Low-Voltage Switchgear and Circuit Breakers:Disconnect Switches

Disconnect Switches

Disconnect switches are commonly used in low- and medium-voltage systems. The application of disconnect switches can be divided into low- voltage (600 V and below) and medium-voltage (601 V through 15 kV) classes. The medium-voltage switches are discussed in Chapter 7.

8.5.1 Low-Voltage Switches

Low-voltage switches can be classified into three broad categories: (1) isolating switches, (2) safety switches, and (3) interrupter switches. The isolating switch has no interrupting or load-carrying capability. It serves only to provide isolation of the circuit or load by manual means after the power flow is cut off by the circuit protective device.

The safety switch is a load-break switch having a quick-make and quick-break contact mechanism. Safety switches are used in small power systems with limited short-circuit capacity. The safety switch may be fused or unfused.

The interrupter switch is of quick-make, quick-break type and is capable of interrupting at least 12 times its continuous current rating. They are assigned horsepower rating. These switches are available in continuous rating from 30 to 1200 A and can be installed in switchboards, panelboards, and grouped motor control centers. The interrupter switch may be utilized with or without fuses depending upon the application.

Some light industrial systems or commercial buildings will use switchgear or switchboards with a high pressure or bolted pressure, three-pole switch acting as incoming service main disconnects. The principal feature of these switches is their continuous current capacities of up to 3000 or 4000 A. At these currents, very high contact pressure is required on the conducting surfaces in order to hold temperature rises to reasonable levels. The switches themselves carry an interrupting rating similar to those for three-pole interrupter switches but not as high as that for power circuit breakers. Interrupting capa- bility for short circuits is almost always handled by current-limiting fuses, which are an integral part of the switch. Most manufacturers provide single phase, blown fuse, and ground fault accessories so that the switches can be used on low-voltage service entrances. Unlike trip devices applied to circuit breakers, these protective devices are not self-powered. Rather, they take oper- ating voltage from a small control power transformer on the source side of the switch. Generally, the mechanical design of these switches is based on a min- imum of operations. The number of operations is expected to be less than for insulated-case breakers; usually limited to isolation during maintenance or for serious ground faults not cleared successfully by other means.

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