I am considering the use two identical relays with their control "coils" in series.
(I am aware that the normal arrangement would be parallel)
My understanding is that with optocouplers the failure modes are generally in the load switching side (for various internal reasons) and it is unlikely that the control circuit fails open circuit. Hence if one of the devices goes faulty the other (which will be rated for the full voltage should the control short circuit) will be unaffected. Does this match with operational experience?
From an operational perspective, what are the common failure modes for optical and mechanical relay? Assuming a well-selected device, what kind of faults do control circuits see most frequently, or is it so rare (compared with the switching end) as to not be worth considering?