The Protection Engineer is concerned with limiting the effects of disturbances in a power system. These disturbances, if allowed to persist, may damage plant and interrupt the supply of electric energy. They are described as faults (short and open circuits) or power swings, and result from natural hazards (for instance lightning), plant failure or human error. To facilitate rapid removal of a disturbance from a power system, the system is divided into 'protection zones'. Protection relays monitor the system quantities (current and voltage) appearing in these zones. If a fault occurs inside a zone, the relays operate to isolate the zone from the remainder of the power system.