This guide explores the different types of protection relays and their testing procedures, with a focus on tools like secondary injection test sets and three-phase relay test sets. When a fault is detected, the relay sends a signal to circuit breakers to isolate the faulty section, preventing damage to equipment and minimizing. 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. This happens because the main function of protection devices is related to operation under fault conditions so these devices cannot be tested under normal operating conditions.