reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

5.1.8 Rated short-circuit making current

In addition to the requirements of BS5311, switching devices are capable of making and latching closed against a prospective current equivalent to that of the

system in which the switchgear will be installed, as li mited in magnitude and duration by the highest rating fuselirik permissible in combination with the switching device.

EARTHING SELECTOR

HANDLE

CLOSING

CONTACTOR

CLOSE COIL PROTECTION

AUX 9

PRESS FOR SLOW

OPENING

ISOLATING MECHANISM

SELECTOR HANDLE

MECHANICAL TRIP BUTTON

AUXILIARY SWITCH 51

ANT PUMPING

RELAY AUX 8

AUXILIARY SWITCH 52

OPERATING MECHANISM

AIR CYLINDERS

Allowing for the fuse derating factor, starting current is assumed to become 1.8 x 1171 = 2108 A.

To carry, without operation, for a period of 2 x s = 40 s, a current of this value, fuselinks of UK manufacture (i.e., fuselinks to BS2692) require to have a continuous current rating of the order of 400-

4{) A .

The 'take-over' current (see definition under 'rated breaking current of switching devices') of fuselinks of he order of 400-450 A, i.e., the pre-arcing current

It follows that the lower the breaking current capability of the switching device, the lower the rating of

11e back-up fuse protection and hence the smaller the rating of motor which may be handled. Typically, in

d so itching device of 5 kA sym rated breaking current, th maximum permissible rating of the fuselinks would

be of the order of 250 A. This in turn would, on the basis of two successive starts each of 20 s duration,

li mit the size of motor to approximately 600 kW.

Pic. 5.45 Principle of co-ordination of fuselink rating with switching device current breaking capability and motor rating

5.2 Design and construction

5.2.1 General

It was remarked at the beginning of Section 4.2 of this chapter, that whereas the interrupter (the circuit switching device) in 11 kV switchgear is invariably a circuitbreaker, at 3.3 kV, depending upon the duty involved, it may be either a circuit-breaker or a fused switching device. It was further pointed out that, whether featuring a circuit-breaker or a fused switching device, the general form of construction of the switchgear and the operational facilities provided, are similar. Accordingly, the only features dealt with here are those particular to the design and construction of fused

switching equipment or those which, in the interest of clarity, are felt to merit further discussion.

In the early 1960s, there were explosions in the terminal boxes of 3.3 kV motors from flashover across the surface of the filling compound then common. As a consequence, in addition to improvement of the design in his area, the concept of limitation of fault current energy by the use of fuses was introduced. Initially the application of this philosophy took the form of series-connected high breaking capacity (HBC) fuselinks in boxes adjacent to the motors, followed by incorporation of the fuselinks in the switchgear.

It should, perhaps, be stated here that the backing of switching devices, e.g., circuit-breakers, contactors, by fuselinks was then, and remains presently, a well established practice — adopted primarily to deal with currents of a magnitude beyond the making and breaking capability of the switching device. Whilst, as will be explained, this is now the situation in CEGB practice, the switchgear into which the fuselinks were fitted initially had the full system fault level capability unaided by the fuselinks.

Since the mid 1960s, the switchgear used to control 3.3 kV motors up to around 1000 kW has incorporated HBC fuse short-circuit protection — advantage being taken of the fuse characteristic to reduce the fault current switching capability required of the switching device, and thereby produce a more compact and less expensive design of switchgear for the duty. However, a pre-requisite of the development was maintenance of the operational facilities of the existing switchgear with no reduction in reliability.

5.2.2 Duty of switching device and circuit earthing facilities

The duty of interrupting the higher values of overcurrent (which may be many times the current manifested during the starting of motors direct-on-line) having been taken over by the fuselinks, such capability on the part of the circuit switching device became superfluous. The 'scaling down' of the switching device thus possible, permitted appreciable reduction in the overall size of the switchgear together with significant saving of capital cost. However, the reduction in fault current capability of the switching device meant that it could not itself serve as the circuit earthing device. (Note: busbar earthing facilities are not required on switchgear controlling motors.) Accordingly, each switchgear equipment of this type is, in addition to the circuit s witching device, equipped with a circuit earthing switch capable of making and carrying, until the operation of protection elsewhere, any value of fault current which could appear accidentally on the circuit. The circuit earthing switch is padlockable in the closed position. Because in an earthing operation it is essential that the earth path must at all times be electrically continuous, the earth switch is arranged to by-pass the circuit HBC fuselinks.

5.2.3 Switching devices

Like circuit-breakers, fused switching devices of same type, current rating and circuit duty are requi, to be interchangeable, as also are those of the same and current rating but of different duty, subject any modification necessary to control, indication an interlocking circuitry.

Whilst, as for circuit-breakers, closing mechanisr of either the dependent power solenoid or stored er ergy spring types are acceptable, manufacturers to date have supplied only the former. Compared with circuit - breakers, the power requirements of the closing solenoids of fused switching devices are relatively modest

— of the order of a few kilowatts.

To meet CEGB operational and performance requirements, the design philosophy of fused switching device equipment follows more closely the principles of circuit-breaker switchgear than that of contactor controlgear. Indeed, interrupters of the air-break type are virtually circuit-breakers, but of limited short-circuit capability. Thus, closing mechanisms are of the 'latch closed' type and comply generally with the requirements specified for those of circuit-breakers.

5.2.4 Switching device operating mechanisms

Closing mechanisms of the dependent power solenoid type are specified to be 'trip-free'. Alternatively, if not trip-free in the generally accepted sense, they must simulate the action by being free to allow opening of the switching device immediately after closure, regardless of whether the closing control circuit remains energised. Stored energy mechanisms must comply generally with the requirements specified for circuit-breakers.

5.2.5 Main circuit fuselinks

Operation of any fuselink initiates opening of the switching device. This is essential in order to preserve the integrity of the `on/off indication and hence, as far as possible, indication of the state of the plant controlled.

The fuselinks for the short-circuit protection of the main circuit are of the HBC type compliant with BS2692. The rating of the fuselinks fitted is normally the highest which will provide satisfactory 'take-over' from the switching device.

Virtually all 3.3 kV motors are switched at full system voltage, i.e., direct-on-line started. Thus the switchgear must be capable of making and carrying, until the drive has run-up to operational speed, a current of several times that of the motor rated full-load.

Thus, the maximum rating of motor which may be controlled by fused switching device equipment is determined by the overcurrent carrying capability of the fuselinks of the highest rating permissible in the switchgear. The highest rating of transformer which may be so controlled, however, is dictated more by the rated continuous current carrying capacity of the fuselinks,