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Switchboards and switchgear are distribution equipment, serve as service entry equipment, and contain overcurrent protection devices. However, they have very important differences that not many are aware of. By the end of this article, an electrical engineer should be able to answer two important questions: “What is the difference between a switchboard and a switchgear?”and “Should I use a switchboard or a switchgear for my project?”

Electrical System Overview

The NFPA 70: National Electrical Code has similar requirements for switchgear and switchboards regarding their working clearance, egress, arc energy reduction, bonding/grounding, and installations. The main differences in their applications are primarily determined by how their construction strategies meet certain design goals and limitations. A brief comparison of the two devices is given in Table 1.

Table 1: General Comparison of Low-Voltage Switchgear and Switchboards.

*Note: Internal relaying and logic can be used to incorporate transfer controls into switchgear designs.

NEC Definitions of Switchgear and Switchboard

NEC Article 100 defines the term “switchgear” as: 
“An assembly completely enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) and containing primary power circuit switching, interrupting devices, or both, with buses and connections. The assembly may include control and auxiliary devices. Access to the interior of the enclosure is provided by doors, removable covers, or both.”

NEC Article 100 defines the term “switchboard” 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 usually instruments. These assemblies are generally accessible from the rear and front and are not intended to be installed in cabinets.”

Generally, switchgears are more reliable and maintainable than switchboards due to their complex constructions and increased variety of configurations, whereas switchboards are generally a lot cheaper than switchgear due to their simpler designs. Another advantage of switchboards is their ability to include transfer switches and panelboards, while switchgear cannot. Additionally, in contrast to their NEC definition, modern switchboards do not often require rear access. The two devices are shown in Figures 1 and 2. The next section describes the industry standards for switchgears and switchboards to highlight their differences.

Figure 1: An Example of a Switchgear

Figure 2: An Example of a Switchgear

Electrical System Standards

NEMA, UL, and ANSI standards define the construction and testing of switchboards and switchgears. However, these standards are typically covered by standard specifications during the design process; therefore, they are sometimes taken for granted or completely ignored by design engineers. These standards are discussed here because they provide engineers with the foundation to understand the significance of the differences between the two types of equipment. The construction of different switchgear and switchboard types are evaluated against the standards and guidelines shown in Table 2 and Table 3, respectively.

Table 2: Standards for Switchgear Constructions

Table 3: Standards for Switchboard Constructions

Low-Voltage Circuit Breakers

Low-voltage circuit breakers are available in different types and sizes, serving a wide range of purposes and applications. These breakers are evaluated against standards and guidelines, as shown in Table 4.

Table 4: Standards for Low-Voltage Circuit Breakers

Low-voltage AC and DC power circuit breakers are defined by UL 1066. These circuit breakers are also referred to as air, iron-frame, and steel-frame circuit breakers. They are designed with a focus on maintenance, not enclosed in an unremovable coat or casing, typically have a draw-out design that enhances unit serviceability, and their parts can be replaced as necessary. Trip units are used to provide these circuit breakers with overcurrent protection control. These trip units may generally be selected to provide various types and ranges of instantaneous, short-time, long-time, and ground protection.

UL 489 defines molded-case circuit breakers as “the type of circuit breakers that are assembled as integral units in enclosed, supportive housings of insulating material”. Essentially, all parts of a molded-case circuit breaker are contained within a single overall enclosure. UL 489 lists different types of model-case circuit breakers, including regular molded-case and insulated-case circuit breakers.

Molded-case circuit breakers can be equipped with either electronic/solid-state or thermal-magnetic trip units. While some molded-case circuit breakers offer a level of disassembly, most must be entirely replaced in case of damage or malfunction.

Figure 3. A Molded-Case Circuit Breaker.

Insulated-case circuit breakers are a popular category within molded-case circuit breakers. These breakers are not formally defined; however, they generally have a charging or a two-stage action to store mechanical energy. This energy can be used to quickly open the breaker n case of faults. Insulated-case circuit breakers can be considered hybrid breakers with the functionality of power circuit breakers and the configuration of molded-case breakers.