Switchgear

Switchgear, in an electric power system, is the combination of disconnect switches, fuses and circuit breakers used together to control, protect and isolate electrical equipment, both to allow work to be carried out safely on de-energized equipment and to clear faults automatically downstream. Early central power stations used simple open knife switches mounted on insulating panels of marble or asbestos, but rising power levels and voltages soon made manually operated open switches too dangerous for anything beyond isolating already de-energized circuits, leading first to oil-filled switchgear that contained and controlled arc energy safely and, by the early twentieth century, to metal-enclosed switchgear lineups using oil circuit breakers; today oil-filled equipment has largely given way to air-blast, vacuum or sulfur hexafluoride designs that let automatic equipment safely control very large currents at voltages up to 1,100 kilovolts. In a substation, switchgear typically sits on both the high-voltage and low-voltage sides of the large power transformers, with the low-voltage side often housed in a building alongside metering, control and protection equipment for the distribution circuits it feeds.