This is a question about the failure modes of an SPDT (i.e. form C) relay. Specifically, I am interested in electromechanical relays for an automotive application. I understand that either contact could be stuck open or stuck closed. But is it realistically possible for both contacts to be stuck in the same state--either open or closed? A note about why I ask: I only need a single throw for my application, but with the SPDT structure, it occurred to me that I could monitor the state of the extra terminal to get an idea of the health of the relay. This is a bit like what is sometimes done with force-guided relays (but a lot cheaper).
If it is possible that both terminals could get stuck in the same state, then the (contact-related) failure modes would be:
- Contact #1 stuck open
- Contact #1open; #2 stuck closed
- Contact #2#1 stuck open
- Contactclosed; #2 stuck closedopen
- Both contacts stuck open
- Both contacts stuck closed
How then would you assign probabilities to each of these failure modes? Ball-park answers are OK.
In addition, there are a few other logically-possible failure modes, but I don't know if they are realistically possible. Any insight into these would also be helpful:
- Contact #1 stuck open; #2 operating normally
- Contact #1 stuck closed; #2 operating normally
- Contact #2 stuck open; #1 operating normally
- Contact #2 stuck closed; #1 operating normally
- Both contact #1 and #2 open but shorted together internally