The design of the high-voltage substation must include consideration for the safe operation and maintenance of the equipment. Switching equipment is used to provide isolation, no load switching, load switching, and/or interruption of fault currents.

The magnitude and duration of the load and fault currents will be significant in the selection of the equipment used. System operations and maintenance must also be considered when equipment is selected.

One significant choice is the decision of single-phase or three-phase operation. High-voltage power systems are generally operated as a three-phase system, and the imbalance that will occur when operating equipment in a single-phase mode must be considered.

Metal-enclosed low-voltage power circuit breaker switchgear indicates a design which contains low voltage ac or dc power circuit breakers in individual grounded metal compartments. The circuit breakers can be either stationary or draw out; manually or electrically operated; fused or unfused; and
either 3-pole, 2-pole or single-pole, construction.

The switchgear may also contain associated control, instruments, metering, protective and regulating equipment as necessary. Definitions, ratings, design and production tests, construction requirements, and guidelines for application, handling, storage, and installation are covered in IEEE C37.20.11.

Low-voltage metal-enclosed switchgear is typically installed in industrial plants, utility and cogeneration facilities, and commercial buildings for the protection and distribution of power for loads such as lighting, machinery, motor control centers, elevators, air conditioning, blowers, compressors, fans, pumps, and motors.

Low-voltage switchgear is available in ac ratings up to 635 V and 5000 A continuous and in dc ratings up to 3200 V and 12000 A continuous. Short-circuit current ratings are available up to 200 kA.

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