What is Switchgear?

The Basics of Low-Voltage Switchgear

Low-voltage switchgear is typically found on the low-voltage or secondary side of a distribution transformer. This combination is often referred to as the substation. It is used to supply electricity for critical, standby, backup power, and critical process applications for a wide variety of industrial, commercial, and utility applications.

Low-voltage metal-enclosed switchgear is a three-phase power distribution product designed to safely, efficiently, and reliably supply electric power at voltages up to 1,000 volts and current up to 6,000 amps. The American National Standards Institute and the National Electrical Manufacturers Association (ANSI/NEMA) rate low-voltage switchgear for up to 635 volts with a continuous current main bus rating of up to 10,000 amps (for supplying power from parallel sources).

Switchgear controls are designed to isolate electrical equipment from normal electrical currents for maintenance purposes, or from abnormal currents (i.e. faults) that could damage it, and potentially cause a fire. Switchgear is typically seen in the form of fuses, electrical disconnects, circuit breakers, and sometimes a combination thereof. The type of gear a company has depends on several factors, such as voltage level, the type of equipment being isolated, and the gear's environment, to name a few. In this entry, we look at circuit breaker models according to their voltage type, construction type, and insulation design.

The Main Parts of Low-voltage Switchgear

A section of low-voltage switchgear typically consists of three distinct and segregated parts:

1. Breaker compartment

Each breaker compartment can normally hold up to four power circuit breakers arranged in a vertical fashion. Each power circuit breaker is individually compartmentalized from other breakers.

2. Bus compartment

Behind the circuit breaker compartment is the bus compartment which is also compartmentalized by solid barriers from the breaker compartment. Adjacent bus compartments are segregated from each other by an insulated barrier between compartments.

3. Cable compartment

The cable compartment is at the rear of the switchgear section and it is optionally compartmentalized with vented or unvented barriers from the bus compartment. The cable compartment has hinged doors or removable covers that enable access to landing lugs for terminating line and load cables. This compartment arrangement is the most typical and may be called rear-accessible switchgear since access to the back of the switchgear enclosure is required.

CODES AND STANDARDS

Low-voltage switchgear is typically built to adhere to the standards set by the IEEE (North America), IEC (Europe and other parts of the world), UL, and ANSI (American National Standards Institute).

• ANSI/IEEE C37.20.1 - Standard for Metal-Enclosed Low-Voltage (1000 Vac and below, 3200 Vdc and below) Power Circuit Breaker Switchgear

• ANSI/IEEE C37.20.7 - Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults

• UL 1558 - Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear

• UL 1066 - Standard for Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures