3.1 GENERAL
Switchgear is required to enable power sources to be connected to and disconnected from the low-voltage distribution system. This switching is necessary both for normal operational purposes and also for the rapid and automatic disconnection of any circuit that becomes faulty. The switchgear also allows any circuit to be isolated from the live system and for that circuit to be made safe so that work may be carried out on the equipment connected to it.
This chapter deals with switching devices as applied to low voltage (typically 415V or 440V). Three types of LV switchgear are considered:
(a) Circuit-breakers
Circuit-breakers are used to control inputs from transformers, section breakers on switchboard sections, interconnectors between LV switchboards and inputs from auxiliary LV generators. All LV circuit-breakers are of the air-break type.
(b) Contactors
Contactors are used to control mainly motor circuits. They are always of the air-break type and are usually enclosed in the individual cubicles which form that part of a low-voltage switchboard referred to as a 'Motor Control Centre' (MCC).
Contactors are designed only to make and carry fault current for a short time, not to break it. Where the system fault level exceeds the limited breaking capacity of the contactor, fuses are inserted in series with the contactor contacts; Contactors are designed for remote operation and to undergo repeated and frequent operation without undue wear.
(c) Moulded Case and Miniature Circuit-breakers
These form a special class of lightweight compact circuit-breakers for mounting onto or behind panels. They are designed for hand operation only but have built-in protective tripping arrangements.
3.2 Main Air Break Circuit-breakers (ACB)
The main low-voltage circuit-breakers are always of the air-break type whose construction and operation are similar to those for high-voltage ACBs described in Chapter 2, and shown in Figured 2.3 Being designed for low-voltage systems their insulation levels are of course lower, but, by the same token, their normal rated currents and their short-circuit current ratings are considerably higher. This leads to generally heavier copperwork, to large arc chutes and especially to heavy switching contacts and isolating contacts.
Like their HV equivalents, LV circuit-breakers are horizontally isolated, with similar interlocks to ensure the correct sequence of operations when being withdrawn or reinserted.
Being smaller in size they are usually mounted in pairs, one above the other in an LV switchboard, presenting a dead-front panel face.
Most LV systems are 4-wire. Some main circuit-breakers are 4-pole, but most are 3-pole with an unswitched neutral connecting link.
LV circuit-breakers are rated from 800A to 4 000A normal current. They come in standard ranges of breaking capacity, which in British Standards is 35kA, 43kA and 70kA rms symmetrical. These currents at 415V are equivalent to 25MVA, 31MVA and 50MVA respectively, or at 440V are equivalent respectively to 27MVA, 33MVA and 53MVA. Because of the heavier normal and short-circuit currents found in LV systems, the circuit-breakers usually have much heavier breaking and isolating contacts than those of the HV types.
The circuit-breaker Closing mechanism may be operated by solenoid or motor/spring.
Tripping is by a separate shunt-trip coil, always powered from an independent battery-supported d.c. supply.
The basic circuit-breaker control circuits are essentially the same as those described in Chapter 2, for high-voltage switchgear. Some circuit-breakers are equipped with an additional release device in series with the main circuit which trips the circuit-breaker instantaneously if it is closed onto a fault; it does not operate under any other circumstance. This series tripping release is part of the circuit-breaker, not a protective relay, and requires to be reset by hand after operation. Where this device is fitted, the anti-pumping circuit is unnecessary and is omitted.
Closing and tripping of low-voltage circuit-breakers is usually carried out by operating switches on a remote electrical control panel. In some cases this remote control facility is not provided, and switching is done locally at the switchboard.
The procedure for disconnecting a circuit-breaker and removing it from its panel depends on the type of switchgear; the maker's instructions must be consulted in each case. To avoid personal injury, the closing spring must always be discharged before removing a spring-closed circuit-breaker from its switchgear panel.
3.3 Contactors
All contactors form a part of the individual distribution cubicles which make up an MCC. Almost all are unlatched.
Each contactor is rated according to the service which it feeds, which may vary from a fractional horsepower motor drawing one ampere to a large 250kW motor drawing over 400A. Consequently the contactor cubicle may vary in size from 'one tier' deep up to 'seven tiers' deep (see para. 3.7.2).
Each contactor operating coil is supplied, through its control circuits, from the switchboard busbar either direct at 415V or 440V, or through a small step-down transformer. This ensures that, if busbar power fails, all connected contactors 'drop off and keep their motors disconnected until each can be individually restarted.
Every contactor is backed up by a set of high rupturing capacity (HRC) fuses housed in the same cubicle. These are rated according to the fault level at the switchboard/(typically 31MVA or 50MVA, equivalent to 43kA or 70kA at 415V). The correct size of fuse is chosen so that, in the event of a fault in the feeder circuit which exceeds the ability of the contactor to clear it, the fuse will blow first, leaving the contactor to open on a dead circuit.
The choice of back-up fuse is further discussed in the manual 'Electrical Protection'.
Facilities are provided for testing the contactor while isolated from the busbar. While so isolated a separate test supply can be applied to the contactor coil to check its operation without actually starting the motor.
3.4 Moulded Case Circuit-breakers
A type of low-voltage circuit-breaker widely used in most installations is the Moulded Case Circuit-breaker( MCCB),Shown in Figure 3.1, it consists of a moulded plastic case containing a switching element which is operated manually by an external handle or 'dolly'. Because the original design was American, the dolly position is down for 'Off and up for 'On'.
MCCBs can be used for switching either a.c. or d.c. circuits. They are usually mounted, when used on distribution panels, behind the panel, and only the dolly shows. Other arrangements however, such as surface mounting, are also found.
Most of the MCCBs used onshore, and offshore are 3-pole, but very occasionally a 4-pole version is fitted. They are also supplied as 2-pole (for example for d.c. switching), but this is usually a 3-pole type with one pole omitted.
MCCBs are very compact and have a high breaking capacity for their small size. Where the system fault level at the point where an MCCB is used exceeds its fault-breaking capacity, separate HRC back-up fuses must be used in series, as described above for contactors.
MCCBs are made by a number of manufacturers, and different makes and sizes are used in several installations. The following description, therefore, can be no more than very general.
A MCCB as used in onshore and offshore installations is normally fitted with two separate overcurrent devices. One is a thermal element in each pole having an inverse-time characteristic, and the other an instantaneous 'high-set electromagnetic element in two of the three poles; this operates instantaneously but only on the highest fault currents and then overrides the thermal element. Both trip the circuit-breaker when the current reaches the set operating level in any of the poles.
Typical MCCBs ,have normal current ratings of either 125A or 250A, for exampel, will have a breaking capacities as given below.
Normal
The maximum currents that can be handled by these two sizes of MCCB are therefore 125A and 250A, but they can be arranged to trip at lower