Switchboard or Switchgear?

03/01/2006


Engineers, architects, contractors and facility owners often use the terms "switchboard" and "switchgear" interchangeably when referring to 480-volt (600-volt class) circuit breaker distribution equipment. But there are notable differences in configurations, components, standards, applications, reliability and selection criteria between these two types of power distribution equipment. What follows is an overview of these differences—and suggestions for selecting the right type for the project.

A major difference between switchboards and switchgear is the type of breakers used. The basic types that we are concerned with here are: sealed, semi-open and open types. Specifically, these are called molded case, insulated case and power circuit breakers.

Molded case circuit breakers. MCCBs are the most common, used in all types of low-voltage switchboards and panel boards. One will find these breakers in ratings from 15 amps to 3,000 amps. The breaker mechanism is totally sealed within external molded case. If the breaker has a failure or problem, it must be replaced. These breakers are typically bolted onto the bus, or may have plug-in designs. The removal or addition of MCCBs to a switchboard should only take place with the switchboard power turned off.

Insulated case breakers. ICCBs are a type of molded case circuit breaker designed to provide features typically available in power circuit breakers. Typical ratings range from 400 amps to 5,000 amps. These breakers are available as options in switchboards, and can be fixed or drawout design. De-signed to the same standards as MCCBs, they provide access to replaceable parts such as contacts.

Power circuit breakers. Typical ratings range from 800 amps to 5,000 amps. PCBs are designed and tested under much different standards from MCCBs or ICCBs. PCBs are connected to the bus in a drawout design, allowing the breakers to be withdrawn partially or fully while the entire switchgear is powered on. PCBs have numerous components that can be inspected and replaced, such as contacts, pole assemblies and arc chutes.

Switchgear, switchboards and associated breakers are designed and tested to different standards as shown in the table below, resulting in different capabilities and application considerations.

Application Considerations

How much continuous current can you put on a 400-amp circuit breaker? It depends on the breaker. With MCCBs and ICCBs, the breaker is typically rated for only 80% of its capacity within a switchboard or panel board. In this case, you could put no more than 320 amps continuously on that breaker. This is a limitation not everyone is aware of. It is possible to specify optional 100% rated MCCBs and ICCBs in some frame sizes. PCBs are 100% rated as standard. (See the National Electrical Code Article 220-10.)

Beyond continuous current there are important differences when considering short circuits and faults. While beyond the scope of this article, we will identify two key issues for the three types of circuit breakers. The first issue is a breaker's interrupting rating, which is the maximum short circuit current the breaker can handle safely and still operate. This is the rating most often discussed as the short circuit rating or withstand. The three types of circuit breakers are available in a wide range of interrupting ratings. However, it is important to note that PCBs are tested and rated to higher level of initial (or asymmetrical fault) than MCCBs or ICCBs. Depending on the engineer's detailed calculations using resistance and impedances, MCCBs or ICCBs listed fault rating may need to be de-rated.

However, beyond a circuit breakers ability to withstand and interrupt a maximum short circuit, there are trip levels or regions. Circuit breakers will open based on various magnitude and durations of current. These trip levels are expressed as a curve on a graph of current vs. time. There are three regions to consider: long time (continuous current range), short time faults and instantaneous fault. The area of difference between MCCBs, ICCBs and PCBs is in the short time regions. Essentially, PCBs have higher short time ratings, which along with the ability to eliminate the instantaneous range, allows PCBs to "wait" for breakers further downstream in the distribution system to trip and isolate faults. This is of particular use in large distribution systems where one doesn't want main circuit breakers to trip when a fault occurs on a smaller down breaker. This is referred to as selective or fully coordinated system. This type of coordination is more readily achieved with the use of PCBs at main service points.

Another consideration is space. Switchgear is larger and requires front and rear access. In addition, the clearance in front must take into account the space needed to drawout a breaker while still maintaining the NEC required clearances. Similarly, rear connected switchboards, depending on specified options, may also require similar space considerations. Front accessible switchboards have the least space requirements, and one may be able to locate it against a wall.

Both switchboards and switchgear are code-compliant and proven in the industry. But there are some advantages to switchgear and rear-connected switchboards that can reduce downtime and failures. First, there is the idea of individual compartments for breakers. In the event of a short circuit on a breaker, the resulting energy will be contained and isolated from other breakers and from bus and cable compartment. Second, the ability to have drawout breakers also permits repair, inspection and replacement of a breaker while the rest of the switchboard or switchgear continues to operate. Third, PCBs, and to a lesser extent ICCBs, have exposed and accessible parts that can be regularly inspected and replaced without having to buy an entirely new breaker. Lastly, PCBs have a more rugged construction and are able to handle more closing and opening operations, including faults, and provide for automatic remote control for transfer schemes.

So how does one make a selection? Initial costs often play a major role in the selection. The cost differences between a low-end switchboard and high-end switchgear can be substantial—as high as two or three times—and has to be weighed against the long-term issues of maintainability, reliability and downtime. Project type and complexity often determines the choice. A simple office facility with no maintenance staff is much different from a manufacturing facility. Recommended applications for switchgear include manufacturing or process facilities with round-the-clock operations, data centers, and telecommunication switching sites, airports, convention centers or skyscrapers. Hybrid or high end rear access switchboards are good medical facilities, laboratories, light manufacturing, large institutional or commercial facilities. Front accessible switchboards are recommended for basic office and commercial buildings, K-12 schools, warehouses or retail facilities.

Further Research

For electrical engineers involved in the design and specification of these systems additional technical information should be sought out from equipment manufacturers and in references such as IEEE standard 1015 "Blue Book" on applying low-voltage circuit breakers and IEEE standards 241 and 141 on commercial and industrial power systems. However, even with the basics, it should be possible for all building professionals to understand that there are differences between switchboards and switchgear.

System and Components

Standards

Switchboards, MCCBs and ICCBs

UL 489 and UL 891

Switchgear with PCBs

ANSI C37 (UL 1066) and UL 1558



Basic Configurations

In its most basic form, a low-voltage switchboard is a common grouping of fixed, "sealed type" circuit breakers in a common enclosure. The breakers are directly connected to the bus and not typically exposed to each other within the entire enclosure. Cable connections are made in the front of the board. They typically require only front access and may be mounted against a wall. These are often seen in small- and medium-size commercial or institutional facilities.

Switchgear consists of individually mounted and compartmentalized draw out "open type" circuit breakers. There are physical barriers between the breaker and between the breakers and the bus. Cable connections are made in the rear compartment. They are larger and require front and rear access. These have been typically used in industrial and large commercial and institutional facilities.

Historically, these two simple explanations have helped to highlight the differences between a low-voltage circuit breaker switchboard and switchgear. However, the lines have been blurred with the availably of "rear connected" switchboards that can provide hybrid options of individual compartments and drawout circuit breakers with "semi-open" or "open type" construction.