How often do most utilities calibrate their relays? Digital and electro-mechanical relays? Is there a national standard most plants follow? thanks

I believe our plant calibrate our low voltage relays every 4 years (under 15KV). High voltage relays over 15KV are on a two year test frequency. If station specific conditions (such as vibration, high temperatures, humidity, or chemicals) warrant, the frequency is increased as needed. Im not aware of a standard but there should be an NERC recommendation or possibly compliance.

We calibrate our digital relays every 6 years and the electro mechanical every 3 years. Digital relays does not have any moving parts which could drift or fail out of calibration like the mechanical ones. Also it would be wise to test the trips to ensure the contacts does roll the lock out properly by performing a full functional test.