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. 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. In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. : 4 The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as. Protective relays and devices have been developed over 100 years ago to provide “last line” of defense for the electrical systems. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. It functions as a watchdog by constantly surveying multiple system components including voltage, current, frequency, and phase angle. Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. This document provides recommendations, background and philosophy on relay protection that is not available in M07.