Engineering use: Relays are used in control panels, motor circuits, PLC interfaces, alarms, breaker trip circuits, and power system protection schemes. 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. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. The selection and applications of. Protective relays can be classified based on their operating principle, construction, or function: 1. Static Relays: Use electronic components without moving parts.