The function of this protection is to detect single-phase, two-phase or three- phase overcurrents. Protective relays and devices have been developed over 100 years ago to provide “lastline”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. Definite time delay means that the protection operate time dose not change or depend on the. A protective relay is an intelligent electrical device designed to detect faults in power systems and initiate corrective actions such as tripping a circuit breaker. Its main purpose is to safeguard electrical equipment like transformers, generators, and transmission lines from damage due to. This handbook covers the code of practice in protection circuitry including standard lead and device numbers, mode of connections at terminal strips, colour codes in multicore cables, dos and donts in execution.