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. Core idea: Protective relays monitor electrical quantities and command protective devices to isolate faults or abnormal operating conditions. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits. This prevents damage to equipment, reduces downtime, and safeguards.