How often should protective relays be tested?

Testing frequency depends on several factors including relay criticality, operating environment, fault history, and regulatory requirements. As a general guideline, critical transmission protection relays should be tested every 2-4 years, while distribution and industrial relays may be tested every 4-6 years. Electromechanical relays typically require more frequent testing than modern digital IEDs. NERC PRC-005-6 (North America) and similar regulations in other regions mandate specific testing intervals — consult your local grid code or regulatory authority for applicable requirements.