TO (rms)-The maximum allowable repetitive rms overload current that may be applied to the output for a specific duration and duty cycle while still maintaining output control. Applies mainly to solid state relays.
Cycle. A monostable relay cycles when it picks up and then drops out, or vice versa.(IEC)
Cycling. The minimum number of hours during which a relay may be switched between the off state and the on state at a fixed, specific cycle rate, load current, and case temperature without failure.

1.7 Relay Terms and Definitions-Glossary

The following definitions do not include terms peculiar to mathematical formulae, statistical analysis, relay reliability studies, and the like. Such terms are defined in the appropriate chapter. When cross-references are made, the preferred terminology is the one under which the definition appears.

Dashpot. A device that employs either a gas or liquid to absorb energy and retard the operation of the relay.
De-energize. To remove power from a relay coil.
Dielectric strength. V_ISO-The maximum allowable ac rms voltage (50/60Hz) which may be applied between two specified test points such as input-output, input-case, output-case in solid state relays, and between current-carrying and non-current-carrying metal members in electromechanical relays.
Disengage. A relay disengages at the instant it terminates a function previously effected in a given output circuit.(IEC)
Disengaging value of the characteristic quantity The threshold value of the characteristic quantity at which the relay disengages under specified conditions.(IEC)
Disengaging ratio. The ratio of the disengaging value to the operating value.(IEC)
Dropout, to drop out. A monostable relay drops out when it changes from an energized to an unenergized condition. (IEC)
Dropout, time. See time, release.
Dropout value, measured. As the current or voltage on an operated relay is decreased, the value at which all contacts restore to their unoperated positions.
Dropout value, specified. As the current or voltage on an operated relay is decreased, the value at or above which all relay contacts must restore to their unoperated positions.
Dry reed relay. See relay, reed.
Duty cycle. A statement of energized and de-energized time in repetitious operation; for example, 2 seconds on, 6 seconds off. Often expressed as the energized percentage of total cycle time.
Dynamic contact resistance. A change in contact electrical resistance due to a variation in contact pressure on a contacts mechanically closed (see Figures 1.1 and 1.2); occurrence is during non-bounce condition.
Energization. The application of power to a coil winding of a relay. With respect to an operating coil winding, use of the word commonly assumes enough power to operate the fully unless otherwise stated.
Energized condition. The specified condition(s) of an appropriately energized monostable relay. (IEC)
Energizing quantity. An electrical quantity (either current or voltage) alone, or combined with other such quantities which when applied to a relay under specified conditions enables it to fulfill its purpose.(IEC)
Energizing quantity, input. For an all-or-nothing relay: The energizing quantity that the relay will respond to when that quantity is applied under specified conditions. For a measuring relay: An energizing quantity which either by itself constitutes the characteristic quantity or helps to constitute it. (IEC)
Final actuation time. See contact, final actuation time.
Fixed contact. See contact, stationary.
Flux, leakage. That portion of the magnetic flux that does not cross the the armature to pole-face-gap.
Follow, contacts. The force exerted by a movable contact against a fixed contact when the contacts are closed.
Force, contact. The force exerted by a movable contact against a fixed contact when the contacts are closed.
Form, contact. See combination, contact.
Frame. The main supporting portion of a relay, which may include parts of the magnetic structure.
Freezing, magnetic. Sticking of the relay armature to the core due to residual magnetism.
Frequency, operating. The rated ac frequency of the supply voltage at which the relay is designed to operate.
Fulcrum (armature). See bearing, armature.
Gaging, relay contact. The setting of relay contact spacing to determine the point in the armature's stoke at which specified contacts function.
Gap, armature The distance between armature and pole face.
Gap, buffer. The space separating the armature buffer and the movable spring, or the space between the movable springs and buffers when the armature is in its unoperated position.
Gap, contact. The distance between a pair of mating relay contacts when the contacts are open.
Gap, heel. A gap or nonmagnetic separation in the magnetic circuit other than between the armature and pole face. Generally, located between the heel piece and pole piece of an ac relay.
Gap, hinge. See hinge gap.
Gap, residual. The thickness of nonmagnetic material in the magnetic circuit between the pole face center and the nearest point on the armature when the armature is in the fully seated position.
Gap, stud. See gap buffer.
Grass. See dynamic contact resistance.
Header. The subassembly that supports and insulates the leads passing through the wall of a sealed relay.
Heel piece. The portion of a magnetic circuit of a relay that is attached to the end of the core remote from the armature.
Hermetically sealed relay. See relay, enclosed.
Hesitation, armature. Delay or momentary reversal of armature motion in either the pickup or dropout stroke.
Hinge, armature. Delay or momentary reversal of armature motion in either the pickup or dropout stroke.
Hinge gap. Space between the armature and the frame at the hinge (allows for armature to move without binding).
Hold value specified. As the current or voltage on an operated relay is decreased, the value at or above which all relay contacts must restore to their unoperated positions.
Homing. See relay, homing.
Housing. An enclosure or cover for one or more relays, with or without accessories; usually provides access to the terminals.
Hum. The sound caused by mechanical vibration resulting from alternating current flowing in the coil, or by unfiltered rectified current.
Humidity, non-operating. That steady state humidity to which the relay may be subjected without electrical malfunction or mechanical damage.
Humidity, operating. That steady state humidity to which the relay may be subjected without electrical malfunction or mechanical damage.
Influencing quantity (factor). Any quantity (factor) likely to modify any of the specified characteristics of a relay (e.g., pickup, dropping out, accuracy, etc.). (IEC)
Initial actuation time. See contact initial actuation time.
Input. That portion of a relay to which a control signal is applied in order to achieve the switching function (generally applied to solid state or hybrid relays).
Insulation resistance. R_ISO The minimum allowable dc resistan between input and output of solid state relays and between contacts and coil for electromechanical and reed relays.
Isolation. The value of insulation resistance, dielectric strength, and capacitance measured between the input and outputs, input to case, output to case, and output to output when applicable.
Lead, finish. The outer termination of the coil winding.
Lead, inside. See lead, start.
Lead, outside. See lead, finish.
Lead, start. The inner termination of the coil winding.
Leakage (max.off state current I_D(rms)). The rms current conducted by the output circuit of the relay at maximum load voltage with zero input voltage/current.
Level. A series of contacts served by one wiper of a stepping switch.
Lifter, armature. See buffer, armature.
Limiting continuous thermal withstand value of an energizing quantity. The highest value (rms if ac) of the energizing quantity that a relay can carry continuously and under specified conditions, while satisfying the temperature rise requirements. (IEC)
Limiting dynamic value of an energizing quantity. The highest value of an energizing quantity that a relay can withstand under specified conditions of wave form and duration without permanent degradation of the specified characteristics due to the resultant dynamic effect. (IEC)
Limiting short-term thermal withstand value of an energizing quantity. The highest value (rms if ac) of an energizing quantity that a relay can withstand under specified conditions for a specified short time without permanent degradation of the specified characteristics due to overheating. (IEC) See rating, short time.
Load, contact. The electrical power encountered by a contact set in any particular application.
Load, curve. The static force/displacement characteristics of the total spring-load of the relay. (See Figure 1.15).

 

Magnet, blowout. A device that establishes a magnetic field in the contact gap to help extinguish the arc by displacing it.
Make. The closure of open contacts to complete an electric circuit.
Mechanical shock, non-operating. The mechanical shock level (amplitude, duration and wave shape) to which the relay may be subjected without permanent electrical or mechanical damage (during storage or transportation).
Mechanical shock, operating. That mechanical shock level (amplitude, duration and wave shape) to which the relay may be subjected without permanent electrical or mechanical damage during its operating mode.
Meter relay. See relay, instrument.
Minimum current. See current, intermediate.
Miss, contact. Failure of a contact mating pair to establish the intended circuit. This may be a contact resistance in excess of a specified maximum value.
Mounting plane. The surface to which the relay is fastened.
Nonbridging. See contact, nonbridging.
Nondropout, specified. See operating characteristics, hold value.
Nonfreeze pin. See residual.
Nonoperate, specified. See operating characteristics, nonpickup.
Nonpickup value, specified. As the current or voltage on an unoperated relay is increased, the value which must be reached before any contact change occurs.
Nonrelease, specified. See operating characteristics, hold value.
Normal condition. The de-energized condition of the relay.
Normal position. The de-energized position of contacts, open or closed, due to spring tension, gravity, or magnetic polarity. The term is also used for the home position of a stepping switch.
Normal sequence of operation. The intended contact operation sequence built into a relay unaffected by wear or dimensional change. See gaging, relay contact.
Offstate dv/dt. The application of both position and negative voltages with maximum specified rate of rise to the output terminals.
Operate. A relay operates when sequentially it starts, it passes from an initial condition towards the prescribed operated condition, and it switches.(IEC)
Operate time. See contact operate time.
Operate value. See operating characteristics, pickup value.
Operate value, just. See operating characteristics, pickup value, measured.
Operate value, must. See operating characteristics, pickup value, specified.
Operating characteristics. Pickup, nonpickup, hold and dropout, voltage and current.
Operative range of an input energizing quantity of an all-or-nothing relay. The range of values of the given input energizing quantity for which the relay, under specified conditions is able to perform its intended function(s) according to the specified requirements. (IEC)
Operative range of an input energizing quantity of a measuring relay. The range of values of the given input energizing quantity for which the relay, under specified conditions, is able to perform its intended function(s) according to the specified requirements. (IEC)
Output. That portion of a relay which performs the switching function required. Generally applied to solid state and hybrid relays.
Output circuit. The whole of the electrically conductive parts within a relay connected to the terminals between which the predetermined change is produced.(IEC)
Overdrive. A term used to indicate use of greater than normal coil current(applied voltage), and usually employed in obtaining fast operate time or pulse response.
Overtravel armature dropout. The portion of the armature travel that occurs between closure of the normally closed contact(s) and the fully released static position of the armature.
Overtravel armature pickup. The portion of the armature travel occurring between closure of the normally open contact(s) and the fully operated static position of the armature.
Overtravel contact. See follow, contact.
Pickup, to pick up. A monostable relay picks up when it changes from the unenergized condition to an energized condition.(IEC)
Pickup value, measured. As the current or voltage on an unoperated relay is increased, the value at or below which all contacts function.
Pickup pulse. A short, high-level pulse applied to a relay; usually employed to obtain fast-operate time.
Pickup value, specified. As the current or voltage on an unoperated relay is increased, the value at or below which all contacts must function.
Pickup (or pull-in) time. See time, operate.
Pileup. An assembly of contact springs or combinations fastened one on top of the other with insulation between them.
Pileup, contact. The total assembly of contacts in one stack on a relay.
Pressure, contact. See force, contact.
Pneumatic bellows. Gas-filled bellows, sometimes used with plunger type relays to obtain time delay.
Pole, double. A term applied to a contact arrangement to denote that it includes two separate contact combinations, that is, two single-pole contact assemblies.
Pole face. The part of the magnetic structure on the end of the core nearest the armature.
Pole gap. See gap, armature.
Pole piece. The end of an electromagnet, sometimes separable from the main section, and usually shaped so as to distribute the magnetic field in a pattern best suited to the application.
Pole, single A term applied to a contact arrangement to denote that all contacts in the arrangement connect in one position or another to a common state.
Power dissipation P_T The maximum average power dissipation at a given load current.
Pretravel armature. The initial movement of the armature prior to engagement with current.
Pull curve. The force-displacement characteristics of the actuating system of the relay.
Pull-in time. See time, operate.
Pull-in (or pull-on)value measured See operating characteristics, pickup value, measured.
Pull-in (or pull-on) value specified See operating characteristics, pickup value, specified. Pusher See buffer, armature.

1.7 Relay Terms and Definitions-Glossary

The following definitions do not include terms peculiar to mathematical formulae, statistical analysis, relay reliability studies, and the like. Such terms are defined in the appropriate chapter. When cross-references are made, the preferred terminology is the one under which the definition appears.

Race, relay. A deficient circuit condition wherein successful operation depends upon a sequence of two or more independent contacts and in which the sequence is not insured by electrical or mechanical interlocking restraints. Ratchet relay. See relay, stepping.
Rated power of an energizing circuit, rated burden of an energizing circuit. The power of burden (watts if dc,volt-amperes if ac) absorbed under the reference conditions by a given energizing circuit of a relay and determined under specified conditions. (IEC)
Rated value of an energizing quantity The value of an energizing quantity to which some of the specified characteristics are referred.(IEC)
Rating, contact. The electrical load-handling capability of relay contacts under specified conditions and for a prescribed number of operations.
Rating, short time. The value of current or voltage that the relay can stand, without injury, for specified short time intervals. (For ac circuits, the rms total value, including the dc component, should be used). The rating recognized the limitations imposed by both thermal and electromagnetic effects.
Ratio, armature lever. The distance through which the armature buffer moves divided by the armature travel (See travel armature). Also, the ratio of the distance from the armature bearing pin (or fulcrum) to the armature buffer in relation to the distance from the bearing pin (or fulcrum) to the center of the pole face.
Rebound, armature. (1) The return motion or bounce-back toward the unoperated position after the armature strikes the pole face during pickup, referred to as armature pickup rebound; (2) The forward motion or bounce in the direction of the operated position when the armature strikes its backstop on dropout, referred to as armature dropout rebound.
Relay. An electric device that is designed to interpret input conditions in a prescribed manner and after specified conditions are met to respond to cause contact operation or similar abrupt change in an associated electric control circuit. Notes: (a) Inputs are usually electric, but may be mechanical, thermal or other quantities. (b) A relay may consist of several units, when responsive to specified inputs, the combination providing the desired performance characteristic.
Relay, all-or-nothing. An electrical relay which is intended to be energized by a quantity, whose value is either: (1) higher than that at which it picks up; (2) or lower than that at which it drops out. (IEC) Note: The adjective "all-or-nothing" can be deleted when no ambiguity will occur.
Relay, alternating current(ac). A relay designed for operation from an alternating-current source.
Relay, annunciator. A relay that indicates the present or former state of a circuit or circuits.
Relay, antenna switching. A special RF relay used to switch antenna circuits.
Relay, automatic homing A stepping relay that returns to its home or starting position under certain prescribed conditions.
Relay, automatic-reset. See reset, automatic.
Relay, auxiliary. A relay that operates to assist another relay or device in the performance of a function.
Relay, bar. A relay so designed that a bar actuates several contact simultaneously. Not to be confused with a relay having bar-shaped contact.(See card, armature.)
Relay, bidirectional or add and subtract relay. A stepping relay in which the rotating wiper contacts may be driven in either direction.
Relay, bimetal. A form of thermal relay using a bimetallic element to activate contacts when heated electrically.
Relay, bistable. An electrical relay which, having responded to an input energizing quantity (or characteristic quantity) and having changed its condition remains in that condition after the quantity has been removed. Another appropriate further energization is required to make it change over. (IEC)
Relay, center-off. See relay, center stable.
Relay, center-stable. A relay that is operated in one of two energized positions and that returns to a third, off position when the operating winding is de-energized. Also referred to as center-off.
Relay, chopper. A relay designed to produce a modified square wave output of the same frequency and bearing a definite phase relationship to a driving sine wave.
Relay, close. differential-A relay having its dropout value specified close to its pickup value.
Relay, coaxial. A special RF relay that opens or closes a coaxial cable or line. It is generally a low impedance device.
Relay, continuous-duty. A relay that may be energized with rated input power and carry a rated load indefinitely without exceeding specified limitations.
Relay, crystal can. A term used to identify a relay housed in a hermetically sealed enclosure that was originally used to enclose a frequency-control type of quartz crystal.
Relay, current-balance. A relay that operates when the magnitude of one current exceeds the magnitude of another current by a predetermined degree.
Relay, current-sensing. A relay that functions as a predetermined value of current; an overcurrent or an undercurrent relay, or a combination of both.
Relay, dashpot. A relay employing the dashpot principle to effect a time delay.
Relay, dependent-time measuring. A specified time measuring relay for which times depend, in a specified manner, on the value of the characteristic quantity. (IEC)
Relay, differential. A relay with multiple windings that functions when the voltage, current, or power difference between the windings reaches a predetermined value.
Relay, direct current (dc). A relay designed for operation from a direct-current source.
Relay, double arm. A relay with two pileups, each actuated by a separate armature arm.
Relay, electrical. A device designed to produce sudden, predetermined changes in one or more electrical output circuits, when certain conditions are fulfilled in the electrical input circuits controlling the device.(IEC)
Note 1-The term relay shall be restricted to a relay unit having a single relaying function between its input circuits and its output circuits.
Note 2-The term relay includes all the components which are necessary for its specified operation.
Note 3-The adjective "electrical" can be deleted when no ambiguity may occur.
Relay, electrical interlock. See relay, interlock.
Relay, electromagnetic. A relay whose operation depends upon the electromagnetic effects of current flowing in an energizing winding.
Relay, electromechanical power controller (EMPC). An electromechanical relay which contains electronic circuitry that monitors electrical characteristics of its containing system and causes the EMPC to open or close based upon its specified parameters. An EMPC combines the traditional function of a relay and a circuit breaker or fuse.
Relay, electromagnetic time delay. A relay in which the actuation of the contacts is delayed by the inductive effect of a conducting sleeve or slug (usually nonmagnetic) or a short-circuited winding over the core.
Relay, electromechanical An electrical relay in which the designed response is developed by the relative movement of mechanical elements under the action of a current in the input circuits. (IEC)
Relay, electrostatic. A relay in which operation depends upon motion of two or more insulated conductors caused by electrostatic effects.
Relay, electrostrictive. A relay in which operation depends upon the dimensional changes of an electrostrictive dielectric.
Relay, enclosed.
(1)Hermetically sealed-A relay contained within an enclosure that is sealed by fusion or other comparable means to ensure a low rate of gas leakage.(Generally metal-to-metal or metal-to-glass sealing is employed.)
(2)Encapsulated-A relay embedded in a suitable potting compound.
(3)Sealed-A relay contained in an unsealed housing.
(4)Covered-A relay contained in an unsealed housing.
Note: The coil and contact assemblies may be separately enclosed and isolated from each other by various combinations of the above enclosures.
Relay, ferreed. Coined name for a special form of dry reed having a return magnetic path of high remanence material that provides a bistable contact.
Relay, flasher. A self-interrupting relay, usually by the thermal type.
Relay, frequency sensitive. A relay that operates when energized electrically at a particular frequency or within specific frequency bands; for example, a resonant reed relay.
Relay, homing. A self-interrupting relay, usually of the thermal type.
Relay, hot-wire. A stepping relay that returns to a specified starting position,(home) prior to each operating cycle.
Relay, hybrid electromechanical-(HEMR). A relay with isolated input and output in which electromechanical and electronic devices are combined to perform a switching function with an electromechanical relay specifications.
Relay, hybrid solid state-(HSSR) A relay with isolated input and output in which electromechanical an electronic devices are combined to perform a switching function with a solid state output.
Relay, impulse.
(1) A relay that follows and repeats pulses, as from a telephone dial;
(2) A relay that operates on the stored energy of a short pulses after the pulse ends;
(3) A relay that discriminates between length and strength of pulses, operating on long or strong pulses and not operating on short or weak ones;
(4) A relay that alternately assumes one of two positions as pulsed.
Relay, independent-timer measuring. A specified-time measuring relay the specified time for which can be considered as being independent, within specified limits, of the value of the characteristic quantity. (IEC)
Relay, inertia. A relay with added weights or other modifications that increase the moment of inertia of its moving parts, either to slow its operation or to cause it to continue in motion after the energizing force ends.
Relay, instruments. A sensitive relay in which the principle of operation is similar to that of instruments such as the electrodynamometer, iron vane, galvanometer, and moving magnet.
Relay, integrating. A relay that operates on the energy stored from a long pulse or a series of pulses of the same or varying magnitude, for example, a thermal relay.
Relay, interlock. A relay with two or more armatures having a mechanical lindage or an electrical interconnection, or both, whereby the position of one armature permits, prevents, or causes motion of another.
Relay, intermittent-duty. A relay which must be de-energized at intervals to avoid excessive temperature or a relay that is energized at intervals, as in pulsing.
Relay, latching. A relay that maintains its contacts in the last position assumed without the need of maintaining coil energization.
(1)Magnetic latching- A relay that remains operated, held either by remanent magnetism in the structure or by the influence of a permanent magnet, until reset. (See relay, bistable.)
(2)Mechanical latching- A relay in which the armature or contacts may be latched mechanically in the operated or unoperated position until reset manually or electrically.
Relay, lockup. See relay, latching.
Relay, magnetostrictive. A relay in which operation depends upon dimensional changes of a magnetic material in a magnetic field.
Relay, manual reset. A relay that may be restored manually to its unoperated position.
Relay, marginal. A relay that functions in response to predetermined changes in the value of coil current or voltage.
Relay, measuring. An electrical relay intended to switch when its characteristics quantity, under specified conditions and with a specified accuracy, attains its operating value. (IEC)
Relay, mechanical time delay. A relay in which operate or release action is delayed by a clockwork, escapement, bellows, dashpot, or other mechanical device.
Relay memory.
(1) A relay having two or more coils and a set of contacts that remain in a position determined by the coil last energized;
(2) Sometimes used for a latching relay.
Relay, mercury contact.
(1) Mercury-wetted contact-A form of reed relay in which the reeds and contacts are glass enclosed and are wetted by a film of mercury obtained by capillary action from a mercury pool in the base of a capsule vertically mounted.
(2) Mercury contact-A relay mechanism in which mercury establishes contact between electrodes in a sealed capsule.
Relay, meter. See relay, instrument.
Relay, monostable. An electrical relay which, having responded to an input energizing quantity (or characteristic quantity) and having changed its conditions, returns to its previous condition when the quantity is removed. (IEC)
Relay, motor-driven. A relay whose contacts are actuated by the rotation of a motor shaft.
Relay, moving coil. See relay, instrument.
Relay, multiposition. A relay that has more than one operate or nonoperate position; e.g., a stepping relay.
Relay, neutral. A relay whose operation is independent of the direction of the coil current, in contrast to a polarized relay.
Relay, non-specified-time. An electrical relay for which the times are not subject to any accuracy specification.(IEC) Relay, open An unenclosed relay.
Relay, over current. A relay that is specifically designed to operate when its coil voltage reaches or exceeds a predetermined value.
Relay, plunger. A relay operated by a movable core or plunger through solenoid action.
Relay, polarized. A relay whose operation is dependent upon the polarity of the energizing current.
(1)Bistable.(1) A tow-position relay that will remain in its last operated position keeping the operated contacts closed after the operating winding is de-energized.
(2)Center-stable. A polarized relay that is operated in one of two energized positions, depending on the polarity of the energizing current, and that returns to a third, off position, when the operating winding is de-energized.
(3)Double-biased. See bistable.
(4)Magnetic latching. See bistable.
(5)Monostable. A monostable polarized relay is a two-position relay that requires current of a pre-determined polarity for operation and returns to the off position when the operating winding is de-energized or is energized with reversed polarity.
(6)Single-biased. See monostable.
(7)Single-side-stable. See center-stable.
(8)Three-position center-off. See center-stable.
(9)Un-biased. See center-stable.
Relay, power. A relay with heavy-duty contacts; usually rated 25 amperes or higher. Sometimes called a contactor.
Relay, reed. A relay using glass-enclosed, magnetic reeds as the contact members.
Relay, RF switching. A relay designed to switch electrical ac energy with frequencies higher than audio range.
Relay, rotary. A relay whose armature is rotated to close the gap between two or more pole faces. (Usually has a balanced armature.)
Relay, rotary solenoid. A relay in which the plunger's or armature's linear motion is mechanically converted into rotary motion.
Relay, sensitive. A relay that operates on comparatively low input power.
Relay, solenoid. See relay plunger.
Relay, solid state (SSR). A relay with isolated input and output whose functions are achieved by means of electronic components and without moving parts.
Relay, solid state power controller (SSPC). A solid state relay which contains electronic circuitry that monitors electrical characteristics of its containing system and causes the SSPC to open or close based upon its specific parameters. A SSPC combines the traditional functions of a relay and a circuit breaker or fuse.
Relay, static. An electrical relay in which the designed response is developed by electronic, magnetic, optical or other components, without mechanical motion. (IEC) See relay, solid state.
Relay, static with output contact, static relay with output contact. A static relay having a contact in one or more of its output circuits. (IEC) See relay, hybrid electromechanical.
Relay, static without output contact, static relay without output contact. A static relay having no output contacts in its output circuits. (IEC) See relay, solid state.
Relay, specified time. An electrical relay in which one or more of the times which characterize it (e.g., operating time) are subject to specified requirements; in particular, accuracy. (IEC)
Relay, stepping. A relay having many rotary positions, ratchet actuated, moving from one step to the next in succesive operations, and usually operating its contacts by means of cams.
Relay, telephone-type. A term sometimes applied to relay with an end -on armature, an L-shaped heel piece, and contact springs mounted parallel to the long axis of the relay coil. Originally used in telephone systems.
Relay, thermal. A relay that is actuated by the heating effects of an electrical current.
Relay, three position. A relay in which operation or release is delayed internally (coil slugs or sleeves), mechanically, (clockwork, bellows, dashpot, etc.), or by an accompanying solid state timing circuit.
Relay, undercurrent. A relay specifically designed to function when its energizing current falls below a predetermined value. (See relay, current sensing.)
Relay, undervoltage. A relay specifically designed to function when its energizing voltage falls below a predetermined value.
Relay, vacuum. A relay whose contacts are sealed in a low pressure environment.
Relay, vibrating reed. See relay, frequency sensitive.
Relay, voltage sensing. A relay that functions as a predetermined voltage relay, an overvoltage or undervoltage relay, or combination of both.
Relay, wire spring. A relay whose contact springs are made of spring wire.
Relay, zero-voltage-turn-on. A relay with isolated input and output in which added control circuitry delays the output turn-on until a zero voltage transition of the ac sine wave is detected. Construction may be all solid state or hybrid with a solid state output.
Release, measured. See operating characteristics, dropout measured.
Release, overtravel. See overtravel armature dropout.
Release specified. See operating characteristics, dropout specified.
Release time. See contact, release time.
Release value. See operating characteristics, dropout value.
Reset. The return of contacts or a mechanism to the normal state (initial conditions).
Reset, automatic. A qualifying term applied to:
(1) A stepping relay that returns to its home position either when it reaches a predetermined contact position or when a pulsing circuit fails to energize the driving coil within a given time. May either pulse forward or be spring reset to the home position.
(2)An overload relay that restores the circuit as soon as an over-current situation is corrected.
Reset, electrical. A qualifying term applied to a relay to indicate the it may be reset electrically after operation.
Reset, manual. A qualifying term applied to a relay to indicate that it may be reset manually after operation.
Residual, armature. The protrusion from the armature or pole face that provides the residual gap ( see residual screw, pin, plate, stud or shim). Residual screw, pin, plate, stud or shim. Non-magnetic screw, pin, plate, stud or shim attached to either the armature or the pole face of a relay to prevent the armature from directly contacting the magnetic core.
Residual setting. See gap residual.
Resistance, contact. The electrical resistance of closed contacts measured at their associated contact terminals.
Resistance, dynamic contact. Variation in contact resistance due to changes in contact pressure during the period in which contacts are motion, before opening or after closing (See Figures 1.1 and 1.2).
Resistance, winding. The total terminal-to-terminal resistance of a winding at a specified temperature.
Resonant reed relay. See relay, frequency sensitive.
Restoring spring. See spring, return.
Retractile spring. See spring, return.
Return. A relay returns when sequentially: (1) it disengages; (2) it passes from an operated condition towards the prescribed initial condition;(3) and it resets. (IEC)
Returning ratio. The ratio of the returning value to the operating value.(IEC)
Ring, mechanical damping. A loose member mounted on a contact spring to reduce contact chatter and bounce.
Ring, shading. A shorted turn surrounding a portion of the pole of an alternating-current electromagnet that delays the change of the magnetic field in that part, thereby tending to prevent chatter and reduce hum.
Rotary stepping relay. See relay, stepping.
Rotary stepping switch. See switch, stepping.

1.7 Relay Terms and Definitions-Glossary

The following definitions do not include terms peculiar to mathematical formula, statistical analysis, relay reliability studies, and the like. Such terms are defined in the appropriate chapter. When cross-references are made, the preferred terminology is the one under which the definition appears.

Seating. The magnetic positioning of an armature in its final desired location.
Self de-energize. The removal of power from a relay coil by an auxiliary switch or contact within the relay itself. Usually applies to latching relays only.
Self de-energizing switch. A secondary relay or auxiliary contact usually enclosed within the primary relay which removes power from the primary relay coil after it has transferred position. Usually applies to latching relays only.
Sensitivity. Specified pickup expressed in watts.
Separation, contact. See gap, contact.
Sequence, contact. The order in which contacts open and close in relation to other contacts and armature motion.
Set, contact. See combination, contact.
Shading coil. See ring, shading.
Shield, electrostatic. Grounded conducting member located between two or more mutually insulated elements to minimize electrostatic coupling.
Shield, magnetic. A ferromagnetic member used to reduce magnetic coupling.
Shim, nonmagnetic. A nonmagnetic member introduced in series with the magnetic circuit. See residual.
Sleeve, coil. A conducting tube placed around the full length of the coil core as a shortcircuited winding to retard the establishment or decay of flux within the magnetic path.
Slow-release time characteristic. See characteristic, slow release time.
Slug, coil. A conducting tube placed around a portion of the core to retard the establishment or decay of flux within the magnetic path.
Soak magnetic. The conditioning of a relay to a predetermined magnetic state (usually saturation).
Soak value, magnetic. The voltage, current, or power applied to the relay coil to ensure a predetermined magnetic state (usually saturation).
Solenoid relay. See relay, plunger.
Spring, armature. See contact movable.
Spring, balance. A member used in relays with permissive make contacts to provide restoring force.
Spring, brush. The spring in a rotary stepping-switch bank that contacts the associated wiper.
Spring, buffer. See spring, damper.
Spring compliant (contact). A contact spring that moves appreciably when mating contact spring.
Spring, contact. A current-carrying spring to which a contact is fastened or which in itself serves as a contact.
Spring, damper. An auxiliary spring added to prevent unwanted movement of some relay member because of vibration or shock.
Spring, driving. The spring that drives the wipers of a stepping relay or stepping switch.
Spring, noncompliant (contact). A contact spring that does not move appreciably when contacted by a mating contact spring.
Spring, restoring. See spring return.
Spring, retractile. See spring, return.
Spring, return. A spring that moves or tends to move the armature to the released (normal) position; i.e., a spring that provides a force to move the armature to the released (normal) position and hold it there.
Stack. See pileup.
Stack contact. See pileup, contact.
Start. A relay starts at the instant it leaves an initial condition. (IEC)
Step, applied power. The sudden application of RMS load voltage to the relay when in the off condition.
Stop, nonmagnetic armature. See residual.
Stop, spring. A member that controls the position of a pretensioned spring.
Stroke. See travel, armature.
Stud, armature. See buffer, armature.
Stud, spring. Stud, gap. See gap, buffer.
Switch. A relay switches at the instant it completes its designated function in a given output circuit. (IEC)
Switch, crossbar. A switching device employing multiple relay elements in a matrix of grid arrangement to connect selected crosspoints electrically.
Switch, dry reed. See contact, reed. Switch, stepping. A class of electromagnetically operated, multiposition switching devices. Their wipers are rotated in steps so that contact is successively made between the wiper tips and contacts that are separated electrically and mounted in a circular arc called a bank.
Temperature case (T_C). The temperature at a specified point on the relay which is used to evaluate the current capabilities of the relay.
Temperature, maximum, allowable case (T_{C max}). The maximum allowable case temperature for a given load current at rated line voltage.
Temperature, maximum junction T_J(MAX). The maximum junction temperature of the output switching semiconductors, expressed in ° C. (Applies to solid state relays.)
Temperature, operating (T_A). The ambient temperature range over which operation is specified with relays unmounted or mounted to a heat sink as specified.
Temperature, storage (T_STG). The ambient temperature range to which a nonoperating relay may be subjected without permanent electrical or mechanical damage.
Terminal contact spring. The portion of a contact spring to which the current-carrying conductors are attached.
Terminal coil. A device such as a solder lug, tab, binding post, or similar fitting, on which the coil winding lead is terminated and to which the coil power supply is connected.
Thermal resistance maximum, junction to ambient (R_OJA). The maximum thermal resistance between the semiconductor junction and still air ambient. (Expressed in ° C/watt).
Thermal resistance maximum, junction to case (R_OJC). The maximum thermal resistance between the output switch semiconductor(s) junction to point of measurement on the relay case (expressed in ° C/watt).Thermal shock non-operating. These temperature extremes between which the relay may be rapidly cycled without permanent electrical or mechanical damage.
Time, actuation. The time interval from coil energization or de-energization to the functioning of a specified contact; same as time, contact actuation, subdivided as follows:
(1)Time, final actuation-The sum of the initial actuation time and the contact bounce intervals following such actuation.
(2)Time, initial actuation-The time from coil energization or de- energization to the first closing of a previously open contact or the first opening of a previously closed contact. (See time, operate; time, release.)
Time, bridging. The time in which all contacts of a continuity transfer combination are electrically connected during the transfer.
Time, contact bounce. The time interval from initial actuation of a contact to the end of bounce.
Time, contact stagger. The time interval between the functioning of contacts on the same relay.(For example, the time difference between the opening of two normally closed contacts on pickup.)
Time, operate. (1) The time interval from coil energization to the functioning of the last contact to function. Where not otherwise stated, the functioning time of the contact in question is taken as its initial actuation time (that is, it does not include contact bounce time). (See Figures 1.01 and 1.02.) (2) For a solid state or hybrid relay in a non-operated state, the time from the application of the pickup voltage to the change of state of the output.
Time, release. (1) The time interval from coil de-energization to the functioning of the last contact to function. Where not otherwise stated, the functioning time of the contact in question is taken as its initial actuation time (that is, it does not include contact bounce time). (2) For a solid state or hybrid relay in an operated state, the time from the application of the dropout voltage to the change of state of the output.
Time, seating. The time interval from coil energization to the seating of the armature.
Time transfer. The time interval between opening the closed contact and closing the open contact of a break-before-make contact combination.
Travel, armature. The distance traveled during operation by a specified point on the armature.
Trip pulse. A short, high level pulse applied to the tip coil of a latching relay.
Trip, to trip. A latching relay trips when it changes from the operated to the unoperated condition. Usually refers to the breaking of continuity of the main contacts.
Trip value, specified. As the current or voltage on an operated latching relay is step applied, the value at or below which all contacts return to their unoperated positions.
Tube, coil. An insulated tube on which a coil is wound.
Unergized condition. The specified condition of an unergized monostable relay. (IEC)
Vibration, nonoperating. The vibration level and frequency span to which the relay may be subjected without permanent electrical or mechanical damage.
Voltage, nominal A single value of voltage (or a narrow voltage range) intended to be applied to the coil or input. See voltage, rated coil.
Voltage, off state. In solid state relay, the following determine whether the relay will stay off under each load voltage condition:
(1) Critical rate of rise of commutation voltage, dv/dt. The maximum value of the rate of rise of principal voltage which will cause switching from the off state to the on state.
(2) Maximum off state voltage (V_DMax)(RMS). The maximum effective steady state voltage that the output is capable of withstanding when in off state.
(3) Maximum rate of rise of off state voltage, dv/dt. The rate of rise of the off-state voltage which the output can withstand without false operation.
(4) Minimum off state voltage (V_DMin)(RMS). The minimum effective voltage which the relay will switch.
(5) Non-repetitive peak voltage (V_DSM).The maximum off-state voltage that the output terminals are capable of withstanding without breakover or damage.
Voltage, on state. In solid state relays, the output terminal wave form at rated current consists of repetitive half-cycles (+and-) of distinctive voltage drops. Each voltage state is necessary for load current conduction and may be specified for specific applications, as follows:
(1) Instantaneous on state voltage (V_T) The instantaneous voltage across the output when in the on condition.
(2) Maximum RMS on state voltage (V_T)(RMS). Maximum RMS voltage drop across the relay output at maximum load current ITRMS.
(3) Minimum power factor load (PF_MIN). The minimum power factor load the relay will switch and still meet all of its electrical specifications.
(4) Peak on state voltage (V_TM). The maximum value of V_T excluding +/- 20degrees of zero crossing of the voltage wave form.
Voltage, rated coil. The coil voltage at which the relay is intended to operate for the prescribed duty cycle. Note: The use of any coil voltage less than rated may compromise the performance of the relay.
Voltage, reverse polarity. The maximum allowable reverse voltage which may be applied to the input of a solid state relay without permanent damage.
Winding. An electrically continuous length of insulated wire wound on a bobbin, spool or form.
Winding, bias. An auxiliary winding to produce an electromagnetic bias.
Winding, bifilar. Two windings with the wire of each winding alongside the other, matching turn for turn; may be either inductive or noninductive. (See coil, parallel wound.)
Winding, noninductive. A winding in which the magnetic fields produced by two parts of the winding cancel each other and provide noninductive resistance.
Wipe, contact. The sliding or tangential motion between two mating contact surfaces as they open or close.
Wiper. The moving contact on a rotary stepping switch.
Zero-voltage-turn-on voltage (zero synchronous turn-on) (V_TO). The maximum voltage across the output terminals following initial turn-on.

Priniciples of Electromechanical Relay Operation

3.1 Introduction

This section is concerned with the fundamental operating principles of "light duty" electromagnetic relay. Such relays are for use in communication, control, monitoring, or alarm switching circuits in which load currents are normally fractions of an ampere to 25 amperes. The questions of importance in the use of such relays tend to be not only how much power they can switch but more importantly how often, how fast, how reliably, or how consistently. For the best understanding of the electrical and mechanical characteristics of such relays, one should consider separately the electromagnetic switching "actuator" and the contact performance. Principles of operation for solid state relays are discussed in Chapter 14.

The principles of operation discussed here are those that might reasonably be of concern to a user of relays or a circuit designer faced with the selection and specification of a relay that will best satisfy his needs in terms of functions to be performed and modes of operation and environments to be encountered. Since a relay usually performs a switching function, an understanding of contact phenomena should also aid in the selection of a relay well suited to a particular application in terms of reliability and optimum life. Relay performance follows precise rules of physics, chemistry, and metallurgy. When these rules are observed or violated, the relay behaves accordingly.

The texts listed in the bibliography provide a comprehensive treatment of relay design and contact theory.

Priniciples of Electromechanical Relay Operation

3.2 Direct-Current Relay Actuator Systems

A relay is a remotely controlled operated switch; it consists of one or more contact pairs that serve to open, close or transfer external circuits. Consideration of the electrical phenomena occurring at the coils during the switching function is given in Paragraph 3.13, and contact(s) considerations start at Paragraph 3.14. Some mechanical requirements for their operation must be considered first.

Priniciples of Electromechanical Relay Operation

3.3 Mechanical Requirements for Operation

The switch usually consists of combinations of movable and fixed, low resistance contacts which do the actual circuit opening and closing. The fixed contacts may be mounted on compliant springs or fixed brackets. The movable contacts are mounted on some form of spring that can be deflected or on a hinge arm. The force and travel needed for these motions must serve a number of purposes.

Before the armature is actuated for a relay with double throw contacts, the movable contacts must be held against the normally closed fixed contacts by a spring force sufficient to establish good electrical connection. When the armature is actuated, a number of things happen. By some means, each movable contact is pushed or pulled away from the corresponding, normally closed, fixed contact. This requires a force sufficient to overcome one or more springs. Also, there is friction between the contacts if they slide before they separate, and in actuator pivots, if they are present. As contact motion takes place, various springs deflect according to Hook's low, and inertial forces must be overcome.

After the accelerated motion of contact transfer, there is an impact and deceleration as the movable contact reaches its normally open fixed contact. Both must deflect or deform to some degree as the desired contact force builds ups. In many designs, some further overtravel will occur to provide contact cleaning action, through sliding, and to compensate for contact wear or erosion. Also, when there are multiple sets of contacts, allowance must be made for manufacturing tolerances for the various stages of travel required for and the spring forces associated with them.

Understanding these forces and travels requires a consideration of the contact mechanism and the motions involved. The discussion below will deal, in an elementary way, with both the static and the dynamic characteristics required of a mechanism for providing these forces. Detailed analysis of design consideration are available in the reference mentioned at the end of this section.

Priniciples of Electromechanical Relay Operation

3.4 Power-Force-Stroke Relationship DC Electromagnets

The most common form of actuator or motor system for electromagnetic relays consist of an energizing coil and a permeable iron circuit. It has both a fixed portion (open loop) and a movable member, called the armature, that completes the magnetic circuit by closing the air gap. This armature must be hinged, pivoted, spring-mounted, or somehow free to move within certain constraints so that its motion can do useful work, namely, causing the contacts of the controlled circuit to perform a switching function. In addition, it must normally store some energy in a spring (or springs) for the return stroke and for holding selected contacts closed when the relay coil is in the de-energized condition. The factors of the greatest importance over the life of the relay are the following:

(1) To provide, at a minimum reasonable expenditure of power, adequate magnetic pull to assure reliable closing of contacts.
(2) To provide sufficient separation between open contacts.
(3) To provide the desired operate and release time characteristics.
Contact Actuating Systems. Actuation of contacts in electromagnetic relays is accomplished by four methods:
(1) Direct armature clapper actuation (simplest construction)
(2) Multiple spring flexure (as in low power relays)
(3) Permissive make actuation (typical of wire spring relays)
(4) Direct solenoid actuation (typical of power contactors)
Only the first three systems will be analyzed here in as much as solenoid actuation is readily understood and generally not applicable to relays switching less than 25 amperes.

Direct Armature Clapper Operation. One of the simplest contact arrangement employed in relays is the Break Before Make or transfer (form C) contact combination employing a clapper type armature-(see Fig.3.1)
It uses a flat or helical coil (armature return spring) to provide the return or restoring force on the armature in the de-energized position. The break (or normally closed) contact is normally closed under action of the return spring. The sequence of spring forces to be overcome by the armature in transferring contacts to the fully operated position is graphed in Fig.3.2(broken curve 1,2,3,4).

 

The moving contact spring attached to the armature is held against the break contact with a force created by the return spring that also causes flexure in the movable contact spring. When the coil is energized, the armature will immediately begin to move in proportion to the energizing power. Between points 1 and 2, the flexed armature spring is being relieved, but the force to be overcome by the armature is also determined by the spring rate (deflection per unit loading) of the return spring. At point 2, the movable contact spring is completely relieved, and the normally closed (break) contacts open. The armature continues to overcome the spring rate of the return spring until the normally open (make) contact is met at point 3. Load on this contact is built up at the combined spring rates until the armature bottoms on the core at point 4.

 

Flexure Operation. In low power relays, the basic spring arrangement are typically those illustrated in Fig.3.3. In this so-called flexure operation, a return spring is tensioned to hold the armature in the de-energized position. The moving contact spring is independently tensioned against the normally closed (break) contact. A slight separation, X₁, is provided between the insulated actuator card or buffer and the moving contact spring to assure that the full force of the moving contact spring is exerted against the stationary break contact. When the relay is energized, the armature, through the actuator, lifts the moving spring contact off the break contact and pushes is toward the stationary, normally open (make) contact. After the contacts touch, further armature travel builds up contact force until the armature seats on the core or against the armature stop. The force developed at the contact interface is a function of the flexure of the moving contact spring and the stiffness of the stationary contact spring system.

 

In Fig. 3.4, at point 2, the moving contact spring is picked up by the armature buffer, and between points 2 and 3, the normally closed contact preload is overcome. Between points 3 and 4, the contact gap, X₂, is closed and normally open contact is met at point 4. Pressure is built up on the Make contacts between points 4 and 5. Armature overtravel, or contact wear allowance, ends at point 6, where the armature is seated against the pole face.
It should be evident from this force curve that contact pressures are the result of spring deflections caused by overtravel or built-in preload (the adjustment process). Therefore, unless spring preloading is expressed in the relay design, contact force will be reduced proportionally during relay life as a result of mechanical deformation, electrical erosion of contacts, and armature bearing wear.

 

Fig. 3.5 shows a contact construction in which initial tension or preloaded is built into the spring. With such an arrangement, any contact lift-off at all requires a minimum force equal to the initial tension. Furthermore, the contact force does not diminish appreciably during the wear-out process. On the other hand, the force to be overcome by the armature during the pickup stroke is a step function. This makes it possible for the pickup power measured at the start of the armature travel to be less than that required to complete the stroke after the engagement of the contacts. In this case, such a relay is not applied in circuits where the full specified pickup power is abruptly applied to the coil as in "all or nothing" relays.

 

Permissive Make Actuation. This form of contact transfer is illustrated Fig. 3.6. In this case, two moving springs and one stationary contact, from a contact set or a transfer. The moving spring (B) for the break contact is tensed against the stationary contact, and a slight separation is provided between it and the insulating actuator to assure that the full force of the pretensioned moving contact is exerted against the stationary contact. As the armature moves toward the core, the moving contact is lifted off the stationary contact.

 

The moving Make contact spring (A) is tensed against the insulating actuator with a force equal to the desired make contact force. As the armature moves toward the core, the actuating insulator follows. After a prescribed travel, it leaves the moving spring as the moving contact comes to rest on the stationary contact. The armature continues to move toward the core. This provides separation between the actuating member and moving spring and assures that the full pretensioned force of the moving spring is exerted against the stationary contact.

The moving springs of the system usually are extremely compliant. Card wear and erosion of the contacts have virtually no effect on the contact force until the erosion or wear has progressed to the point that the movable contacts rest on the card and their force is not exerted against the fixed contacts. This effect is independent of spring compliance. Balance springs are required with permissive make actuation to overcome the force of the moving Make contacts is restoring the armature to its de-energized position.

Permissive make actuation can provide somewhat longer life than flexure actuation. Contacts, however, are more susceptible to contact chatter under the severe vibration and shock conditions that are encountered in many applications. Another advantage of permissive make actuation is that any tendency of an individual contact to stick or weld is overcome, to some extent, by the forces of the other contact springs acting on the system. In the case of the flexure system, only the restoring force of each individual moving spring is available to overcome any sticking tendency of an individual, normally open, contact pair.

Priniciples of Electromechanical Relay Operation

3.5 Matching Mechanical And Electrical Characteristics

The area under the load curve is a measure of the work that the electromagnet is required to perform. Curve B of Fig. 3.2 represents typical pull curve for an electromagnet. It shows the pull relative to the distance between the armature and core for a particular value or coil ampere-turns. This figure demonstrates that in order for the relay to operate, the magnetic force developed at all points of the armature travel will have to exceed the mechanical forces tending to restrain motion of the armature.The force developed by the electromagnet may be expressed by:

in which N is ampere turns; A is pole piece area: x is distance between armature and core in de-energized position and R_o is reluctance of iron portion of magnetic circuit.
Thus the pull of an electromagnet of fixed dimensional constraints varies with the energizing ampere-turns squared. The ampere-turn value at which the relay just operates is known as the ampere-turn sensitivity. For many circuit applications, however, it is more convenient to express sensitivity in terms of power, P, required to operate (P=I^2R, in which R is the coil resistance). Whereas ampere-turn sensitivity is independent of coil dimensions, power sensitivity varies with volume and the proportion of conductor space occupied by the winding. For a given coil volume and proportion, the ratio N₂/R is constant. It is known as coil conductance and is symbolized by Gc. By inversion, R = N₂/G_c and P=I^2R=N^2I^2/G_c watts or power required is inversely proportional to G_c. Coil conductance may be expressed in terms of the dimensions of the coil by the equation,

 

in which e = winding space factor(decrease only slightly for fine wire windings); p = wire resistivity; l = length of winding cross section; h = depth of winding cross section and d = diameter of core.
Since power is N^2/I^2/G_c, it is clear that the power required to operate varies inversely with coil length and directly with winding depth. Do not assume that power sensitivity will be the same for all values of coil resistance. Using standard magnet wire sizes it is not possible to obtain equal fullness of winding for all desired resistance values.
The required ampere-turns of magnetizing force to be generated in a coil of given available volume may be obtained by many turns of fine wire or much fewer turns of coarse wire that carries correspondingly larger currents. Using wire of a given size and of a known resistance per unit length, the resistance of the coil can be computed from the number of turns that can be wound in the particular dimensions available.
The coil conductance G_c also relates to the amount of heat generated in the coil when providing a given magnetizing force, as well as to the inductance of the coil (the principle factor in determining the response speed of a relay of given design to a given electrical drive).
Scaling down the size of a relay proportionally in all directions affects the performance in a number of ways:
1. More magnetizing power (ampere-turns) is required for a given armature force and stroke.
2. Magnetic saturation of the iron circuit is reached at lower armature force values and little further gain in pull is possible.
3. The coil gives fewer ampere-turns of magnetizing force for a given power input.
4. The ability of the relay to dissipate internally generated heat fall off sharply.
5. The relay is more resistance to vibration, particularly at the higher frequencies, because of the lower moments of inertia of its various parts.
Changing the coil wire one gauge size finer is a coil of given dimensions increases the coil resistance by approximately 60% if the bobbin is wound to the same fullness. Such a change does not affect the power required for a given number of ampere-turns nor, therefore, the corresponding switching performance. It does reduce the current required for equal ampere-turns by 20% and increases the voltage required by 25%. Incremental changes of this order do not ordinarily introduce circuit mismatch inefficiencies of sufficient amount to be of concern, but the use of standard wire sizes (and resistance tolerances) automatically sets the resistance interval for fully wound coils at approximately 60% and the resistance tolerance at plus or minus 10% fir all but the very fine wire sizes (above No. 45 AWG, a tolerance of 15% is sometimes encountered).
Consideration of the force curves shown in Figs. 3.02 and 3.04 indicates that the ease of obtaining contact action with large forces and adequate travel is dependent on how low the dropout power may be as well as the value of pickup power available. Also, the choice of the return spring force and contact spring rates permits the designer to make a trade-off between armature travel and the contact forces for given pickup and drop out values.
In such a trade-off, there are least five variables related to the desired relay performance: 1) pickup values, 2) dropout values, 3) contact gap, 4) pole gap, and 5) contact pressure force. Three may be selected arbitrarily. The remaining two then become dependent variables and must be accepted, or else the first three choices must be revised until all five variables are satisfactory.
A simple relay with a compliant normally open contact has certain predictable characteristics:
1. Pickup values are adjustable within the limitations of spring force and contact position stability.
2. Dropout values cannot be precisely adjusted without special provisions.
3. Contact erosion results in variations in both pickup and dropout values and usually determines the effective end of contact life.
4. Contact impact usually results in some bounce and provides some degree of contact wipe, roll or scrubbing motion.
5. The normally closed contacts become less stable as the relay coil current approaches the pickup value. This condition tends to make many relays prone to contact chatter when subjected to environmental vibration or to coil current ripple when partially energized close to the pickup value.
6. The normally open contact set, commonly having greater overtravel, is less subject to chatter and bounce.
7. Erratic pickup and dropout values can result from switching loads with heavy inrush current transients sufficient to cause contact sticking. Use of modern magnetic iron, annealing techniques, and relay design has resulted in relays seldom troubled with erratic performance due to magnetic hysterics.
Dropout. Curve C Fig. 3.2 represents the pull of a relay electromagnet in the release or dropout range. As indicated, the armature will restore to its de-energized position when the energizing ampere-turns produce a pull throughout the release stroke that is less than the mechanical load.

Priniciples of Electromechanical Relay Operation

3.6 Dynamic Considerations

In addition to satisfying the requirements for operations under various steady-state conditions of circuit and environment, the relay must be considered in its dynamic characteristics, e.g. operate time, tendency to chatter caused by current ripple, and bounce caused by contact impact. Relay response to a step change in applied voltage is subject to delay because of two factors:

1) The first is the time required to change the flux in the magnetic circuit from some initial value to whatever value will cause the desired operation (pickup or dropout). This delay is related to inductance (L) and resistance R in the coil circuit and to applied voltage (E) through the expression for current:

Since, however, the value of inductance (L) is highly variable as a function of the degree of magnetic saturation of the iron, an empirical solution is normally required.
2) The second is the mechanical transfer time of the moving elements. The time required for the accelerated motion of transfer in most small relays is short compared to the time required for the inductive current buildup except for applications in which the effect of the relay inductance is minimized by the other circuit parameters.
The pickup delay due to coil inductance, particularly for sensitive relays with a large value of coil constant, can be considerable, with 5 to 50 milliseconds being common, whereas transfer times will usually fall within the range of 1 to a few milliseconds. For relays of a given design, the speed of response can be improved by:
Although relay inductance is quite variable, dependable operate time curves can be made that are based on the coil conductance relationship, G_c = N^2/R, in which the actual time for operation is plotted against coil "overdrive". Coil overdrive consists of two factors- the ratio of the final steady-state current to the relay pickup current, and the ratio of the open-circuit voltage to the voltage actually required for pickup current, and the ratio of the open-circuit voltage actually required for pickup of the relay. Graphic representation of coil overdrive versus operate time requires a family of curves; one form is shown in Fig. 3.7. Using these curves for a given design of relay, one can start with the desired performance and determine the necessary circuit power as well as the relay specifications or start with given circuit conditions and determine the speed of response after the coil current is reduced below the dropout value. If, however, a relay is shunted with a diode such as might be used for transient voltage suppression in transistor or integrated circuits, the delay can be as great as 20 to 300 milliseconds or longer. This is expressed approximately by the conventional, logarithmic, current decay curve from the steady-state value to the relay dropout value.
In the design of a relay for high speed service, the bounce of the contacts resulting from the closure impact is another dynamic characteristic to consider. In general, some mechanism for the absorption of the moving parts kinetic energy at the instant of impact must be provided in order to keep bounce time short compared to the operating time. This is normally accomplished by providing a small amount of spring compliance and contact rubbing action in addition to making the movable contact member as light as possible and its resonant frequency high. In any power switching application, bounce control is one of the most important factors in obtaining good contact life. Visible arcing at contact closure is normally evidence of contact bounce.

 

Priniciples of Electromechanical Relay Operation

3.7 Design Analysis

The armature type electromagnetic relay is a complex electromechanical device in which electrical energy is converted, through linkages, into mechanical motion that actuates electrical contacts. Its performance and reliability, therefore, depend upon the interactions of many design parameters related to applications and performance criteria. It is essential, then, that the suitability of a design be evaluated in terms of the factors that may affect its performance under specified environments, winding circuit conditions, mechanical life requirements, and contact loads.

The initial phase in the evaluation and application of any relay should be an analysis of its design and study of the controls exercised to assure consistent performance. To a large extent, design criteria depend upon the type of application, the economic considerations, and the consequences of failure. A design that is adequate for most applications might not be suitable for use in critical circuits requiring close controls and a high degree of stability. In such cases, performances studies of limited quantities of relays cannot always be relied upon as the sole means of establishing the suitability of a design. Whether a design is basically sound, employs quality materials, and is a product of good workmanship and controlled techniques must also be determined. Relay design analysis is an art in involving good engineering judgment. This judgment must be based, to a large extent, on experience and an appreciation of cost factors related to performance requirements and the consequences of failure. It is not possible to present a comprehensive treatment of the subject in this handbook. The following discussion, however, outlines a few design factors that warrant consideration in specifying relays for critical applications.

1.Friction. Friction can significantly affect performance. Plated armature bearings that tend to rust often produce excessive friction. Excessive slide or wipe between actuating or actuated members also may cause variable friction.

2. Finishes. Some finishes intended for corrosion protection are susceptible to a whisker-like growth that can cause voltage breakdown between plated parts and other metallic members of a relay having small clearances. Wear-resistant finishes on pole faces and backstop surfaces are often necessary on relays requiring a long mechanical life or high degree of electrical characteristic stability.

3. Contact Adjustments. Contact force and contact overtravel may be affected significantly be environment, wear, and contact margins. It is essential, therefore, that the minimum adjustments provide sufficient margin to assure reliable contact performance for the required life and applicable environments. The design and contact adjusting techniques should assure also that the full force of movable contact springs is exerted against the fixed contacts. With buffer-actuated contact springs it is essential that there be a gap between the actuator and movable spring when the armature is fully released. If a buffer gap is not provided, appreciable contact follow is required to assure adequate contact force. For relays with armature card lift-off actuation, clearance is necessary between the card and movable springs of closed contacts in both the energized and de-energized positions. In many instances (such as normally open contacts of permissive make actuated designs), contact force cannot be measured directly, however, the overtravel or follow through distance after contact make can be measured instead. To determine the type of measurement required to assure reliable contact one needs to study the actuating system.

4. Manufacturing Cleanliness. Relay parts-particularly contacts, bearing, and polefaces-should free form particulate contamination, and contacts should be free from organic films when controlling low energy circuits.

5. Contact Materials. The contact materials employed should be suitable for contact load, environment, and other performance requirements. Wherever possible, materials that tend to stick should be avoided.

6. Insulating Materials. Insulating materials within contact chambers, or adjacent to contacts of open relays, should emit a minimum amount of vapors that might impair contact performance. Design employing inorganic materials, or in which the contacts are isolated from organic materials, offer greater assurance of reliable contact performance. Insulating materials should be A) free form corrosion promoting impurities, B) dimensionally stable to minimize adjustment changes with fluctuation in temperature and humidity, C) free from a tendency to shed particular to shed particles that may contaminate contacts or become entrapped in bearings or air gaps, and D) suitable for the environmental temperature ranges.

7. Soldering Fluxes. Virtually all liquid or paste soldering fluxes and chemical strippers for enameled wire are highly corrosive. Unless the design lends itself to thorough washing and neutralizing processes, the use of the fluxes and strippers should be avoided. Sealing techniques for hermetically sealed relays should insure that flux or vapors are not entrapped within the contact chamber.

Priniciples of Electromechanical Relay Operation

3.8 Winding Circuit Design

There are four operating characteristics for electromagnetic realys, namely, nonpick-up and pick-up on the operate stroke and hold and dropout on the release stroke. Whether only one or a combination of these characteristics is specified depends on circuit complexity and the relay's function. Definitions for these characteristics, which were given in Chapter 1, shall be restated here briefly. When a winding in energized:

1. Nonpick-up is the voltage (or current) at or below which (a) The armature shall not move from its de-energized position, or (b) the normally closed contacts shall not open and the normally open contact shall not close

2. Pick-up is the voltage (or current) at or below which: (a) The armature shall assume its fully operated position (seated against the core), or (b) normally closed contacts shall open and all normally open contact shall close.

When the energizing voltage (or current) of an operated relay is reduced:

1. Hold is the voltage (or current) at or above which: (a) the armature shall not move from its operated position, or (b) all normally open contacts shall open and all normally closed contacts shall close.

2. Dropout is the voltage (or current) at or above which: (a) The armature shall restore to its de-energized position, or (b) all normally open contacts shall open and all normally contacts shall close.

For most applications only pick-up and drop out requirements are essential. Special circuit conditions or other application considerations sometimes necessitate specifying nonpick-up and hold requirements. These will be discussed later. It is usually advisable to specify requirements in terms of the armature position. In some designs, however, specified only in terms of contact operation. Relays in which the armature hesitates while on the verge of just closing or opening contacts or in which the contacts have barely opened or closed could be in an extremely unstable state. A slight mechanical disturbance or voltage fluctuation could cause intermittent opening or closing of contacts. It is advisable, therefore, to specify requirements assuring that the desired performance will be obtained, that is, that the contacts will be fully operated. Specifying that the armature shall not move at a particular energizing value and shall complete its stroke at another value will provide this assurance. In some designs, such as certain sensitive relays under marginal requirements, it may not be possible to guarantee that the armature will seat on the pole piece fully, even though the contacts have transferred properly.

Priniciples of Electromechanical Relay Operation

3.9 Winding Resistance and Sensitivity

To define relays either as voltage sensitive or current sensitive devices is to use common misnomers. When a relay is to be operated from a power source with no series components, it is convenient to specify performance requirements in terms of voltage. When the winding circuit includes other fixed or variable components, however, it is preferabble to specify relay and circuit requirements in terms of current. In either case, it is the ampere-turns-the product of turns and current (NI)-that determines the sensitivity and operating characteristics or a relay. Only for convenience, or to reflect circuit usage, are relays designated voltage or current relays.

Efficiency or sensitivity of an electromagnetic armature type relay may be expressed in ampere-turns or watts required to get full armature closure. For a particular design and contact arrangement, ampere-turn sensitivity is dependent of wire size or fullness of winding. On the other hand, power sensitivity (I^2R or EI) varies with the fullness of winding since it is a function of coil conductance, G_c=N^2/R^2. In the absence of information on a desired winding, it is important that the manufacturer be consulted with respect to sensitivity or operating characteristics. Almost all electromagnetic relay coils are wound with copper magnet wire. Since winding resistance is proportional to the absolute temperature for copper windings:

In these expressions, T_O and T₁ are winding temperatures, and 234.5 is the implied point of zero resistance on the Celsius scale. Since the current required to perform a particular function remains constant, assuming no instability, the power required (I^2R) is proportional to absolute temperature. Ampere-turn sensitivity, again assuming no instability, is unaffected by temperature.

Priniciples of Electromechanical Relay Operation

3.10 Dielectric Considerations

Dielectric ratings for relays are a function of size, the separation between contacts, and the separation between various parts of the structure. The ability of a relay to withstand impressed voltage depends on the type of insulation employed and the severity of the in-service environment. Cold flow of certain types of insulation at high temperatures or prolonged exposure to extremely high humidity may result in a substantial reduction of the dielectric withstanding voltage. Note also that self-induced or externally developed voltage transients as well as the steady-state voltages impressed on the winding or contacts should be well within the capability of a design.

Priniciples of Electromechanical Relay Operation

3.11 Timing Characteristics

Operate Time. Determination of operate time of electromagnetic armature type relays is treated comprehensively in various tests. This discussion is limited, therefore, to circuit applications using the circuit parameters and basic characteristics or a relay to predict the probable range of operate time. Although the method neglects some secondary factors, it is sufficiently accurate for most purposes. Operate time is a function of relay adjustments and coil conductance, Gc = N2/R, which varies with fullness of winding but is substantially constant for a given fullness or winding. The resistance term includes any resistance, which varies with temperature, i.e., operate time will vary with temperature. The following discussion of operate time considers only time to first closure and disregards contact bounce time, which is influenced by many factors relating to the dynamics of a relay design. Operate time of a relay is comprised of waiting time and transit time. Waiting time is the interval, after closure of the winding circuit, in which the flux builds up sufficiently to start the movement of the armature toward the core. Transit time is the interval between the start of movement of the armature and the completion of the switching function. As the energizing power is increased, waiting time decreases but finally levels off. Transit time also decreases somewhat with an increase in energizing power, but it, too, finally tends to become a constant at a certain power level. Thus, total operate time decreases with increased power and approaches a value at which there will be little or no benefit derived from increasing the driving power.

Fig 3.8 illustrates typical operate time curves obtained at three ambient temperatures on several samples of a particular type of four transfer (4 form C) relay. The data were obtained using a bounce free, mercury wetted contact to control the winding circuit. In this figure, Q is the ratio I_j/I_a (or E_j/E_a, in which I_j (or E_j) is the measured pick-up point in current (or voltage) and I_a (or E_a) is the current (or voltage) at which the winding is energized. The upper and lower limits of operate time for a family of relays at given values of Q are illustrated by sets of curves, A and B. Curves such as these provide the basic data for predicting operate time. A family of relays refers to a group of relays having essentially the same armature travel and load and the same fullness of winding or coil conductance (G_c=N^2/R). For another fullness of winding, the timing characteristic will be different. For example, for shallower windings, operate time will be shorter because of N^2 term of G_c decreases more rapidly than the resistance, and varies directly with G_c. As Q decreases (it is the equivalent of increasing power), operate time decreases until it approaches a minimum time. At this point, very little increase in speed is derived from large increases in power. At the other extreme, operate time approaches maximum at Q=1, the point at which energizing current (or voltage) is equal to the value at which the relay just picks up. It is inadvisable to operate a relay in the steep region of these curves if operate time is critical.

If a relay is to be used over a range of ambient temperatures, curves should be obtained at the applicable temperature extremes, as illustrated by Fig. 3.8. Likewise, if a winding reaches an elevated temperature as the result of internal heating, and then is de-energized and re-energized shortly thereafter, timing data should be obtained for this condition. Heating effects are particularly important in rapidly pulsing circuits. It may be seen from Fig. 3.8 that for the same value of Q, operate time is shorter at higher temperatures and longer at lower temperatures. The effect is explained by G_c or N^2/R, being inversely proportional to resistance and absolute temperature. As G_c decreases, operate time becomes faster. With the nonpick-up and pick-up points controlled by initial requirements and stability limits, it is possible to use operate time versus Q data to predict, with reasonable accuracy, the range of operate time for relays adjusted within the specified nonpick-up and pick-up limits. Using curve A fo Fig. 3.8, maximum operate time for this family of relays will result when a relay adjusted to pick-up at the specified pick-up value (I_j is maximum) is energized the minimum circuit current.

In a similar manner, minimum operate time is obtained from curve B when Q is minimum, that is, when a relay adjusted to pick-up at the minimum value, just above the nonpick-up requirement, is energized at maximum circuit current. The operate time range can be determined in this manner for any combination of ambient temperatures by obtaining Q curves at the applicable temperature extremes. The curves of Fig. 3.8 were obtained with no resistance in series with the winding. For the series resistance case, G_c=N^2/(R_c+R_s); R_c is the winding resistance and R_s is the series resistance. For a given Q and a given winding, the relay will be faster when resistance is in series with the winding. This method is used to reduce operate time and avoid the overheating that would result from applying a higher voltage directly across the winding. Operating time characteristics for the series resistance case can be obtained by using the method illustrated in Fig.3.8 Operate time curves expressed in terms of input watts frequently offer greater flexibility in circuit design. The Q data of Fig.3.8 may be converted into watts versus time curves (see Fig. 3.9) as explained in the following Paragraphs.

Case A: Maximum operate time at a given temperature (D curves of Fig. 3.9). An input wattage, W_a, is selected and a value for Q is calculated using either of the following formulas, depending on whether the operating voltage characteristics are expressed in current or in voltage:

Imax = specified pick-up current, plus applicable stability limit
Rmax = nominal winding resistance plus specified tolerance (corrected to applicable temperature)
Emax = specified pick-up voltage plus applicable stability limit (corrected to applicable temperature)
Rmin = nominal winding resistance minus specified tolerance (corrected to applicable temperature)
To obtain the maximum operate time value for the calculated Q, use curve A of Fig. 3.8. This value is plotted against W_a as one point of curve D of Fig. 3.9. In the same manner other points may be calculated using several values of W_a.
Case B: Minimum operate time at a given temperature (C curves of Fig. 3.9). As in case A, an input wattage is selected and the corresponding Q is calculated from the following formulas:

In this case, E_min is the specified nonpick-up voltage minus the applicable stability limit (corrected to the applicable temperature). Likewise I_min is the specified nonpick-up current minus the applicable stability limit.
Minimum operate time is obtained for the calculated Q from curve A of Fig 3.8 and then plotted against W_a as one point of curve C of Fig 3.9.
1. Input power operate time curves established, it is possible to determine minimum and maximum operate time for the applicable temperature by calculating maximum circuit watts and reading the minimum time from curve C and calculating minimum circuit watts, and reading maximum time from curve D.
When timing is critical, a relay should be used only within the fairly flat range of the upper and lower curves.
The methods described also may be used to determine operate time of relays employing copper slugs or short circuited turns to a limited range of the delays.
Conditions requiring special study are:
1) Timing during shock or vibration.
2) Operation in capacitor charging or discharging circuits.
3) Operation when a relay is in parallel with a capacitor and in series with a resistor.

Release Time. If you disregard the dynamic considerations affecting contact bounce, release time of an electromagnet relay is comprised of two stages, waiting time and transit time. The former is the time, after the opening of the winding circuit, for the flux to decay to a level at which the magnetic pull can no longer sustain the mechanical force acting on the armature and the magnetic pull can no longer sustain the mechanical force acting on the armature and the armature starts to move from its operated position. Unless the relay is equipped with a sleeve, copper slug, or short circuiting winding to delay decay of flux, waiting time on dropout will be appreciable shorter than on pick-up since the rate of decay of flux is faster than flux buildup. The second stage, transit time, is the interval between the start of movement of the armature and contact actuation. Transit time usually is shorter during release than during operate because of the more rapid flux decay and because the forces acting on the armature aid release and oppose operation. Release time for many designs, therefore, is shorter than operate time. Contact protection in the form of semiconductor diodes, short circuiting windings, or capacitor resistor networks may substantially increase release time by retarding decay of flux but occasionally a capacitor resistor network may decrease release time.

In a particular design, the parameters affecting release time are variations in the armature residual gap, armature load, contact separation, and magnitude of the interrupted energizing current. Waiting time will be maximum when the interrupted energizing current is maximum and the residual gap and armature load are minimum. It will be minimum when the interrupted energizing current is minimum and the other parameters are maximum. In the case of unprotected windings, this effect usually is small. It may be appreciable, however, for protected winding.

Usually when protection is employed, the range of release current adjustments has an appreciable effect on release time. Transit time will be minimum when the armature load is maximum and the contact separation of normally closed contacts is minimum.

Thus in evaluating release time characteristics, it is essential that studies be conducted on samples representing the two extremes of adjustments for production relays. From this type of study it should be possible to obtain correlation between dropout current or dropout ampere turns and release time.

By specifying dropout and hold requirements and controlling their stability, it is possible to predict the range of release time within limits suitable for practical application. Any variation in dropout current caused by environment, operation, or acceleration will affect release time. In determining release time capability of relays equipped with time delay features such as sleeves, copper slugs, short circuited windings, and the like, the maximum conductance sleeve, copper slug or short circuited winding should be used on relays adjusted to provide minimum release time.

Priniciples of Electromechanical Relay Operation

3.12 Heating Considerations

The primary heating considerations in the evaluation and application of relays are the effects of winding temperature on relay performance under normal circuit conditions and under trouble conditions.

Normal heating is the condition imposed on a coil with respect to duration of energization and wattage dissipation when the circuit is functioning in a normal manner. Relay coils should be capable of withstanding normal heating for their required life.

"Trouble" heating is a condition imposed on a coil when the circuit ceases to function normally, resulting in a dissipation of power greater than that for which the coil was designed. Circuit design should not impose a trouble heating condition that could create a fire hazard; i.e., the coil and circuit design should assure that the coil will be self-protecting.

Three factors should be considered in establishing normal operating temperature limits and corresponding maximum safe normal operating voltages, current, and power dissipation:
1) Ability of the coil to withstand the cumulative hours of heating likely to be imposed during its required life.
2) Ability of other parts of the relay structure-such as insulation, actuators, springs, and contacts-to withstand the temperatures imposed without impairing performance of the relay.
3) The possibility of contamination of contacts from volatile substances in the coil and relay structure and the effect on adjacent components.

Design criteria for trouble temperature limits may vary with the application. In many fields, it is imperative that fire hazards be avoided. Trouble heating is, therefore, limited to a temperature that the coil will be capable of withstanding for a period of time well in excess of the likely duration of the trouble. The safe trouble temperature limit is set at a value below the temperature at which progressive short circuited turns may develop as a result of deterioration of the wire or coil insulation. Relays experiencing a trouble condition that causes heating above the normal operating temperature limit are not relied on to function satisfactorily thereafter. In fact, such relays should be removed from service even though they may still function adequately because the extent of deterioration and its affect on future performance cannot be determined.

Methods of evaluating heat resisting properties of coil and wire insulations will not be discussed in this section. Note, however, that finer gauge wires have a shorter life than coarser gauges. Safe operating limits, therefore, should be based on the heat resisting properties of the finest wire or should be determined for ranges of wire size.
In many systems, curves showing the relation between mean winding temperature and power dissipation are required for the application of relay. These curves are obtained at one or more ambient temperatures depending on the application data required.

Typical curves showing final mean winding temperature (temperature at thermal equilibrium) related to power dissipation are plotted in Fig. 3.10. On the initial wattage curves W_o = E_o^2/R_o in which E_o is the applied voltage and R_o is the resistance at ambient temperature T_o. On the final wattage curves, W₁ = E_oI₁ and I₁ is the current at thermal equilibrium. Since the applied voltage remains constant, power dissipation is inversely proportional to absolute temperature, i.e.

in which T_o and T₁ are in degrees C.

It will be noted that over a temperature rise range of +37.78°C to +65.56°C the relationship between final wattage and temperature is essentially linear. This linearity makes it possible to calculate, with a reasonable degree of accuracy, the temperature rise O, and final mean winding temperature for a variety of circuit conditions. This is done by determining the thermal conductance (p) of the coil, expressed in watts/degree rise. To illustrate, thermal conductance of the coil of Fig. 3.10, obtained at the +121.11°C point on the +37.78° C ambient temperature curve, is 10.3 watts divided by 65.56 or 0.1571 watts/°C.

Substitution of p in the applicable formulas of Table 3.11 permits calculation of limiting circuit conditions that will provide margins to assure reliable operation and to prevent overheating. To avoid errors in calculation temperatures and circuit constants where the final watts/mean winding temperature curve departs significantly from a straight line, it may be necessary to choose a series of values for p, each over a limited range.

When a relay is energized under pulsing conditions, the allowable power dissipation is greater than for continuous energization. Experiments have shown that for pulse up to one second duration at any duty cycle, allowable wattage dissipation is obtained from the expression P₂ = P₁ a+b/a in which a is the on time b the off time, and P₁ the power that may be dissipated continuously. For other pulsing conditions, a more comprehensive treatment in involving the load factor, thermal conductance, and thermal capacitance is required. Unless such information is needed for a large variety of circuit functions, it would be more logical, for isolated cases, to conduct studies of several representative samples under the worst possible circuit conditions to determine the winding temperature and circuit margins for the limiting condition. The worst circuit conditions for heating are maximum voltage and minimum winding resistance, or maximum current through a maximum resistance winding. For determining pickup margin, the limiting case is excitation of the maximum resistance winding at maximum voltage or current followed by application of minimum voltage or current when thermal equilibrium has been reached.

Some contactors have energy saving windings tied to the contacts. A low resistance winding is in series aiding with the high resistance holding winding. When the coil circuit in energized, a normally closed (back) contact shunts the high resistance winding so that a high inrush current picks up the armature, and opens the holding high resistance "economizer" winding shunt, thereby conserving power during the hold period.

Fig. 3.11 General temperature rise formulas for coils of electromagnets.

E= applied volts
I= current in amperes
R= main winding resistance at initial temp. (Oo) in ohms
Rs=extemal or internal shunt resistance (zero temp. coefficient), in ohms
r=extemal or internal series resistance (zero temp. coefficient), in ohms
Wo=initial watts		                        0 = temperature in deg F
	=EI watts (constant power)	                0o initial temp. in 0F
	—I2R watts (constant current)	K = thermal conductance in watts per F
	=E2R watts (constant)	8=temperature rise above o, in 0F
a=temperature coefficient of resistance of
copper (based on inferred absolute zero
resistivity of copper at —390F)

    1
=________
 390 + 0o

Priniciples of Electromechanical Relay Operation

3.13 Contact Performance

Contact theory and its applications to the design of relays constitute a complex subject that is treated comprehensively in texts. Since this section is intended to acquaint the reader with the factors that influence contact performance in service, the discussion is largely limited to presenting the practical aspects of contact design and circuit engineering. For a detailed discussion of constriction resistance, plastic flow, arc initiation glow discharge, tunnel effects, film breakdown, and the like see Reference 5.

Priniciples of Electromechanical Relay Operation

3.14 Contact Characteristics

Contact characteristics that affect switching performance are:

1. Electrical conductivity
2. Thermal conductivity
3. Hardness, limit of elasticity: Young's Modulus
4. Resistance to are erosion, welding or electrical sticking, cold welding, mechanical wear, oxidation, atmosphere contamination (chemically active).
5. Tendency to bounce on impact, gaseous absorption, catalytic polymerization of hydrocarbons, metal transfer at contact closure and arcing at opening.

Besides the physical and chemical properties of the metal, there are some geometrical and dynamic considerations:

1. Shape of contacts
2. Force between contacts
3. Amount of slide or "wipe"
4. Amount of rolling or twisting motion
5. Resiliency of the supporting structure and its tendency to enhance or inhibit bounce or chatter.

When contacts meet, the metal at the point of the contact deforms until the actual touching area supports the contact force and provides metal-to-metal contact unless some foreign material interferes. Deformation is at the point of contact, which can be either in the elastic or plastic modes. This is one of several factors that contribute to the amount of contact bounce. On a microscopic scale, many actual points of contact (often referred to as a-spots) form the electrical conductor and carry the current. The contact interface is also subject to mechanical abrasion and metal "galling" as it rubs, and "cold welding". The surface will absorb a monomolecular layer of volatile molecules in direction proportion to the molecular weight and concentration of the volatile material and the ambient pressure and inversely proportional to the temperature. (Water vapor is also a particularly common substance forming very thin absorbed layers).

Each metal has its own pertinent chemical properties. Silver and silver alloys, which have excellent electrical and thermal characteristics, tend to combine chemically with gaseous compounds of sulphur, the halogens (flourine, chlorine, bromine, and iodine), and silicones to form high resistance, usually hard coatings. Unlike other "noble" metals (gold, platinum, rhodium, iridium, palladium, and ruthenium, all of which are used in contacts), silver has no measurable catalytic effect (polymerization) in the sense of changing, under sliding pressure, the absorbed hydrocarbon molecules into some solid hydrocarbon material. Arcing, however, can accomplish the precipitation of solid carbon or carbonaceous products, usually in a ring around the actual point of contact.

Some more active metals, either pure or in alloys, find special areas of usefulness due to particular mechanical properties. Molybdenum, tungsten, nickel and mercury, for example, are used alone or as alloying or sintering ingredients. Cadmium oxide, tungsten carbide, tin, magnesium, and carbon are sometimes added to silver to inhibit sticking or welding particularly in high current relays or contactors. When contacts are surrounded by an inert gas, like nitrogen, consideration can be given to contact materials that could not be used in open style relays.