Electrical installations usually have several loads, often on a single circuit. Panelboards,
switchboards, switchgear and motor-control centers are methods of consolidating control
of those circuits into a common enclosure on common bus. One of the basic functions
of switchgear is protection, which is interruption of short-circuit and overload
fault currents while maintaining service to unaffected circuits. Switchgear also
provides isolation of circuits from power supplies. In addition, switchgear is used
to enhance system availability by allowing more than one source to feed a load.
Switchgear is a broad term that covers electrical distribution systems made up of metal-clad or metal-enclosed cabinets containing switches or circuit breakers. The switches might be fused or non-fused. They might be air break or vacuum break. While manual break devices are the most common, medium-voltage switchgear could be electrically closed and or opened. Circuit breakers may be air break, oil break or vacuum break, even SF6. These will always be electrically closed and opened. Where the breakers or switches are electrically closed, opened or both, there will be a requirement for control power source and control power wiring. The source might be a remote separate source of low voltage brought in from outside the switchgear, or it might be provided by transforming line power down to the necessary control voltages. This standard covers the cabinets, busing, relaying and control circuits only.