Electromagnetic relays are widely used to communicate safety signals in critical applications, where the failure of the relay leads to risk. Traditionally, there are various techniques for relay health monitoring, that is, finite impulse test (FIT), read back and spare contact reading. These techniques are either intrusive (e.g. FIT and spare contact reading) or does not test the relay completely (e.g. read back). Recently it has been shown that current signature of energizing/de-energizing relay can be used to estimate health of relay contacts. The relays used in safety applications are normally kept energized to ensure fail-safeness. So, of all prominent failure modes of electromagnetic relay, welded contact (i.e. no changeover on relay de-energization) is the most critical one. The current signature of de-energizing relay is employed to detect welded contacts. The paper compares three diverse relay health monitoring schemes and one scheme is analysed for effect of component failure using FMEA (Failure Mode and Effects Analysis).
