The subject of ‘Switchgear’ is regarded as covering all types of switching devices such as circuit-breakers, contactors and hand-operated switches, as well as fuses and protective devices like relays.
Switchgear is used on both high-voltage and low-voltage systems. It is required to enable generators, feeders, transformers and motors to be connected to and disconnected from the high-voltage or low-voltage system. This switching is necessary both for normal operational purposes and for the rapid 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 equipment connected to it.
This chapter deals with switching devices as applied to high-voltage systems; low voltage is covered in Chapter 3. Fuses and relays are dealt with in the manual ‘Electrical Protection’. Two types of HV switchgear are considered in this chapter:
Circuit-breakers are used to control generators, transformers, bus-sections, bus-couplers, interconnecting cables between switchboards and the starting and running of very large motors; they are designed to make and break full fault currents. A circuit-breaker may be of the oil-break, air-break or vacuum-break type. Because of the fire hazard only air-break and vacuum-break units are used on most offshore installations, but oil-break circuit-breakers are widely used onshore. They are not designed for continuously repeated operation.
Contactors are used to control motor circuits and sometimes transformers. They may be of the air- or vacuum- break type; each type is used both onshore and offshore.
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 to undergo repeated and frequent operation without undue wear.
A switchboard may be made up of a mixture of circuit-breaker and contactor cubicles, depending on the nature of the individual loads and the distribution requirements.
A circuit-breaker or contactor has five ratings:
Voltage. This is the nominal system voltage at which the switch will operate without breakdown.
Normal Current. This is the current which the switch will carry continuously without overheating.
Breaking Capacity. This is the maximum fault current (expressed in kA (rms) or MVA) which the switch will interrupt on all three phases.
Making Current. This is the maximum peak asymmetrical current (expressed in kA) that the switch can carry in any pole during a making operation.
Short-time Rating. This is the maximum time (usually specified as 3 sec or 1 sec) for which the switch will carry, without damage, the full fault current before that current is broken.
The theory and manner in which the various types of circuit-breaker and contactor extinguish the arc and interrupt the current is dealt with fully in Chapter 1. The descriptions which follow are concerned with the actual hardware, its operation and its assembly into switchboards.
In an oil circuit-breaker the contacts operate in a tank of oil (sometimes in three separate but smaller tanks). There are usually two breaks in series per phase, the moving contacts moving together in a downward vertical direction inside the tank. Around each of the six fixed contact tips is placed an arc-control device, usually of the ‘cross-jet explosion pot’ type. A sectioned view showing one phase of an OCB is seen in Figure 2.1, which shows the circuit-breaker unit in its fixed housing and in the ‘Service’ position.
TYPICAL HIGH VOLTAGE OIL BREAK SWITCHGEAR UNIT (VERTICAL ISOLATION)
The withdrawable truck is shown in blue, the main copperwork, busbars and conductors red, and the oil is shaded yellow.
The circuit-breaker and its tank, with its six external terminals (two per phase), can be withdrawn vertically downwards clear of the corresponding plug-type connections in the main housing. When so withdrawn the circuit-breaker proper is electrically isolated from the busbars and feeder, and automatic shutters close over the six fixed live connections or ‘spouts’. Isolation may be vertical (as in Figure 2.1) or horizontal. After isolation the whole circuit-breaker unit in its tank, together with its operating mechanism, can be drawn out clear of the housing horizontally on its wheels for examination or maintenance.
Mechanical interlocks are provided to ensure that the circuit-breaker unit can never be isolated unless it has first been opened, and that it cannot be reinserted into the housing unless it is already open.
Where a panel is fitted with a voltage transformer (VT), this is mounted in a separate compartment above the cable connection compartment. In Figure 2.1 the VT is oil-immersed in a tank, which can be withdrawn for isolation. A VT may be connected either to the feeder side or to the busbar side of a circuit-breaker, depending on its application, and it is protected by high-voltage fuses mounted inside the VT compartment (also shown in Figure 2.1).
TYPICAL HIGH VOLTAGE OIL BREAK SWITCHGEAR UNIT
ON DUPLICATE BUSBARS (VERTICAL ISOLATION)
Operating mechanisms to close the breaker may be solenoid, motor/spring, pneumatic or hydraulic and are described more fully in para 2.3.3. The mechanism must be strong enough to close the breaker positively against the ‘throw-off’ forces of the maximum short-circuit current that can possibly occur at the point in the system where the circuit-breaker is installed (see the manual ‘Electrical Protection’). Circuit-breakers latch-in when closed and are tripped by a separate shunt-trip coil, always energised from an independent d.c. battery-supported source. In a few cases a no-volt coil is used for tripping.
After clearing a full-scale fault an oil circuit-breaker should be able to continue in operation without attention, but it is customary to withdraw it at the first suitable opportunity to examine the contacts and replace the oil.
Oil circuit-breakers, being usually large, are not often made up into switchboards but are assembled into sets with common busbars running through and, where possible, mounted out-of-doors. They are sometimes totally enclosed, with their busbar system, in individual iron enclosures - the so-called ‘iron-clad’ or ‘metal-clad’ switchgear. The controlling switchboard would be a separate unit indoors.
On some installations the switchboard contains two independent busbar systems, and the circuit-breaker can be connected to either set. This is the so-called ‘duplicate busbar’ system.
A typical duplicate busbar arrangement, using vertical isolation, is shown in Figure 2.2. Selection between the rear and front busbars is achieved by moving the circuit-breaker truck backwards or forwards to its correct position under the rear or front pairs of fixed isolation contacts (or ‘spouts’). One set of spouts, the feeder connection, is common to both rear and front sets. In the figure the circuit-breaker is shown in the position to engage the front busbars. When in position under the selected spouts, the circuit-breaker is raised to make contact with the chosen busbar system. Interlocks ensure exact positioning before the breaker is raised.
Duplicate busbar systems may also be arranged for horizontal isolation, but the vertical arrangement is more common. They are used more often with oil circuit-breakers than with other types.
Duplicate busbar systems are fairly common in onshore installations. They are also used on some offshore platforms.
2.3.1 Air Break Circuit-breaker Panel
An air-break circuit-breaker panel is made up of two parts: a fixed portion or ‘housing’ and a removable circuit-breaker truck. The precise layout varies from one manufacturer to another; a typical arrangement is shown in Figure 2.3.
The unit shown in the figure is horizontally isolated. The circuit-breaker truck (blue) can be clearly seen in the ‘Service’ position with the breaker’s moving contacts open and with the arc chutes above. In this case the busbars run along the bottom of the housing, with the feeder cable box at the back. The busbars and other main copperwork are shown red.
The fixed part is divided into four compartments, each one separated from the others by an earthed sheet-metal barrier. At the front of the panel are two compartments; the upper one is a low-voltage compartment housing low-voltage control equipment, associated fuses and auxiliary cable terminations. The lower one houses the circuit-breaker truck. A door is fitted at the front of each of the compartments. At the rear of the panel are two further compartments each with a bolted cover. The lower compartment houses the busbars and the upper the feeder cable terminations and current transformers, as shown in Figure 2.3.
TYPICAL HIGH VOLTAGE AIR BREAK SWITCHGEAR UNIT
The circuit-breaker is mounted on a truck supported on rollers which can be moved in a fore-and-aft direction within the circuit-breaker compartment, or removed from it. At the back of the circuit-breaker compartment are two steel shutters, one to cover the fixed feeder contacts and the other the busbar contacts. When the circuit-breaker is moved into the ‘Service’ position, the shutters open to allow plugging contacts at the rear of the circuit-breaker to enter the insulated spouts which house the fixed feeder and busbar contacts. The shutters provide a safety barrier to prevent human contact with live metal when the circuit-breaker is disconnected or removed from the panel.
Where a panel is fitted with a VT, this is mounted in a compartment above the cable connection compartment. A voltage transformer may be connected either to the feeder side or to the busbar side of a circuit-breaker, depending on its application, and it is protected by high-voltage fuses mounted inside the VT compartment (also shown in Figure 2.3). The cover of this compartment can only be removed when the VT has been isolated.
On the dead-front panel in front of the VT compartment may be mounted indicating instruments, controls and the protective relays concerned with that item of switchgear.
2.3.2 Air Break Circuit-breaker Truck
A typical circuit-breaker on its truck is shown in Figure 2.4. The switching contacts are shown open, and above the fixed contacts are the arc chutes. On the left are the two sets of three isolating contacts, with their heavy contact tips, which engage with the fixed busbar and feeder sets of isolating contacts in the housing. When the truck is withdrawn, safety shutters drop to cover the live fixed contact spouts; these shutters are automatically lifted as the truck is reinserted into the ‘Service’ position.
TYPICAL AIR BREAK CIRCUIT-BREAKERAND TRUCK
The motor/spring operating mechanism is housed in the base. On the face on the right-hand side (not seen in the figure) are the handle for manually charging the spring, mechanical indicators for showing whether the spring is charged or discharged, indicators for showing whether the breaker is open or closed, and mechanical pushbuttons for releasing the spring (i.e. closing) and for tripping the breaker; these are marked ‘l’ and ‘O’ respectively.
2.3.3 Operating Mechanisms
The circuit-breaker closing mechanism for either an oil-break or air-break circuit-breaker is usually solenoid or motor/spring powered. The solenoid uses d.c., usually at 110V, and operates on the breaker linkage. Alternative hand operation is provided.
The closing mechanism operates through a toggle link which, when closed, is ‘over-toggled’ and therefore solid. When a trip signal is given, a trip coil breaks the toggle and allows the circuit-breaker to open under its trip springs. If the breaker should be closed onto a fault,
the toggle breaks immediately, so allowing the breaker to re-open freely. This arrangement, known as a ‘trip-free’ mechanism, protects the operator against injury by the breaker’s throw-off forces if he unwittingly attempts to close it by hand onto a fault.
A motor/spring mechanism charges a powerful helical spring by motor. When charged, the spring latches and remains so until released. On receipt of a closing signal, the latch is released and the spring closes the breaker through its linkage. Immediately the breaker has closed, a limit switch causes the motor, usually d.c. operated, to recharge the spring ready for the next closing operation. In the event of motor power failing, the spring can be recharged manually by a detachable handle. The full operating circuit is explained in para.
An essential difference between the solenoid and the motor/spring closing systems is that the solenoid takes a very large current during its quick stroke, necessitating heavy cable leads, whereas the motor takes about five seconds to recharge the spring - that is, to deliver the same energy - so that the motor draws far less current. The motors can, if the system requires it, operate on a.c.
Both systems require a separate trip signal to actuate the shunt-trip coil; this releases the closed mechanism and causes the breaker to open under its own separate trip springs. It can also be tripped mechanically by hand.
A number of complete housings, together with their withdrawable circuit-breakers, can be assembled into a single switchboard, as shown typically in Figure 2.5.
TYPICAL HIGH VOLTAGE SWITCHBOARD
The panels, whose upper parts carry the local controls, protective relays and indicating instruments, form a continuous switchboard. The circuit-breakers are at the bottom behind doors which can be opened for mechanical control of the breaker or for withdrawing it.
The switchboard acts as an electrical ‘manifold’, with a common set of busbars running the length of the board. In most offshore boards the busbars run along the bottom, sometimes encapsulated in epoxy resin, and tee-offs are made from them to the lower contacts of each circuit-breaker (Panels 1-8) and contactor (Panels 10-13). This is shown in red in Figure 2.5 for a typical high-voltage switchboard.
At a busbar transfer panel, such as Panel 9, the copperwork run is changed from the bottom level (for the circuit-breakers) to two levels for the tiers of contactors. This arrangement is also shown in Figure 2.5. Sometimes (as here) use is made of the front of a narrow transfer panel to accommodate extra relays.
At a bus-section panel, Panel 5, the bottom run on one side passes into the bus-section circuit-breaker, and from its top contacts back to the bottom level to feed the circuit-breakers on the other side. By studying Figure 2.5 it will be seen that, even when the busbars on one side of a section breaker have been made safe for maintenance by complete isolation and earthing down, the bus-section cubicle may still have live copperwork on the other side of the circuit-breaker. Particular care is therefore necessary when working on, or near to, bus-section cubicles.
Switchboards other than the one described and made by other manufacturers, will differ in the detail of their busbar arrangements. Nevertheless the principles explained above will apply in all cases.
Contactors (sometimes called Motor Switching Devices (MSD) when used with motors) are generally smaller and lighter than circuit-breakers operating on the same system because they carry less load current and, more important, they do not have to break full system fault current. However they must be able to carry it for the specified time (3 sec or 1 sec), to close onto it positively without bounce, arcing, or welding, and to operate repeatedly.
For these reasons high-voltage contactors only occupy half the space taken by the circuit-breaker, and they are usually mounted in pairs, one above the other, in a panel the same size as for one circuit-breaker. Four such contactor panels can be seen on the right of the switchboard in Figure 2.5.
The contactor itself operates on the same principle as the air-break circuit-breaker, with contacts in air and an arc chute. The closing mechanism is always solenoid-operated, and particular attention is paid to the linkage to make it robust and suitable for repeated operation, such as for motor starting.
Contactors may be ‘latched’ or ‘unlatched’. In the former case they hold-in by latch after closing, and the closing solenoid is then de-energised. They require a separate trip signal to open them by shunt-trip coil, as with the circuit-breaker.
Much more common, however, is the unlatched contactor. It is held closed by its closing solenoid, usually with reduced current through an economy resistor. The solenoid remains energised throughout. When it is desired to trip the contactor, the solenoid is simply de-energised and the contactor ‘falls off’. It also falls off if the supply to the solenoid fails. Such contactors are said to have an ‘inherent undervoltage’ feature, which means that, unlike the latched type, they will open automatically if the operating voltage fails.
Because a contactor breaks normal currents but cannot break a heavy fault current, it is usually backed up by fuses (see the manual ‘Electrical Protection’). These are a set of high rupturing capacity (HRC) fuses in series. They are so chosen that, with currents in excess of those which can be safely broken by the contactor, the fuses will blow first, so
interrupting the current and leaving the contactor to open on a dead circuit. These fuses, which are quite large, are usually mounted at the bottom of the contactor unit, which must be withdrawn in order to get at them.
HIGH VOLTAGE CONTACTOR UNIT
Figure 2.6 shows a typical half-height HV contactor unit. The fuses can be seen at the bottom. The contactor unit, with fuses, is horizontally isolated and can be withdrawn on rails for servicing, maintenance or fuse changing. A special carriage with rails is required to withdraw an upper unit. The whole unit shown in Figure 2.6 is withdrawable clear of the switchboard.
The HRC fuses are of the ‘trigger’ type. The blowing of any one fuse will mechanically release the contactor, so ensuring a break in all three phases.
Circuit-breakers and contactors which use vacuum interrupters form another class of switching device. The principle upon which vacuum interrupters control the arc and break heavy currents is described fully in Chapter 1. The paragraphs which follow describe how the interrupters are actuated and how the circuit-breaker or contactor is integrated into a switchboard panel.
The only important differences between a vacuum circuit-breaker and a vacuum contactor are that the circuit-breaker has a much larger rated breaking capacity and is latched, whereas the contactor is usually unlatched. Apart from these features and, of course, size, the description which follows applies to both.
A typical vacuum interrupter element, together with its operating mechanism, is shown in Figure 2.7. Figure 2.7(a) shows the interrupter in the open position. The armature arm (green) is held open by a compression spring, and the moving contact (yellow) is lifted by a boss on it into the open position and held there. The vacuum area is coloured blue; it is contained on its upper side by a bellows to take up the movement of the upper contact.
When the closing coil is energised it attracts the armature downwards away from the boss (Figure 2.7(b)). The moving contact is then free to move and is forced downwards by atmospheric pressure, opposed only by the vacuum. It should be particularly noted that, with this design, contact pressure is maintained by atmospheric pressure only.
To open the contacts the closing coil is de-energised, the spring takes over and drives the armature upwards. It strikes the boss and takes the moving contact sharply with it against the atmospheric pressure, so returning to the position of Figure 2.7(a). This mechanism will be seen to be of the normal unlatched type, used when operating as a contractor, which opens on the de-energising of the operating coil. When used as a circuit-breaker the motion would be latched and a shunt-trip coil used to release it.
Other vacuum switches, particularly vacuum circuit-breakers, have different types of mechanism and do not always use atmospheric pressure. Those which latch require a separate tripping coil which usually operates by breaking the latching toggle.
Because vacuum interrupters are sealed for life, there is no question of contact replacement. Since the vaporised metal is always redeposited on the contacts, wear is in any case small. It can be detected by indicator or the use of feeler gauges. If the wear of any one of the set of three exceeds the manufacturer’s recommendation, all three interrupters must be replaced since the small contact travel makes the adjustment critical.
The 3-phase vacuum interrupter unit, together with its operating mechanism and back-up fuses, is vertically isolated within its own panel. The whole unit is drawn downwards by operation of an external handle, as shown in Figure 2.8.
TYPICAL VACUUM CONTACTOR SWITCHBOARD
Interlocks are provided to prevent isolation unless the interrupter is open. In those contactor panels which feed motor circuits the act of downward isolation also automatically puts an earth on the isolated feeder cable. In contactor panels which feed transformers the feeder earth is separately applied by an external earthing handle which cannot be moved until the interrupter unit is fully isolated.
A set of vacuum contactor panels, sometimes also with vacuum circuit-breaker panels, can be assembled to form a complete switchboard, as shown in Figure 2.8. Vacuum switch panels are very narrow compared to the equivalent air-break circuit-breaker panels and lend themselves well to very compact, space-saving switchboards.
A high-voltage switchboard is an assembly point which receives power from the HV generators or other sources, controls it and distributes it. A common busbar system runs through the board to which the power sources are connected through switchgear. The busbars act as a ‘manifold’, and feeders are taken from it, through circuit-breakers or contactors, to all power-consuming services such as transformers, motors or interconnectors.
TYPICAL OFFSHORE HIGH VOLTAGE SYSTEM
Figure 2.9 shows, in diagram form, a typical air-break HV switchboard. It is in fact the diagram of the HV switchboard shown pictorially in Figure 2.5. It shows the two generator incomer breakers (3 and 6), the bus-section breaker (5), four feeder breakers (1, 4, 7 and 8) supplying very large motors which are too big for contactors, and one interconnector breaker (2). On the right are the four contactor panels. In Panel 9 (a bus transfer panel) the busbar splits into two - one for the high-level and one for the low-level contactors. There are three contactors (10, 12 and 13) feeding medium-sized motors and one (11) feeding a transformer; all four contactors have back-up fuses.
The closing mechanisms of circuit-breakers met with on offshore and onshore installations are almost always solenoid or motor/spring operated, as described in para. 2.3.3, and all breakers have separate shunt-trip coils.
Solenoids are operated through their own contactors, usually from a 110V d.c. supply. The control circuits are simple and need no explanation here.
Motor/spring mechanisms are more complicated. Their circuits require switches and contacts to control the close and trip initiation and to ensure the correct sequence of operation. Circuits are also necessary for indication, for charging the closing spring and for operating the closing release. Auxiliary switches are therefore provided, mechanically linked to the circuit-breaker mechanism. Also fitted are carriage switches or isolating contacts which are operated by the movement of the truck to the ‘Service’ or ‘Isolated’ positions.
TYPICAL CIRCUIT-BREAKER CONTROL CIRCUIT (MOTOR/SPRING OPERATED)
A typical circuit-breaker motor/spring control arrangement is shown, in diagram form, in Figure 2.10. Contacts are shown with the circuit-breaker open and in the ‘Service’ position, all relays de-energised and the closing spring discharged.
When the 110V d.c. supply is connected, the spring charging motor runs; after the closing spring has been charged and automatically latched, auxiliary limit contacts operate to stop the motor and prepare the closing circuit.
Some circuit-breakers may only be closed remotely by the manual operation of a switch at a remote electrical control panel; others may be closed either remotely or by a local switch on the switchgear panel, in which case a ‘Local/Remote’ selector switch is fitted. The circuit illustrated in Figure 2.10 is arranged for remote closing only. Once the spring is charged, operation of the remote switch to the CLOSE position completes the closing circuit and energises the closing coil; this releases the latch and the spring closes the breaker. When the circuit-breaker has closed, an auxiliary switch opens, disconnecting the closing coil, and at the same time the motor, reconnected by the limit switch, runs to recharge and latch the spring.
Normally the circuit-breaker is tripped by operation of the remote switch; in addition a local ‘Emergency Trip’ pushbutton is fitted to the switchgear panel. If a system fault causes the protection to operate, the trip relay tripping contact closes. Closing any of these tripping contacts energises the trip coil, releases the holding latch and allows the breaker to open.
If the circuit-breaker closes onto a faulty circuit it receives an immediate trip signal from the protection, which causes it to trip; if the operator continued to hold the operating switch to CLOSE, the circuit-breaker would ‘pump’ in and out until the control switch was released. To prevent this, an anti-pumping circuit is provided. When the circuit-breaker closes, and while the control switch is still held to CLOSE, an auxiliary switch completes a circuit to energise the anti-pumping relay. The contacts of this relay change over to break the closing coil circuit and complete a hold circuit for the anti-pumping relay. Thus the breaker will open if tripped by an immediate fault condition, but no further closing operations can take place until the control switch has been released. This provides a ‘one-shot’ closing facility.
For maintenance it may be necessary to close and trip the circuit-breaker by local control while it is disconnected and isolated from the busbars. When the truck is in the ‘Isolated’ position, carriage switch contacts operate to change over the closing and tripping circuits from their normal connections to a local test switch. This may now be used to operate the circuit-breaker.
Auxiliary contacts operate lamps to indicate when the circuit-breaker is open or closed. Additional contacts may provide control features special to a particular circuit, such as the automatic start of standby motors, load shedding, sequence starting or other special requirements.
Where solenoid closing is used alone, its high d.c. current requires the closing solenoid itself to be switched by a separate contactor.
Separate fuses are provided in each switchgear cubicle for the closing and tripping supplies so that each may be isolated without affecting the other, and to provide protection to each auxiliary circuit appropriate to its loading.
The circuit-breaker motor/spring control system shown in Figure 2.10 and described in this section is typical of what may be found in many installations, but variations exist.
There may be long periods of time when circuit-breakers are not called upon to trip. It is vital however that, when the occasion arises such as the onset of a fault, the trip circuit operates and the breaker trips successfully. A ‘Trip Circuit Supervision’ relay is provided on each circuit-breaker unit. It monitors the trip circuit and its power supply continuously, both when the circuit-breaker is closed and also when it is open.
A trip circuit supervision schematic diagram is shown in Figure 2.11. The trip circuit supervision relay consists of three elements: ‘a’, ‘b’ and ‘c’. The trip circuit is monitored by passing a small current derived from the tripping supply through the trip coil and all associated trip circuit wiring in series with the coils of relay ‘a’ or ‘b’ or both. This current is sufficient to operate relays ‘a’ and ‘b’, even when they are connected in series, but, being limited by resistors, is not high enough to trip the breaker. Relays ‘a’ and ‘b’ are energised individually when the contacts across which they are connected are not closed. Relay ‘c’ is a hand-reset flag relay with alarm contacts which are closed if the trip circuit is healthy, but they open to give an alarm if it is not.
If the circuit-breaker is open, both relays ‘a’ and ‘b’ are energised in series. If the circuit-breaker is open but a trip initiation signal is present, say from a hand-reset tripping relay, then relay ‘b’ alone is energised. When the circuit-breaker is closed, relay ‘b’ is de-energised and only relay ‘a’ monitors the trip circuit.
TRIP CIRCUIT SUPERVISION
If the trip supply fails or the breaker fails to open when a trip signal is initiated, then both relays ‘a’ and ‘b’ are de-energised and an alarm is initiated when relay ‘c’ releases. This happens when:
· the trip supply fails;
· any part of the trip circuit is open-circuited;
· the breaker fails to open;
· the trip circuit supervision relay auxiliary supply fails.
The opening of the contacts of relays ‘a’ and ‘b’ is delayed by about 400ms to allow for transient dips in the 110V d.c. supply and to allow time for the circuit-breaker to open without initiating an alarm.
The only high-voltage fuses fitted in an HV switchboard are those which form a back-up to a contactor (air-break or vacuum) and those on the HV side of a voltage transformer. All are of the high rupturing capacity (HRC) type.
The contactor fuses are of the open type but are embodied in the contactor unit itself. This forms adequate protection since it is necessary to isolate the unit, and in the case of the air-break type to withdraw it, in order to gain access to the fuses. They can be seen in Figure 2.6.
Where fuses, whether HV or LV, are used in series with a contactor, their purpose is to protect the contactor itself against having to open on a fault current which is in excess of its rating. Fuses used in this manner are termed ‘back-up fuses’ and are selected with reference to the contactor’s own inverse-time characteristic. This process is fully described in the manual ‘Electrical Protection’.
Voltage transformer HV fuses form part of the VT itself. The VT compartment can only be opened after isolation, after which the fuses are accessible. Although the VT fuses have a very small current rating, they still have to be able to break a full-scale short-circuit current if a fault should develop on the VT itself.
There are many low-voltage fuses in the control and instrumentation circuits. These are of the type described in Chapter 3 for LV switchgear.
To ensure that a high-voltage circuit is safe to work on, it must be disconnected (isolated) from all sources of supply, and it must also be solidly earthed down. Earthing is carried out at the isolating switchgear.
FEEDER AND BUSBAR EARTHING
Earthing-down procedures vary from one type of switchgear to another; the manufacturer’s operating instructions must be consulted in each case.
Four examples of the use of switchgear for safety earthing are shown in Figure 2.12. In each case the circuit-breaker or contactor truck must be moved out of the ‘Service’ position before a permissive interlock key can be released which permits earthing to take place.
Figures 2.12(a) and (b) show one type of earthing system. In Figure 2.12(a) the feeder is earthed through an earthing switch that can only be closed after the permissive interlock key has been inserted and operated. The permissive interlock key is released only when the switchgear truck is either locked in the ‘Isolated’ position or completely removed from the panel. For busbar earthing the same switchgear uses a special earthing circuit-breaker truck shown in Figure 2.12(b) which temporarily replaces the normal truck. Interlocks described below ensure that the busbar cannot be earthed until it is disconnected from any possible source of supply.
Figures 2.12(c) and (d) show a different type of switchgear with ‘integral earthing’, where the busbar or the feeder contacts on the circuit-breaker are moved mechanically to connect with a set of earthed contacts mounted between the busbar and the feeder spouts. The busbar or feeder contacts can only be moved after the appropriate permissive interlock key has been inserted into the front panel of the switch-truck and operated. This key, either busbar or feeder, is captive on the shutter operating mechanism inside the switchgear panel and is only accessible when the truck is removed. When either key is withdrawn, the appropriate shutter will not open when the breaker truck is pushed in, so preventing accidental access to live metal. To earth a feeder, the busbar shutters must be immobilised, and vice versa.
Where any earth is applied, a key is released from the switchgear unit concerned, to be placed in a Lockout Box. The earth cannot be removed until the key is returned.
Before a section of busbars is earthed down, all possible in-feeds must be disconnected. For example, one-half of a busbar system must be isolated not only from its generator incomers but also from the other half through the section breaker; also from any interconnectors and from any transformer that could feed back into the busbar. To ensure this, the individual keys released when all the possible in-feed breakers have been isolated are inserted into a key-box. When all the keys are home, a master key is released which permits the busbar earthing connection to be made or the earthing switch to be inserted.
Full and detailed instructions for isolating and earthing down are contained in ‘Standing Instructions Electrical’.
Each switchboard panel is fitted with an anti-condensation heater; this is usually energised at 240V or 250V a.c. and controlled either by a hand switch or by an auxiliary switch that connects the heater when the circuit-breaker is open.