The Electro Mechanical Protective Relay was once the backbone of higher voltage electrical power distribution systems.  Although digital relays are used today, there remains a high percentage of older equipment that still relies on these legacy devices.  Terms like amp tap, time dial, instantaneous, IAC, CO, IFC all are part of the legacy or electro-mechanical relays.  This video is based on material found in electrical power training classes at: Brainfiller Courses