MOV Failure Mode 2

[buzzp]

I am interested to hear what failure modes people have found failed MOV's in, ie open or short circuit. I am questioning the use of an MOV across a DC relay coil vs. a diode with a series resistor. I am concerned with an application (safety device) which is calling out to put an MOV across the coil. If it were up to me, I would put in a diode and series resistor for the following reasons:
- series resistor will minimize the time to de-energize vs. just a diode or just an MOV
- MOV is one device so it could potentiall fail shorted causing the safety device to not function when called upon to do so
- MOV's lose their energy dissipation abilities with each 'surge' of voltage making it less affective as it absorbs more 'spikes'
- Their is concerns with an 'short' occurring with either configuration but there is two devices that will have to fail shorted and I have not seen a resistor fail short circuited but I have seen a diode fail shorted

Any thoughts on failure modes of MOV's or other additional input will be appreciated.

 

[buzzp]

Yes there are issues universally with these lock-outs (and others). If proper maintainance practices are implemented, the lock-outs will be exercised once a year to assure they will not stick when called upon to energize. I do know of several companies that do not exercise these relays and they are the ones that have problems.
There is a need for protecting the solid state relays from the high voltage transients developed when the coil is de-energized. Granted, the relays generally have some of their own protection but it is always better to handle these kinds of issues at the source. As far as EMI, the MOV would be useless for anything that is not conducted. Any radiated emissions from neighboring equipment would still affect the unit(or could). Thanks for the input.