Switchboard Basics

Power distribution systems in commercial and industrial facilities are electrically more complex than that of a residential home. As a result, these larger power distribution systems must be capable of handling higher levels of current and voltage. Generally, medium to large facilities require switchboards to safely circulate power to transformers, panelboards, control equipment and finally to system loads.

The role of the switchboard, therefore, is to divide the main current that is channeled into the switchboard into smaller currents for further dissemination. In addition, the switchboard also provides switching, current protection, and metering to these diverse currents.

Switchboard Definition

The National Electrical Code (NEC) defines the switchboards as a large single panel, frame, or assembly of panels on which are mounted, on the face, back, or both – switches, overcurrent and other protective devices, buses and other instruments.

Switchboard Construction

The frame of the switchboard houses and supports the other components. The dimensions of a switchboard can range from 70-90 inches high, 32-46 inches wide, and depths from 20-58 inches.

Buses are mounted within the frame. Horizontal bus bars are used to distribute power to each switchboard section. Vertical bus bars are used to distribute power via overcurrent devices to the load devices. A bus connector makes a mechanical and electrical connection between a vertical bus bar and its corresponding horizontal bus bar.

Splice plates are used to join the horizontal bus bars of adjoining switchboard sections. To make additional distribution sections easier to install when they are needed, the horizontal bus is often extended and pre-drilled to accept splice plates. A new section is set flush against an existing section. The old and new sections are thus connected with splice plates.

Operator components are mounted on the front side of the switchboard. This includes overcurrent protective devices such as circuit breakers and disconnect switches. These devices are mounted to the bus bars using straps connected to the line side of the devices.

Cover panels are installed on the switchboard so that no live parts are exposed to the operator. The front cover is referred to as the dead front. The panels are also used as trim to provide a finished look to the switchboard. A product information label identifies the switchboard type, catalog number, and voltage and current ratings.

Switchboard Ratings

When selecting a switchboard, it is important to know the maximum continuous current required and the available fault current. These elements are crucial to determining the various ratings for this equipment.

Interrupting rating – is the level of current that a protective device can safely interrupt without damage under specified conditions. The interrupting rating of the switchboard depends on the interrupting rating of the circuit protection devices and the rating method used. There are essentially two ways to meet this requirement: full rating method and series rating method.

Full Rating Method: This requires selecting a circuit protection device with individual ratings equal to greater than the available fault current.

Series Rating Method: Since the full rating method adds expense to a switchboard design, series-rated switchboards are often available for many applications at a lower cost. The series rating method also requires the interrupting rating of the main upstream circuit protection device to be equal to or greater than the available fault current of the system, but subsequent downstream circuit protection devices connected in series can be rated at lower values.