Assemblies of switchgear and control panels

Where are these assemblies used?

Switchgear and control panels are found in power generating stations, transformer stations, distribution substations, commercial and institutional buildings, industrial plants and factories, refineries, paper mills, metal smelters and any other place where electric energy is utilized or electric power is distributed to any number of zones.c

There are a few common types of assemblies that cover almost any application. A broad classification according to the location of the switchgear assemblies is whether the gear is installed indoor or outdoor.

A further classification for the indoor gear is standard ventilated indoor, indoor with drip hood, indoor for location in sprinklers area, indoor in hazardous location, arc­proof (type a, b or c) and indoor in corrosive environment. Outdoor switchgear can also be further classified as whether it has an isle or not (walk ­in vs. non­walk ­in types) with or without a working area.

Another method of classifying the switchgear is whether it is a bottom or top entry , the power and control cables are entering from the bottom or the top of the gear.

For switchgear assemblies, as heat is generated from the high current flowing, a dust tight enclosure is almost impossible thus a dust proof one is used, instead. The major standards that govern the design, manufacture and testing of these assemblies are: CSA, ANSI, IEC, EEMAC, NEMA.

[pullquote_left]Switchgear assemblies and motor control centers (starters) line­ups can be classified according to their voltage classes. [/pullquote_left]The l.v. equipment is rated in the range of 120/208V single or three phase to 750V AC. The medium voltage ones are 2400/4160V to 44KV . The enclosures for all low voltage construction are metal enclosed. Some constructions include partitions and barriers to give a safer operation over and above the standard construction.

For medium voltage switchgear both construction types, metal clad and metal enclosed, are common. In this section the major components of the switchgear assemblies, motor starters and associated ancillaries will be given.

Major components

Low voltage switchgear assemblies

The main components are:

• Molded case (MCB) or power (air magnetic) circuit breakers (draw­out or fixed) complete with or without integral overcurrent protection device (thermal­ magnetic,
• Solid state or microprocessor based) with or without zone interlocking provision
• Cradle (for drawout versions only)
• Fixed potential and control power transformers
• Current transformers
• Interframe and back barriers
• Main bus bars, run backs, bus risers and cable lugs (compression or mechanical)
• Protective fuses for breaker control circuits, instrument compartments
• Circuit breaker accessories, breaker lifting devices (integral with gear)
• Fused or unfused interrupter switches
• Protective and monitoring auxiliary relays
• Metering devices (ammeters, voltmeters, watthourmeters, wattmeters and digital metering packages ­D.M.P.)
• Control switches and indicating lights

Medium Voltage switchgear

The main components are:

• Draw­out circuit breakers (oil, air, SF6, vacuum)
• Switching device (load break or disconnect) with or w/o fuses (power current limiting or power expulsion type) with or w/o blown fuse indicator/actuator
• Instrument transformers
• Capacitor tripping device
• Relays
• Meters & instruments
• Main bus
• Control & indicating devices, jumpers, cable lugs, cable supports and potheads
• Barriers and shutters
• Instrument compartments
• Test switches and breaker accessories

Low voltage motor control centers

For LV motor control centers the components are:

• Combination motor starters
• Instrument transformers
• Motor protection devices
• Main bus, cable entry compartments, vertical bus, control cables
• Push buttons, pilot lights
• Breaker or switch handle
• Mounting pans
• Programmable logic controllers PLC (processor module, I/O, network interface and other associated modules)

Medium voltage starters

For MV starters the main components are:

• Contactors (vacuum and air)
• Fused control transformer
• Overload  protection
• Current transformers
• Instrument compartments
• Meters
• Auxilliary relays
• Timers
• Control devices
• Multifunction motor protection relays
• Indicating lights
• Power fuses
• Surge arresters

Switchgear Cubicle Types

There are a few distinct types of switchgear cubicles and are listed hereafter:

• Incoming (main) cubicles
• Transformer feeders
• Bus tie (sectionalization)
• Generator exciter control
• Generator neutral cubicles
• Induction motor control cubicle
• Synchronous motor switching board
• Potential transformer cubicles (used in conjunction with isolated phase bus in generating stations).

If the transformers are close coupled and installed indoor , then dry type transformers are used. If they are located outdoor and connected to the switchgear by non­segregated bus duct or cable duct or by a throat (with o/d switchgear) than they are of the oil filled type.

Another type of transformers that can be found indoors is the cast resin coil type.

To be continued…

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