Primary side is the line current and secondary side is connected to the relay. Multiple relays can use the same CT. Combines protection, sensors, control power, and circuit breaker in a single package Typically added to a breaker close circuit to prevent accidental reclosure after a trip. Three fundamental components required for each circuit breaker. CT's transform line current down to a signal level that is. These parameters define exactly when a relay starts operating and when it resets, ensuring reliable protection system performance. These tests are performed using relay test equipment, either through secondary injection or through primary injection using high-current injection equipment. The selection and applications of. Relay protection is essential to ensure the stability, reliability, and safety of electrical power systems. The protective relaying is used to give an alarm or to cause prompt removal of any element of power system from service. Plug Setting Multiplier (PSM): The ratio of the fault current to the relay's pickup current, critical for relay operation.