Digital relays also have self-diagnostics that check the health of the relay and can immediately detect internal failures; this is perhaps their most important single feature.
The ability to detect a failure before the protection system has to operate contrasts with traditional protection where a failed or defective relay remains undetected until it does not operate correctly during a fault or until the next maintenance test.
It is important that the completeness of self-diagnostics be considered in developing a maintenance/test program for multifunction digital relays and that relay failure be alarmed to a manned location so that personnel can immediately take appropriate action.