The most important thing to note is that protection relays are always powered by a current transformer's secondary winding, which is of a low current value. So, protection relays are mostly required in MV (medium voltage) and HV (high voltage panels) where it's combination with the breaker will prevent overcurrent (because a relay can just detect and give a signal, but cannot trip the circuit, which requires the mechanical action of breaker to do the. Protective relays can monitor large AC currents by means of current transformers (CT's), which encircle the current-carrying conductors exiting a large circuit breaker, transformer, generator, or other devices. This signal level is typically 5A nominal. Multiple relays can use the same CT. The limit is defined by the electrical load (burden) of. The first part of the circuit consists of the primary winding of a CT which is also called a current transformer. This CT is connected with the transmission line in series to be protected. The second part includes the secondary winding of the current transformer, CB (Circuit Breaker) & the. A Protective Relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.
