The four key characteristics of protective relays are speed, sensitivity, reliability (dependability), and selectivity (security).

Speed (Operating Time): Protective relays must respond quickly to abnormal conditions to minimize damage to equipment and maintain system stability. Fast operation ensures that faults are isolated promptly, reducing the risk of cascading failures in the power system . Sensitivity: Relays must detect even small deviations in current, voltage, or other electrical quantities that indicate a fault. High sensitivity ensures that the relay can respond to low-magnitude faults without delay, protecting equipment effectively . Reliability (Dependability): A relay must operate correctly whenever a fault occurs. Dependability ensures that the relay will trip the circuit breaker under fault conditions, preventing equipment damage and maintaining system integrity . Selectivity (Security): Relays must avoid false operations due to external disturbances or normal system fluctuations. Security ensures that only the faulty section is isolated, preventing unnecessary outages and maintaining continuity of service . These four characteristics together ensure that protective relays perform their essential role in safeguarding electrical power systems, coordinating with circuit breakers to isolate faults efficiently while minimizing disruption to the rest of the network.