This guide represents a short overview of fundamentals of a power system protection, operating principles and relay characteristics as well as description of main switchgear components like various types of circuit breakers, CTs and PTs, relays etc. 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 Relay Definition: A protective relay is an automatic device that senses abnormal conditions in electrical circuits and triggers actions to isolate faults. Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. Electrical switchgear protection fundamentally involves the integrated deployment of equipment, primarily protective relays, circuit breakers, and fuses, to actively safeguard an entire electrical system by isolating and meticulously controlling power flow. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system.