Low-voltage switchgear fundamentals

Electrical switchgear refers to a centralized collection of circuit breakers, fuses and switches (circuit protection devices) that function to protect, control and isolate electrical equipment. The circuit protection devices are mounted in metal structures. A collection of one or more of these structures is called a switchgear line-up or assembly.

Switchgear is commonly found throughout electric utility transmission and distribution systems as well as in medium to large sized commercial or industrial facilities. Standards for electrical switchgear are defined by IEEE in North America and by IEC in Europe and other parts of the world.

Each breaker compartment can normally hold up to four power circuit breakers arranged in a vertical fashion. Each power circuit breaker is individually compartmentalized from other breakers. Behind the circuit breaker compartment is the bus compartment which is also compartmentalized by solid barriers from the breaker compartment.

Adjacent bus compartments are segregated from each other by an insulated barrier between compartments. Finally, the cable compartment is at the rear of the switchgear section and it is optionally compartmentalized with vented or unvented barriers from the bus compartment.

The cable compartment has hinged doors or removable covers that enable access to landing lugs for terminating line and load cables. This compartment arrangement is the most typical and may be called rear-accessible switchgear since access to the back of the switchgear enclosure is required.

A variation of this arrangement of compartments is?front-accessible switchgear?where the cable compartment is adjacent to the breaker compartment with the cable compartment doors located on the front of the equipment. This arrangement results in a much shallower design that requires no rear access and allows the switchgear to be placed up against a wall, similar to a switchboard

The extensive compartmentalization of low-voltage switchgear is designed to increase the safety, reliability and serviceability of the switchgear by preventing, for example, accidental contact with certain conductors such as the main bus or circuit breakers in adjacent cells while performing maintenance. The compartmentalization could also limit some of the damage from an arcing fault and reduce the risk of the fault propagating to other parts of the switchgear.

Power flows through the low-voltage switchgear enclosure via silver- or tin-plated copper bus. Vertical sections (“risers”) of copper bus connect the breaker stabs which run horizontally into the breaker cells in a switchgear section and connect to the line sides of the feeder breakers via finger clusters. Horizontal (main) bus electrically connects adjacent switchgear sections to one another.

Power flows through the low-voltage switchgear enclosure via silver- or tin-plated copper bus. Vertical sections (“risers”) of copper bus connect the breaker stabs which run horizontally into the breaker cells in a switchgear section and connect to the line sides of the feeder breakers via finger clusters. Horizontal (main) bus electrically connects adjacent switchgear sections to one another.

Low-voltage switchgear provides short-circuit and overload protection via low-voltage power circuit breakers (LV-PCB) with integral trip units. These low-voltage circuit breakers are typically through-the-door, draw-out devices. “Through-the-door” means that the faceplate of the circuit breaker along with the breaker-mounted controls are accessible without opening the switchgear. “Draw-out” means the circuit breaker can be easily moved into test and disconnect positions without opening the switchgear and can be fully withdrawn from the switchgear for service. Low-voltage circuit breakers interrupt short-circuit and overload faults via main contacts that part in open air. Consequently, such circuit breakers are also known as air circuit breakers (ACB) in contrast to medium-voltage circuit breakers which typically utilize vacuum interrupters.

?Low-voltage switchgear carries the following ratings:

• Maximum voltage (Typically up to 635V)
• Power frequency (Typically 50Hz and 60Hz)
• Insulation level (Typically 2.2kV)
• Continuous current (Typically up to 10,000A)
• Short-circuit withstand current?(Typically up to 200kA)
• Short-time withstand current?(Typically up to 100kA, 30 cycles)

short-circuit withstand current rating

The short-circuit withstand current rating is also known as the short-circuit current rating (SCCR) of the switchgear. According to the ANSI/IEEE C37.20.1 standard, short-circuit withstand current is defined as:

The designated limit of available (prospective) current at rated maximum voltage that it shall be required to withstand for a period of no less than four cycles on a 60 Hz basis under the prescribed test conditions. The rated short-circuit withstand current determines the minimum bus bracing which is required of the design. The bus bracing may be able to withstand higher values than the stated rating.

In other words, the short-circuit withstand current rating is the maximum short-circuit current that the switchgear assembly can safely withstand for at least four cycles when protected by an overcurrent protective device (OCPD). This rating will affect how the bus bar is mechanically braced to prevent bending and damage during a short-circuit event.

The purpose of this rating is to coordinate with the short-circuit withstand current rating of the circuit breakers used in the switchgear. The short-circuit withstand current rating of the switchgear must equal the short-circuit withstand current rating of the lowest rated breaker used in the switchgear assembly. For example, if the main circuit breaker has a 100 kA short-circuit withstand current rating but a feeder breaker has a 65 kA short-circuit withstand current rating, the switchgear will carry a 65 kA rating.

interrupt rating

Interrupt rating is a rating that applies to overcurrent protective devices such as low-voltage power circuit breakers. It is defined as the maximum current the overcurrent protective device is rated to safely interrupt at a specific voltage.?

The interrupt rating of the circuit breaker must meet or exceed the short-circuit withstand current rating of the circuit breaker. In some cases, the interrupt rating will exceed the short-time current (of the circuit breaker and the switchgear) resulting in a circuit breaker that will trip instantaneously (within 3-4 cycles), instead of with a short-time delay, in the presence of particularly high fault current.

Finally, the interrupt rating of the circuit breaker must meet or exceed the maximum available fault current that the upstream power source could supply in the event of a short-circuit fault.

short-time withstand current rating

According to the ANSI/IEEE C37.20.1 standard, short-time withstand current is defined as: "The designated limit of available (prospective) current at which it shall be required to withstand its short-time current duty cycle (two periods of 0.5 s current flow, separated by a 15 s interval of zero current) at rated maximum voltage under the prescribed test conditions."

In other words, this rating consists of two quantities: time (typically measured in cycles) and current (typically measured in kiloamps, kA). For low-voltage switchgear, the time rating is 30 cycles (0.5 seconds) and the current rating is the amount of short-circuit fault current that the mechanical assembly, electrical bus bar and bracing can endure for 30 cycles without sustaining damage at the tested voltage.

Low-voltage power circuit breakers also have a short-time withstand current rating and the switchgear must equal the short-time withstand current rating of the lowest rated breaker used in the switchgear assembly. For example, if the main circuit breaker has a 100 kA, 30 cycle short-time withstand current rating but a feeder breaker has a 65 kA, 30 cycle short-time withstand current rating, the switchgear will carry a 65 kA, 30 cycle rating.

Low-voltage power circuit breakers are specially designed to be able to withstand a fault of a given magnitude, without tripping, for up to 30 cycles. Compare this to molded case circuit breakers (MCCB) which are designed to trip instantaneously (within 3-4 cycles) when subjected to fault current above the instantaneous setting. As a result, MCCBs are tested to withstand a short-circuit fault for only 3 cycles before tripping. The purpose of having circuit breakers and switchgear that can withstand a short-circuit fault for up to 30 cycles is to improve selective coordination.

Difference between switchgear and switchboards

Low-voltage metal-enclosed switchgear and low-voltage switchboards are products used to safely distribute power throughout a facility. Both assemblies utilize free-standing enclosures that house circuit breakers, bus bar and power cables. Both products may contain meters, relays, potential transducers, current transducers and transfer schemes for redundant power. However, that is where the similarities end.

Switchboards are typically constructed with a dead-front, open-chassis design with few or no internal barriers between the cables, circuit breakers and bus bar. When the switchboard dead-front is removed, all bus bar, cables and terminations are exposed.

Switchboards are tested per the UL 891 Switchboards standard and are normally composed of fixed-mounted molded case circuit breakers which comply with the UL 489 MCCB standard. Switchboards tend to be front-accessible which means the incoming and outgoing cable terminations can be accessed from the front so the assembly can be mounted against a wall. These differences result in a smaller footprint than a similar switchgear assembly that contains the same number of circuit breakers.

Switchboards also tend to be less expensive than switchgear. For example, fixed-mounted MCCBs are less expensive than draw-out power circuit breakers. However, MCCBs are not designed to be serviced and if the breakers are fix-mounted, the switchboard must be de-energized in order to replace them. Switchgear, on the other hand, contains draw-out power circuit breakers which can be removed from the equipment while it is energized and are designed to be fully serviceable.

Switchboards only have a 3 cycle short-time current withstand rating, versus a 30 cycle rating for switchgear. This is due to the fact that MCCBs also only have a 3 cycle short-time current withstand rating. This means that achieving selective coordination is more difficult since short-time delays cannot be programmed in to provide time for circuit breakers farther downstream to clear faults.

Certain arc-flash safety technologies are also not available in switchboards. Such technologies available only in low-voltage switchgear include Arc Quenching technology?and arc-resistant construction.

In facilities that consume large amounts of power and facilities that require reliable power, switchgear and switchboards both play important roles. The switchgear may provide primary low-voltage power distribution and protection, often residing at the service entrance or on the secondary of a transformer substation, feeding power to various switchboards and low-voltage MCCs located throughout the facility which in turn feed smaller branch circuits such as lighting, HVAC and process-specific loads.

Reference : Eaton website