A protective relay is an electrical device used to detect and isolate faults in a power system. It is a critical component of a power system protection scheme, and its main function is to trip or open the circuit breaker to isolate the faulty section of the power system when a fault. What is a protective relay ? Ans. The. Which of the following is NOT a primary function of a protective relay? Which of the following types of relays is LEAST suited for protecting overhead lines against lightning strikes? What is the primary advantage of using solid-state relays compared to electromechanical relays for protection. Protective Relay Definition: A protective relay is an automatic device that senses abnormal conditions in electrical circuits and triggers actions to isolate faults.