What is switchgear and protection pdf
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- switchgear and protection interview questions and answers pdf
- switchgear and protection interview questions and answers pdf
In an electric power system, switchgear is the combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply. The very earliest central power stations used simple open knife switch, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making opening manually operated switches too dangerous for anything other than isolation of a de-energized circuit.
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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.
Low-voltage metal-enclosed switchgear is a three-phase power distribution product designed to safely, efficiently and reliably supply electric power at voltages up to 1, volts and current up to 6, amps. Low-voltage switchgear is often found on the secondary low-voltage side of a power distribution transformer. This transformer and switchgear combination is known as a substation. Low-voltage switchgear is typically used to feed low-voltage motor control centers LV-MCC , low-voltage switchboards and other branch and feeder circuits.
A typical structure or section of low-voltage switchgear consists of three distinct and segregated parts:. 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 removeable 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 switchgea r since access to the back of the switchgear enclosure is required. 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. Horizontal main bus electrically connects adjacent switchgear sections to one another.
In most cases, insulation or dielectric strength between the three bus phases is provided via an adequate air gap. In locations where bus clearances are not sufficient to provide the necessary dielectric strength, insulation is applied to the bus. 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.
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. The short-circuit withstand current rating is also known as the short-circuit current rating SCCR of the switchgear.
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 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 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 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. 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. 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 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 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.
Selective coordination is achieved by setting the overcurrent devices in a system such that the device closest to the fault opens first, as opposed to all devices that the see fault current opening.
Selective coordination applies for the full range of overcurrents on the system and the full range of interrupting durations associated with those overcurrents. In other words, a short-circuit fault can occur downstream in a power distribution system with upstream circuit breakers experiencing but withstanding the fault current and remaining closed while only the circuit breaker closest to the fault opens.
The purpose of selective coordination is to increase the reliability and uptime of a power distribution system. Theoretically, a relatively minor downstream fault in a system that is not selectively coordinated could result in multiple breakers tripping on overcurrent and causing a widespread outage in the facility. Such an outage could be catastrophic for emergency systems, life safety systems and other critical power applications. Consequently, the National Electric Code NEC devotes numerous articles to requirements around selective-coordination for certain applications.
Such selective coordination is achieved with circuit breakers by modifying the instantaneous trip settings if available and short-time delay settings of the circuit breakers in the power distribution system. Even though molded case circuit breakers may have electronic trip units with short-time delay settings, these devices are rated to interrupt maximum rated fault current within 3 cycles so only a limited amount of coordination can be achieved by adjusting the instantaneous trip levels for circuit breakers that are farther downstream and farther upstream.
However, if a fault exceeds the instantaneous trip current of the breakers, the system is non-selective and a larger outage will occur. Low-voltage power circuit breakers, on the other hand, have a short-time current withstand rating which enables them to delay tripping for a preset time up to 30 cycles. Consequently, a LV-PCB can be programed to delay tripping in order to give downstream circuit breakers a chance to clear the fault first.
Hence, low-voltage switchgear with power circuit breakers can be used to greatly improve the reliability of a power distribution system. A properly coordinated system can also increase worker safety because the fault will be isolated downstream, reducing the likelihood that the worker will have to interface with upstream equipment which typically has a higher arc flash hazard.
Unfortunately, there is a downside to a power distribution system that has selective coordination implemented.
Since molded case circuit breaker instantaneous settings may be increased and power circuit breaker short-time delays programmed in order to achieve selective coordination, the incident energy at certain points of the system may increase.
Incident energy is directly related to clearing time and available fault current among other variables , so increasing the fault current or clearing time will result in increased incident energy. Therefore, it is even more important to follow the NEC Article Article 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 Switchboards standard and are normally composed of fixed-mounted molded case circuit breakers which comply with the UL 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. 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.
Low-voltage switchgear fundamentals. What is switchgear? Quick links to fundamentals of low-voltage switchgear: Metal-enclosed switchgear Parts of a low-voltage switchgear What is short-circuit withstand current rating? What is interrupt rating? What is the difference between switchgear and switchboards? Unit substation.
What is low-voltage metal-enclosed switchgear? What are the main parts of low-voltage switchgear? A typical structure or section of low-voltage switchgear consists of three distinct and segregated parts: Breaker compartment Bus compartment Cable compartment. Side view of a single switchgear section. Magnum DS breaker cell in low-voltage switchgear.
switchgear and protection interview questions and answers pdf
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. Low-voltage metal-enclosed switchgear is a three-phase power distribution product designed to safely, efficiently and reliably supply electric power at voltages up to 1, volts and current up to 6, amps.
switchgear and protection interview questions and answers pdf
In an electric power system , switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply. The earliest central power stations used simple open knife switches , mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making opening manually operated switches too dangerous for anything other than isolation of a de-energized circuit.
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