Many modern buildings with heavy electrical loads, such as IT equipment and air-conditioning plant, have electricity substations close to the load centre. Sometimes these are inside a high-rise building or at the heart of a complex such as a hospital, so the switchgear within must be safe and environmentally neutral. Electrical supplies entering the building or site are typically llkV or 33kV and controlled by medium-voltage switchgear. At this voltage, normal air-break switchgear is too bulky. Breaking the electrical arc drawn by the opening contacts may involve a blast of air or gas or switching in a vacuum. To reduce the risk of flashover between the current-carrying parts of the circuit-breaker itself, the device may be immersed in oil or gas.
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