These four fundamental requirements serve as the basis for designing, configuring, and maintaining relay protection systems and are fundamental to analyzing and evaluating relay protection systems. While these requirements are interrelated, they often involve. AC voltage is generally 220V or 110V as per "GB50053-2013 Design Code for Substations of 20kV and Below". Quadrants of Relay Protection For relay protection that operates by tripping, four basic requirements are generally considered: Selectivity, Speed, Sensitivity, and Reliability. Every protection system which isolates a faulty element is required to satisfy four basic requirements: (i). Fingrid's application guideline for relay protection presents the operating principles of the relay protection in Fingrid's 110, 220 and 400 kV power networks and the requirements for operation of the protection systems of Fingrid customers (hereinafter referred to as 'customer')., generator, line, transformer, bus, etc. A fuse performs both detection and interruption functions automatically but its use is limited for the protection of low-voltage circuits only.