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

FIG. 5.47 3.3 kV fused units of Whipp and Bourne manufacture, generally as depicted in Fig 5.46 but

Shown withdrawn and tilted for inspection/maintenance. The left hand unit features a vacuum switching device and that on the right an air-break interrupter.

Mechanical endurance — 1 million no-load operating cycles.

• Substantially non-inductive loads switched on for long periods

Rated duty — uninterrupted. Utilisation category — AC1 Mechanical endurance — 0.3 million no-load operating cycles.

Additionally, contactors shall be capable of making tt nd carrying the specified prospective short-circuit current of the system, as limited in magnitude and duration by the associated circuit short-circuit protective device, i.e., fuselinks.

The co-ordination between the contactor and the circuit short-circuit protective device, in accordance

oith 13S4941: Part 1: Appendix C, is basically as follows:

•Type b co-ordination for equipment utilising overload devices to BS4941: Part 1.

•Type c co-ordination for equipment utilising overload relays to BS142.

Type b co-ordination accepts that the characteristics of any overload relay may be altered per-

manently. Type c co-ordination requires that no damage (including permanent alteration of the characteristics of any overload relay) shall occur.

However, light burning of contacts, and risk of their welding — the latter provided that no flashover occurs — is accepted.

6.2 Design and construction

6.2.1 General

This section describes the design and construction of the switchgear, controlgear and fusegear found on the 415 V and lower voltage systems. The requirements outlined are those for equipment concerned directly with the control of operational plant. They are not necessarily applied in non-operational plant areas, e.g., offices, welfare blocks, stores, etc. Henceforth in the text, the switchgear, controlgear and fusegear will be

Duty Class I For use in systems where the power factor of short-circuit current is extremely low, e.g., where the switchgear is connected in relatively close proximity to the source of generation. Duty Class 1 is the normal requirement.

Duty Class 2 For use in 'distribution' systems in which the power factor of short-circuit is not likely to be lower than 0.3.

Mostly the switchgear is arranged in multi-circuit switchboard formations accommodated in purpose built switchrooms. However, it is expedient in certain instances to locate items, such as small motor starters and distribution fuseboards, local to the plant involved.

Except in the case of distribution fuseboards, although there is now a tendency towards grouping starters into small multi-motor control centre formations, the requirement 'local to plant' is largely for single-circuit units — floor standing or wall mounted.

Main switchboards, i.e., unit and station auxiliaries switchboards, usually comprise a combination of circuit-breakers, fused motor starters and fused distribution gear. The incoming supply circuits of such switchboards are almost always circuit-breakers. In many cases the supply to the switchboard is derived from a 3.3 kV/415 V transformer housed in the switchboard — the high voltage feed being by cable connected directly to the transformer HV terminals. The switchgear includes all protective and interposing relays, main and any necessary interposing current trans- . formers, transducers, instruments, control, selector and test switches.

The general assembly of the switchgear complies with BS5486: Low voltage switchgear and controlgear assemblies: Part 1, Specification for type-tested and partially type-tested assemblies (general requirements): ■thich is identical with IEC Publication 439-1: 1985.

However, for power station service it is necessary to augment and interpret the British Standard as outlined below. Representative switchboard formations are shown in Figs 5.49 to 5.51. Typical examples of the individual cubicle arrangements, air circuit-breakers

and starter equipment that comprise the switchboard formations are illustrated in Figs 5.52 to 5.54.

Basically, switchgear arranged in switchboards is of Form 4 to BS5486. The 'Form' of construction determines the degree of separation by barriers and partitions (within the switchgear structure) of the major components. 'Form 4' specifies the separation of bus- bars from the functional units, and separation of all functional units, including their outgoing terminals, one from the other. Essentially, the whole of the busbar system up to the point of connection to the short-circuit protective devices of each circuit must be capable of withstanding a short-circuit at any point other than in the immediate vicinity of the short-circuit. There

of course, be damage from arcing at the point of short-circuit. Also, the whole switchboard assembly must withstand any external fault. In the event of an internal arcing fault on any functional unit, the damage

must, as far as possible, be confined to that unit, so that the busbars and all other functional units remain fit for service. In addition to the basic requirements of BS5486, specialised equipment and components, e.g., circuit-breakers, contactors, fuse-switches, etc., must comply with the appropriate British Standard.

Withdrawable and removable parts (see Definitions in BS5486) of the same manufacture, type, rating and duty must be interchangeable. Withdrawable and removable parts of the same manufacture, type and rating, but of different circuit duty must be interchangeable, subject to any necessary modification to auxiliary circuits, protective devices, etc.

Handling equipment, e.g., lifting devices, is provided for withdrawable and removable parts exceeding 25 kg gross weight.

The use of hygroscopic or flammable materials is avoided as far as practicable. Terminals in which a

fusegear and controlgear switchgear,voltage Low

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screw compresses a bare wire conductor directly are not permitted. Whilst, in general, no diversity factor is allowed for Duty Class 1 equipment, the provisions of BS5486: Part 1 in this respect are accepted for Duty Class 2.

As for switchgear at the other voltages, the temperature rise of terminals for external cabling must not, when carrying rated operational current continuously, exceed 50 ° C, or the limits specified in the appropriate

apparatus specification, whichever is the lower. All main conductors and incoming/outgoing terminals must be identified with their phase/polarity colours, presently in accordance with BS158. The colours at the various points of identification must be of durable material and a minimum of 300 mm 2 in area. Because in the course of time the adhesive may fail; thus allowing 'unwinding' and introducing the risk of fault, identification by taping is not accepted.

6.2.3 Cabling arrangements

Wherever practicable, external cabling is arranged to enter equipment from below. To allow some freedom of choice as to the type of cabling used in a particular circumstance, main circuit terminals must be suitable for the reception of conductors of either copper or aluminium, and arranged to minimise the bending of t he cores.

The main circuit terminals of each functional unit in a switchboard are enclosed separately in an air insulated compartment or barriered section/subsection of earthed metal or approved insulation material. Such a compartment may also accommodate current transformers for instrument and protection purposes.

Terminals for the connection of external control and auxiliary circuits are grouped and positioned relative to the main circuit with which they are associated. Whilst not necessarily so in the past, it is in most cases now policy to enclose each group separately, and thus reduce the risk of work or a fault on one group jeopardising another.

Ali terminal arrangements must, with cables connected, ensure achievement of the clearances and creepage distances shown in Tables 5.1 and 5.2.

TABLE 5.1

Indoor equipment

The physical disposition of main circuit cabling relative to auxiliary cabling at the point of entry into the equipment enclosure is arranged to minimise the risk of fault or fire on the former affecting the proper function of the latter. For this reason, where the design of equipment necessitates the installation of lengths of both main and auxiliary incoming/outgoing cabling within the enclosure, the former is segregated from the latter by earthed metal or other mechanically robust

fire-resistant material. Cable armour alone is not cepted as satisfying this requirement.

Terminal boxes and gland plates for single-core L. bles rated in excess of 400 A are required to be designe, specifically to minimise the production of inducet., currents.

Gland plates are supplied on the basis of one plat per cable gland, and must be detachable to facilitate drilling to suit by the cable contractor.

Insulation must be so mounted and the method of attachment of connections thereto such as to minimise the likelihood of mechanical overstressing during normal tightening of the mounting and connection fixings. Particular care is taken to ensure that expansion and contraction of components shall have no damaging effect, having especial regard to the temperatures likely to be attained under fault conditions. Additionally, the configuration of the surfaces of insulation must be designed to minimise the accumulation of airborne pollutants.

6.2.4 Electrical clearances and creepage distances

As a general rule, the electrical clearances and creepage distances shown in Tables 5.1 and 5.2 are observed as minima. However, some relaxation is permitted specifically in the following areas:

•Contactors and associated overload protective devices may have clearances and creepage distances not less than those specified in BS5424: Part 1: Appendix B.

•Circuit disconnectors (isolating devices), fuse-switches, switched and other disconnectors may have clearances not less than 12.5 mm.

The relaxed dimensions may be maintained on connections to the terminals of these components for a distance not exceeding 40 mm.

The clearances shown must be maintained irrespective of any insulation applied, unless such insulation is capable of withstanding the full power frequency high voltage test.

6.2.5 Busbar systems

The busbar circuit of a switchboard comprises the complete conductor system up to its points of connection to the circuit disconnecting/protective devices of the functional units. Typical busbar circuits are illustrated in Fig 5.55.

Main and tee-off busbars are contained in separate compartments within the switchboard. They are usually air-insulated, but may feature solid insulation at the option of the manufacturer. Where insulation is other than by air, it must be in the form of sleeving with joints encased in moulded or similar covering. Because of the possibility of deterioration with age,