Switchboards are usually designed with metering devices, overcurrent protection equipment, and other accessories in grouped segments to avoid isolation. Constructing switchboards in individual/isolated sections like switchgears is possible, but such construction is rare. In this article, we will discuss switchboards based on group-mounted devices.
Switchboards can be equipped with both circuit breakers, listed in UL 489 or UL 1066, or fused switches. If a switchboard is equipped with a UL 1066 breaker, then special care should be given to ensure that the switchboard has appropriate protection against short-circuit faults and conditions. The following sections thoroughly discuss circuit breakers and their key characteristics in switchboards and switchgears.
Two essential properties of a circuit breaker are the breaker’s short-time current rating and the short-circuit interrupting capacity. The short-time current rating of a breaker defines the duration it can withstand a short-time rated current level fault through itself. The short-circuit interrupting capacity rating of a circuit breaker is the maximum fault it can interrupt within a given period. For example, switchgears listed in UL 1558 are equipped with circuit breakers that can withstand short-circuit currents for four cycles and short-time rated current for 30 or 60 cycles.
A circuit breaker may be rated to interrupt fault currents of 100 kA within four cycles and to withstand short-time fault currents of 65 kA for up to 30 or 60 cycles. Similarly, switchboards listed in UL 891 are equipped with circuit breakers rated to withstand short-circuit currents for three cycles; however, they do not have any short-time ratings. This distinction between switchgears and switchboards, especially concerning the absence of short-time ratings in the case of the latter, is significant because a switchboard must be protected from higher magnitude faults that occur for three cycles or longer.
Circuit breakers listed in UL 1066 may have the option to disable their instantaneous trip function. This option can pose a huge risk for switchboard protection. Suppose a switchboard is equipped with a UL 1066 circuit breaker with its instantaneous protection disabled. If we expose such a switchboard to fault currents less than the maximum instantaneous pickup of the breaker, the switchboard may fail completely.
Switchgears are constructed as individual sections of metering equipment, fuses, circuit breakers, relays, and other appurtenances. A complete metal barrier is used to compartmentalize and isolate adjacent vertical sections. The bus bars for each power connection (phase, neutral, and ground) run horizontally through the switchgear and are connected to the individual vertical bus bar in each section. These vertical busbars connect to feeder power circuit breakers that control and regulate power distribution to downstream systems.
Switchgears are designed to employ draw-out circuit breakers, allowing for convenient removal and reconnection capabilities. Switchgears listed in UL 1558, in particular, are designed to use only those circuit breakers listed in UL 1066. Switchgears use circuit breakers with frame ratings ranging from 800 A to 6,000 A. The draw-out design facilitates maintenance and minimizes the need for unnecessary de-energization of downstream systems connected to the switchgear bus.
Figure 1. Draw-Out Case Circuit Breaker in LV Switchgear
Options and Applications
Generally, switchgears are the largest electrical equipment assemblies inside an electrical room. When
sizing up electrical rooms, we must consider the size of the switchgear and give it the required front and
rear access points. Also, the draw-out type circuit breakers need space to operate breaker lifting devices
and draw out the breaker alongside NEC-required working space clearances.
The length of a single switchgear can vary widely depending on the number of circuit breakers and the
overall complexity of the switchgear design, as they both increase the required number of total individual
sections. Switchgears commonly have depths of at least 60 inches, although some designs also
incorporate depths of 72 and 90 inches.
Switchgears that only need front access may have depths of up to 40 inches. Such designs often require
double the width of the gear because extra cable sections have to be installed adjacent to each circuit
breaker section.
The convenience of switchgears is that they can incorporate advanced controls through programmable
logic controller-type intelligent controls, relay logic, or a combination of the two. The “main-tie-main” is
a common configuration for switchgear. In this configuration, the tie circuit breaker is typically “open”,
and each main circuit breaker is “closed”. This setup provides power to each bus section in the switchgear
assembly. Such a switchgear is said to be “dual-ended” because it will often have two separate sources of
power. Loss of power from one source, either due to a fault or maintenance, can trip the main circuit
breaker on that side of the “main-tie-main” setup. The switchgear controls can then “close” the tie circuit
breaker to restore power to the interrupted section.
The protection scheme of switchgear may also incorporate intelligent relays to monitor power quality
parameters like voltage, current, and frequency, amongst others, to observe the electrical system and
detect abnormal scenarios and faults which are far beyond the capabilities of the more basic and modest
overcurrent circuit breakers. Switchboards generally cannot incorporate such relays or intelligent control
devices due to insufficient sectionalized compartments.
Switchboards are generally smaller than switchgears, but large enough to require substantial space within
the electrical room if panelboards, transfer equipment, and similar equipment are included in their lineup.
Additional space is required for some switchboards constructed with UL 1066 draw-out circuit breakers.
An advantage of switchboards is that they allow for installing a wide array of circuit breakers. Depending
on the manufacturer, a single switchboard section may contain breakers ranging from 20 A to 1,200 A or
more. Like switchgears, switchboards also come in a variety of depths. A switchboard may have a depth
ranging from 24 to 60 inches, with 36 inches being the most common.
Switchgears can offer more complex functionalities than switchboards. Switchboards are simpler and less
costly assemblies that may incorporate a panelboard, an automatic transfer switch, and other equipment
into their enclosures. Whereas switchgears may incorporate more complex transfer schemes and controls
of significantly greater depth. This makes switchgears ideal for operating complex electrical systems.
Front-access and rear-access need substantial space requirements that must be incorporated into the
proposed layout of an electrical room; otherwise, certain models of switchgears and switchboards will
have to be ruled out due to space constraints. Assuming the following two conditions from NEC Table
110.26(A)(1) are applied to a switchboard or switchgear in such a way that:
- Condition 2 (grounded parts on one side of the working space, exposed live parts on the other)
applies to the rear side of the unit. - Condition 3 (exposed live parts on both sides of the working space) applies to the front side of the
unit.
Therefore, for a common front-access and rear-access switchgear, the unit would require a minimum total
space of 12.5 feet, and for a front-access-only switchboard, the unit may be installed against a wall and
require a total space of 7 feet.
Electrical System Financial Considerations
Switchgears and switchboards are available in a variety of configurations, functionalities, advantages,
disadvantages, and specialties, making direct comparisons between these units impossible. The
advantages of switchgears are not found with switchboards and vice versa. Some general parameters can
guide engineers in selecting the most appropriate unit for their requirements.
Switchgear offers more complex operations and easier maintenance options. The control and protection
requirements of complex power systems may necessitate the use of switchgear due to their more advanced
relays.
Another important factor is space constraints, especially for existing facilities and buildings, which may
preclude the use of deeper switchgear. Switchboards are also preferable when meeting the needs of a
wider variety of feeder sizes and types. Switchgears are costlier and require more material than
switchboards due to their compartmental design. As a result, limited funding may lead to the preference
for the more affordable switchboard design.
Ultimately, there is no definitive rule to determine the appropriate installation of either a switchgear or a
switchboard. Engineers must understand their needs and constraints and the differences between ANSI
and UL standards associated with each piece of equipment. The optimal combination of the parameters
discussed in this article can help guide an engineer’s decision regarding the preferred type of device for
their specific application requirements.
