Protection Device: Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate corrective actions to ensure continuity of electric service, to limit injury to people and to limit damage to equipment. 
These devices include lightning arresters, surge protectors, fuses and relays with associated circuit breakers, reclosers and so forth.
• Protective relays which respond to electrical quantities,
• Communications systems necessary for correct operation of protective functions
• Voltage and current sensing devices providing inputs to protective relays,
• Station dc supply associated with protective functions (including station batteries, battery chargers, and non-battery-based dc supply), and
• Control circuitry associated with protective functions through the trip coil(s) of the circuit breakers or other interrupting devices.