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

FIG. 5.27 (conrd) Typical II kV switchboard or GEC manufacture

REF NO",-,r.NGI A11114.

FUSES AND i1/KIS 11 HIPPING

CIRCU1T',-;)

2UNDERVOLTAGE PICITECT.ON RELAY (TRIPPING!

3UNDERVOLTAGE PROTECTION RELAY (TIMING)

4UNDERVOLTAGE PROTECTION RELAY

5CUBICLE ILLUMINATION MASTER SWITCH

6I NDICATING LAMP BLUE

CUBICLE ILLUMINATION ON

7I NTERPOSING UNDERVOLTAGE RELAY

SUPPLY SUPERVISION RELAY

9THERMAL RELAY (MOTOR PROTECTION)

10TRIPPING RELAY

11PRE—OVERLOAD ALARM RELAY

12I NDICATING LAMP WHITE TRIP CIRCUIT HEALTHY

13TRIP SUPERVISION RELAY

14OVERCURRENT RELAY

15BUCHHOLZ ALARM RELAY

16AMMETER

17CIRCUIT BREAKER CONTROL SWITCH

18B.E.F. PROTECTION RELAY

19STANDBY EARTH FAULT RELAY 20 1 NTERTRIP CIRCUIT

SUPERVISION

21EARTH FAULT RELAY

22OVERCURRENT AND EARTH FAULT RELAY

23I NTERTRIPPING RELAY

24I NTERLOCK RELAY

25STANDBY INTERLOCK RELAY 26 VOLTAGE SELECTION RELAY

'LOCATED INSIDE CUBICLE

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Fir. 5.31 Interior of cubicle fitted to the type of itchboard illustrated in Fig 5.30; circuit-breaker ithdrawn, busbar and circuit shutters propped open

to) A key is free only when the circuit-breaker (or earthing switch) is closed in an earthing mode, i.e., to earth busbars or a circuit. Removal of the key locks the circuit-breaker (or earthing switch) in the dosed-to-earth condition. Each earthing condition is dealt with by a discrete device.

hi A key, when inserted and entrapped, permits closure of the circuit-breaker — it being free only when the circuit-breaker is open. However, attempted removal of the key when the circuitbreaker is closed must not initiate opening of the circuit-breaker.

3% key when inserted and entrapped, permits the circuit-breaker (or earthing switch) to be closed in tin earthing mode only — it being free only when the circuit-breaker (or earthing switch) is open. As in (b) above, attempted removal of the key when [he circuit-breaker (or earthing switch) is closed must not initiate opening of the circuit-breaker (or earthing switch). Here again, as in (a) above, separate devices are used for the busbar and circuit earthing conditions.

I di A key is free only when the circuit-breaker is disconnected from the service condition. Whilst

removal of the key locks the circuit-breaker against connection in the service mode, it must remain possible to operate the circuit-breaker in either an earthing or the disconnected mode, or to place and operate an earthing switch in an earthing mode.

4.2.6 Identification of conducting parts

All main conductors are arranged, and identified with their phase colours, in accordance with B5158, The identification is specified to be of not less than 650 mm 2 in area at each location. The order of the phase connections is Red-Yellow-Blue. Essentially, for conductors arranged substantially in one plane, the positioning when viewed from the front of the switchgear, is as follows:

•Run of conductors horizontal: Red phase top, left or farthest away.

•Run of conductors vertical: Red phase left or farthest away.

4.2.7 Earthing of structures

Each switchboard formation is equipped with a main earth bar — usually externally at the rear at the base

— to which `tee-off connections are made as necessary. The material may be copper, aluminium or an alloy of aluminium — specifications being B51432 for copper, and BS2898, material ElE or E91E, in the case of aluminium and its alloys. A clearance of not less than 15 mm is specified between the back face of the main bar and the adjacent surface of the switchgear enclosure to permit the attachment (on Site) of individual plant earthing cables.

Joints in earth bars are kept to the minimum practicable. Where necessary, they are secured by screw fasteners, as are the connections thereto. Since experience has shown a need to avoid interfaces of electrotinned/aluminium surfaces — a combination likely to arise from the extensive use of aluminium conductor cable for earth bonding — copper earth bars are specified to be of plain, i.e., uncoated, finish.

I mmediately before assembly of a joint or connection, the faces of aluminium/aluminium alloy conductors have the oxide fil m removed by, for example, steel wire brush, and the cleaned surface coated with petroleum jelly or other approved compound. Similarly, copper joint faces are cleaned and coated with petroleum jelly.

Provision is made at each end of each switchboard main earth bar for connection to the power station main earthing system. This must present a flat area of not less than 50 mm x 50 mm (nominal).

To deal with the fault levels likely to be experienced, earth bars of copper have a minimum sectional area and width — the latter being specified to ensure adequate area at joints, etc., — in accordance with the information tabulated below. Earth bars of

aluminium/aluminium alloy are specified to have a width not less than that for copper, and electrical and mechanical properties not inferior to that material.

Unless the positioning of the main earth bar itself is convenient for the purpose, tee-off connections are provided to facilitate the bonding to earth of cable glands.

The metal framework of all withdrawable parts is connected to the earthing conductor through contacts designed specifically for the purpose. Such contacts are required to establish the connection to earth not less than 25 mm before establishment of the main circuit connections.

Busbars and all conductors connected directly thereto are insulated. The insulation may be moulded or in the form of sleeving. It must be not less than 1 mm thick and capable of withstanding without damage the rated short-time current of the switchgear busbar system. To facilitate extension, busbars are of constant current rating throughout the switchboard. Although traditionally of copper, conductors of aluminium/ aluminium alloy are not precluded. However, where of the latter materials, the jointing procedure specified for earth bars would be adopted.

4.2.8 Circuit and busbar earthing

Each incoming/outgoing feeder circuit in a switchboard, and usually each single-circuit unit, is provided with means for earthing the circuit. Likewise, provision is made for earthing the busbars — usually at two locations on each electrically continuous, i.e., uninterruptable, section.

Traditionally, the earthing on 3.3 kV systems is through the circuit-breaker in association with selector devices forming an integral part of the switchgear. However, on those units where it is necessary to provide for earthing the busbars as well as the circuit, mechanical interlocking is provided to prevent the use of both facilities simultaneously.

At 11 kV, whilst some designs feature earthing through the circuit-breaker in association with selector devices forming an integral part of the switchgear, others use purpose designed earthing switches. Whichever arrangement is adopted it must preclude, at those locations featuring both busbar and circuit earthing, the earthing of busbars and circuit simultaneously.

Earthing switches have closing mechanisms of the dependent power solenoid or the stored energy spring.. operated type (see Section 4.2.18 of this chapter), and a rated short-circuit making current capability of n ot less than that of the circuit-breaker of the associated system switchgear. Closing mechanisms are provided with means for padlocking the switch in the closed position and, most importantly for safety reasons, it is not possible operationally to slow-close an earthing switch when connected for earthing operation. The basic specification for earthing switches is BS5253.

The initial connection of busbars or circuits to earth, i.e., the connection to earth immediately following the making dead of the busbars or circuit, is completed by power closing the circuit-breaker or earthing switch, When earthing through the circuit-breaker, mechanical interlocking between the circuit-breaker and the earth path selector devices ensures that it is possible to prepare the switchgear for an earthing duty only when the circuit-breaker is disconnected from both the busbars and the circuit. Similarly, where earthing is through an earthing switch, mechanical interlocking is employed to permit connection of the switch only

when it is open.

Where access to the busbar and circuit disconnection (isolating) contacts may be obtained in the course of preparation of the switchgear for an earthing operation, provision is made whereby the automatic features of the shutter covering the group of contacts to be earthed remain operative, but that the shutter associated with those contacts not involved in the earthing operation remains closed and padlockable until the switchgear is restored to the service condition.

Where, however, preparation of a switchgear equipment for an earthing operation neither involves nor permits access to the busbar and circuit disconnection contacts, the shutters may remain operable, as in the 'service' condition.

To preserve the integrity of the connection of a busbar or circuit to earth, i.e., to reduce the possibility of inadvertent disconnection, means other than locking are provided whereby, when a switchgear equipment is prepared for an earthing operation through its circuit-breaker, the electrical tripping of the circuitbreaker is rendered inoperative, both during closing and when closed. Thus the circuit-breaker becomes a fault (short-circuit) current making and carrying switch and, as such, transfers elsewhere clearance of any fault current consequent upon inadvertent earthing of live conductors. However, the arrangements adopted for rendering the electrical tripping inoperative must be such that it is not possible to return the equipment to the service condition and to reclose the circuit-breaker without restoration of the electrical tripping. To prevent inadvertent earthing, padlocking or coded-key interlocking facilities are provided for locking earthing devices against selection into the earthing condition.

Indication is provided to show when a switchgear equipment is prepared for 'CIRCUIT EARTH' or,

, appropriate, 'BUSBAR EARTH'. The indi-

diere

L'itions are by mechanical flag or, alternatively, by ▪ litablv positioned labels visible from the front of the equipment, inscribed CIRCUIT EARTH or BUSBAR \ RTH in black letters on a white background. The I . %c arranoements indicate merely that the equip- o prepare'd for an earthing operation, and that

;nenl

He earthed condition will exist only when the circuit-

hreaker or earthing switch is closed.

All conductors in the path to earth are required to

1, o.c a rated short-time current of not less than that

:f he associated system switchgear.

o In addition to the facilities for earthing busbars and circuits through the circuit-breaker or purpose deskilled earthing switches, portable devices are provided ' application to the busbar and circuit spouts of highor voltage switchgear featuring withdrawable circuitbreakers or fused switching devices. The devices may ▪ nre as 'primary earths', 'drain earths' and `metalclad

., itchgear movable earths'.

Used as primary earths, the devices substitute for an earth through the circuit-breaker, thus releasing the cuit-breaker for servicing, etc., whilst work proceeds in parallel on the circuit. The devices are applied to the

.N%ilehgear spouts in accordance with the procedure outlined in Section 1.3 of this chapter. For this duty, he devices must have a short-time current rating eapaHe of carrying, until the operation of protection, any

\aluc of fault current which could materialise at the point of application in the event of energisation of the

spouts to which they are connected. Being applied manually, the devices have no current making capability; hence the need to precede their application by an earthing operation through a circuit-breaker or an earthing s‘v itch having a fault current making capability.

The use of the devices as drain earths and metalclad itch gear movable earths is governed by the procedure described in Section 1.3 of this chapter. Because, when used in the latter capacity, they must be suitable for application one phase at a time, the devices comprise

single-phase units supplied in sets of three.

41.9 Auxiliary switches

Ausilian, switches, i.e., switches for use in control and iliary circuits, are, wherever possible, driven posiiiv eh, in both directions. Only where the requirement iS or a greater number of auxiliary switches than can he dri‘en positively are interposing (i.e., repeat) relays

permitted. However, every endeavour is made to al- lo,:aie the more important circuitry to switches driven

Positively. Where necessary to satisfy, for example, sequence and interlocking arrangements, auxiliary switches arc provided which are responsive to the following , itcligear conditions:

•Service (circuit-breaker connected for service).

•Disconnected (circuit-breaker disconnected from the service position).

•Circuit prepared for earthing.

•Busbars prepared for earthing.

All auxiliary switches are wired, in accordance with 'standard' diagrams, to terminals in the fixed (cubicle) portion of the switchgear. Typical circuit diagrams, together with an associated terminal schedule, are illustrated in Figs 5.32 to 5.35.

4.2.10 Cabling arrangements

All cabling external to the switchgear, together with glands and terminations, is normally supplied under a separate cabling contract, details of which will be found in Chapter 6 of this volume. All cable terminal chambers are positioned at the rear of the switchgear, the entry of cables normally being from below. With cabling connected, the insulation level of the terminal arrangement must be not lower than that of the rated insulation level of the switchgear.

The physical disposition of main circuit cabling relative to that for auxiliary circuits at the point of entry into the switchgear enclosures is arranged to minimise the risk of a fault or fire on the main cabling affecting the function of the auxiliary cabling. Similarly, where the design of the equipment necessitates the installation of lengths of both main and auxiliary 'external' cabling within the enclosure, the arrangement adopted must again minimise the possibility of a fault or fire on the former damaging the latter. Essentially, this means the segregation within the switchgear enclosure of main from auxiliary cabling by earthed metal or mechanically robust fire-resistant material. Cable armour alone does not satisfy this requirement.

Subject to minimum wire conductor sizes of 1.0 mm 2 and 2.5 mm 2 for individual circuits and bus-wiring respectively; ancillary electrical equipment, e.g., control and auxiliary wiring, control and selector switches, relays, etc., is specified in accordance with:

•ESI Standard 50-18 for circuits operating at 100 V and above.

•Engineering Recommendation SI7 for circuits operating at less than 100 V.

These documents specify performance arid constructional requirements, together with guidance in matters of application.

4.2.11 Voltage transformers

Voltage transformers are of the metal-enclosed dry type, complying with BS3941. They are normally installed on the 'fixed' portion, i.e., the cubicle of the switchgear, on the circuit side of the circuit-breaker

— and thus within the protected zone. The method of mounting permits disconnection (isolation) from the main circuit and ready removal whilst the switchgear