Specifying electrical distribution systems

It is useful for both electrical and nonelectrical engineers to understand basic features when selecting, specifying, and applying electrical distribution systems.


This article is peer-reviewed.Learning objectives

  • Understand basic features when selecting and specifying electrical distribution equipment.
  • Know how much current to apply to various equipment, such as circuit breakers.
  • Glean a basic understanding of panelboards, transformers, and other electrical equipment.

To narrow the broad scope of electrical distribution, this discussion will focus on practical considerations for specifying electrical distribution systems. The discussion will be limited to more common low-voltage 480/120 V electrical distribution equipment encountered in most facilities.

Figure 1: Typical switchboard. Courtesy: Eaton

Primary service equipment

In some smaller facilities, panelboards may be used as a primary service; but for larger facilities, the primary service equipment will be based on a switchboard or switchgear. Engineers, architects, contractors, and facility owners often use the terms “switchboard” and “switchgear” interchangeably when referring to 480 V 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.

There are several notable differences between switchboards and switchgear including physical size, front or rear access, and how breakers are mounted and removed. The type of breakers used also is a major difference between switchboards and switchgear. 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 commonly used in all types of low-voltage switchboards and panel boards. One will find these breakers in ratings from 15 to 3,000 amps. The breaker mechanism is totally sealed within an external molded case. If the breaker has a failure or problem, it must be replaced. These breakers typically are bolted onto the bus, or may have plug-in designs. The removal or addition of MCCBs to a switchboard should take place only with the switchboard power turned off (see Figure 1).

Figure 2: Typical switchgear. Courtesy: Eaton

Power circuit breakers. Typical ratings range from 800 to 5,000 amps. PCBs are designed and tested under much different standards than 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 (see Figure 2). PCBs have numerous components that can be inspected and replaced, such as contacts, pole assemblies, and arc chutes.

Insulated-case breakers. ICCBs are a type of MCCB designed to provide features typically available in PCBs. The internal parts are mostly, but not completely, sealed like those in an MCCB. Typical ratings range from 400 to 5,000 amps. These breakers are available as options in switchboards and can be fixed or drawout. Designed to the same standards as MCCBs, they provide access to replaceable parts, such as contacts.

Application considerations

The amount of continuous current that can be put on a 400-amp circuit breaker depends on the breaker. With MCCBs and ICCBs, the breaker typically is rated for only 80% of its capacity within a switchboard or panelboard. In this case, you could put no more than 320 amps continuously on that breaker. This is a limitation that not everyone is aware of. It is possible to specify optional 100%-rated MCCBs and ICCBs in some frame sizes with some cost premium. PCBs are 100%-rated as standard. Refer to NFPA 70-2017: National Electrical Code (NEC), Article 220.10, for more on this topic.

Beyond continuous current, there are important differences when considering short circuits and faults. While beyond the scope of this article, is important to note that PCBs are tested and rated to higher levels of initial (or asymmetrical) fault than MCCBs or ICCBs. Depending on the engineer's detailed calculations, the MCCB’s or ICCB’s listed fault rating may need to be derated.

Beyond a circuit breaker’s ability to withstand and interrupt a maximum short circuit, there are trip levels or regions to consider. Circuit breakers will open based on various magnitudes and durations of current. These trip levels are expressed as a curve on a graph of current versus time. There are three regions to consider: long-time (continuous-current range) faults, 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 downstream breaker. This is referred to as a selective or fully coordinated system. This type of coordination is more readily achieved with the use of PCBs at main service points.

Space is another consideration. Switchgear is larger than switchboards and requires front and rear access. In addition, the clearance in front must take into account the space needed to draw out a breaker. While not covered in the code when withdrawing a drawout breaker, it may occupy the NEC-required clearances—making egress and access difficult. Rear-connected switchboards, depending on specified options, also will require similar careful space considerations. Front-accessible switchboards have the least space requirements and may be located 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 the 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 must be weighed against the long-term issues of maintainability, reliability, and downtime. Project type and complexity often determine 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, telecommunication switching sites, airports, convention centers, or skyscrapers. Hybrid or high-end rear-access switchboards are good choices for 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.

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Anonymous , 04/06/17 04:03 PM:

this is a very good detailed explanation with important information.

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