The electrical quantities that may change under fault conditions include: voltage, current, frequency and phase angle. Relion protection and control relays for several application reduce complexity. Long term cost reduction (TCO) for trainings and maintenance by reduce variety of relays A fast and selective arc fault mitigation for air-insulated LV & MV switchgear and Relion protection and control relays and sensor. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. The selection and applications of. To introduce all kinds of circuit breakers and relays for protection of Generators, Transformers and feeder bus bars from Over voltages and other hazards. To describe neutral grounding for overall protection. Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. 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. Three fundamental components required for each circuit breaker.