What is the effect on rellability by using relays in series to reduce high current arcs ?

First of all, operate and release times of identical relays are not always the same therefore, the last relay to “make” and the first relay to “break” are the only contacts which would see the high current arc. Second, relays should never be used in series since the total reliability is reduced by the following relationship.

Total Reliability = R x R, where R is the reliablity of each unit.