Disconnect switches rapidly disconnect circuits from power supplies in the event of an emergency. Disconnect switches can function in conjunction with circuit breakers, devices which interrupt the flow of electricity along a circuit when the current exceeds the circuit’s capacity. Some disconnect switches are suitable for AC systems. Others are designed for DC systems. Products for single-phase and three-phase power are also available.

Disconnect switches are important safety features on generators and electrical motors. Rural electricity providers or cooperatives often require customers to provide a generator disconnect switch if an auxiliary generator is operating on their system. The generator disconnect switch ensures that a lineman working on the system is in no danger of electrocution. A generator disconnect switch is also called a transfer or isolation switch.

Types of Disconnect Switches

There are two basic types of disconnect switches: battery disconnect switches and electrical disconnect switches.

A battery disconnect switch is a knife switch that prevents unwanted drain from a vehicle battery when the vehicle isn’t in use or is in storage. The battery-disconnect switch installs over either the positive or negative terminals on the battery. Another type of battery disconnect switch comes with a removable knob to protect a vehicle from being stolen when it is parked in a public garage for long periods.

Electrical disconnect switches are often used on electric motors, such as industrial forklifts, agricultural vehicles, and aircraft tow tractors. Electrical disconnect switches are also used on a variety of electrical appliances, such as heaters and air conditioners, to ensure that contractors working on these appliances are protected from the incoming power. Electrical disconnect switches can adhere to international standards developed by the International Electrotechnical Commission (IEC) or to U.S. standard developed the National Electrical Manufacturers Association (NEMA). Both regulating bodies work together to provide quality standards for electrical products manufacturers. 

Specifications

Selecting disconnect switches requires an analysis of performance specifications. Parameters include:

• Current rating
• Operating voltage
• Horsepower rating
• Short-circuit interrupt capacity
• Number of poles

Fuse Types

With fusible disconnect switches, fuse type is also an important consideration. Choices include Class CC, Class H, Class J, Class L, Class T, and Class K.

Mounting Styles

For both fusible disconnect switches and non-fusible disconnect switches, there are many choices for mounting style. These include DIN rail, panel mount, wall or box-mount, base mount, and flange mount.

• DIN rail - The device is designed for mounting on a standard DIN rail.
• Panel mount - Panel mount devices can be placed into, or through, a depression within an existing panel so that its face is flush, or nearly flush, with the panel. These connectors are used in applications where the connector is required to fit a panel cutout.
• Wall / box mount receptacle - Wall or box mount receptacles are typically flanged and threaded for an accessory (backshell), and intended for permanent mounting.
• Base mount - Base mount switches use a mounting pad, normally using screws.
• Flange mount - Flange mounted plugs (flanged inlets) or flange mounted receptacles (flanged outlets) are devices attached to panels by bolting the flange, or riveting the flange in place through holes in the flange surface.

Categories

Disconnect switches may be categorized as fusible, non-fusible, enclosed fusible, and enclosed non-fusible switches.

• Fusible switches support the installation of fuses and provide a greater degree of protection.

• Non-fusible switches do not support fuse installation. 

Standards

Disconnect switches must adhere to certain standards and specifications to ensure proper design and functionality.

ANSI C37.46 - Specification for high-voltage expulsion and current-limiting power class fuses and disconnecting switches.

BS EN 62271-102 - High-voltage switchgear and controlgear. Alternating current disconnectors and earthing switches.

Image credits:

Baker Precision | Leviton | ABB  | Mersen