LOW VOLTAGE SWITCHGEAR BASIC INFORMATION AND TUTORIALS

The variety of l.v. switchgear is also very wide. Typically a switchboard includes circuit-breakers for incoming supplies, a bus-section(s), important feeders and interconnectors, some or all of which may require interlocks.The individual supplies to equipment such as local lighting and small power, battery chargers, trace
heaters and domestic supplies in general tend to be controlled by switch-fuse unitsor fuse-switches.

Where there is a substantial incoming power supply to an industrial complex, an air-break circuit-breaker is often utilised.This can be a single cubicle unit with cables both in and out or it can be the incoming unit on a main switchboard or it can be connected direct to the switchboard busbar system.

Such a unit may be fitted with CT-operated overcurrent trips with inverse time characteristics, an automatic
operating mechanism and other features such as on/off indication, fuses for metering supplies, direct acting earth faults trip, auxiliary switches, shunt trip release and undervoltage release.

Typically these circuit-breakers have thermal ratings up to 4000A, a short time rating of 40–50 kA for one second and a short-circuit breaking capacity of a similar level. One point which is worth noting is that the rated insulation voltage should be about 660V. As the normal insulation level of l.v. cable is 1000 V it is possible to consider the use of 600 V motors and control gear as a means of reducing costs.

Increasingly popular as alternatives to fuses for fault protection are moulded case circuit-breakers. These are now offered in a wide variety with ratings in excess of 2500A and breaking capacities up to 150 kA at 380/415V. They are offered with either thermal/magnetic or electronic protection. Ratings up to 3000A and 660V a.c. are also available. However, the fault breaking capacity of these may be limited to 60 or 65kA.

Open-type power circuit-breakers in compact ranges are available up to 8000A. These compact breakers are available in modular form with a wide range of accessories.

They can be plugged in with terminal shields.They can be locally or remotely controlled and they have arrangements for earth leakage, shunt trip or undervoltage release.Auxiliary and alarm switches are fitted and a visible indication is available confirming that in the ‘off’ position all contacts are separated by the required
distance. The main outgoing cables can therefore be selected taking into account the most onerous of the circuit duties.
Mechanical strength and protection is also important. For example, vibration may be significant if the substation is adjacent to a road or heavy machinery. Fire precautions are also necessary if the cables are being run in a confined space where personnel may be in danger from heat or smoke. Corrosion may also be a hazard.

Single-core cables may cause overheating of adjacent metalwork and can also produce high induced voltages under fault conditions. They are also a source of interference unless suitable precautions are taken to segregate them from sensitive equipment.

The alternative to a circuit-breaker is an isolating switch with or without backup fuses.The isolating switch should preferably have a making capacity of about ten times and a breaking capacity of about eight times normal full load current at say 0.3 p.f.

This is to deal with the stalled rotor current or starting current of any motor connected to the board. Alternatively on-load switches can be obtained which provide a capability of making onto a fault and breaking a current of about three times normal rating.

On the incomer an alternative may be to use a fuse-switch. It is important in this case that the moving parts are connected to the busbars and not to the incoming Substations and Control Rooms 51 cable, otherwise the fuses are still alive when the switch is open.

On those boards which are protected by a circuit-breaker or fuse at the sending end of the incoming cable no fusegear is required on the incomer itself. This can then be an isolating switch using perhaps a fuse-switch with fuses replaced by links of adequate throughfault rating.

For outgoing units the same range of equipment is available, such as air circuitbreakers, mccbs, fuse-switches and isolators, but with the addition of switch-fuses, i.e. an isolator associated with separate fuses, distribution fuseboards and motor starters for the control of rotating equipment.

All such gear is connected through busbars of appropriate rating to the incoming supply. It is imperative that the busbars are adequately insulated and protected from vermin, insects, dust and damp but most important that they should not be adversely affected by a fault on an outgoing circuit.

There are many ways of cabling a switchboard but the majority have rear access from the bottom. However, a modern trend is to have front access to the cable compartments allowing the board to be set against the rear wall.

Additional space is required for aluminium cables, and shrouded terminals are a useful safeguard. Other variations and accessories available are bus-section switches, packaged substations where a Class C transformer is directly connected to the switchboard, power factor correction equipment including the capacitors, relays, contactors and fuse-switches for automatic multi-stage operation, together with instrumentation, metering, protective relaying and control switches. A 400V switchboard is shown in Fig. 2.12.

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