Protective relays sense electrical information and issue trip commands to protect electrical lines and apparatus. If power to the relay is lost, or if diagnostics fail, the alarm contact will close. It functions as a watchdog by constantly surveying multiple system components including voltage, current, frequency, and phase angle. It. 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.