Chapter 11 Busbar and Modular Wiring Systems 11.1 BUSBAR SYSTEM The wide range of busbar system is available and can be used for single or three-phase distribution to many types of applications ranging from lighting to heavy-duty machines in factories. This consists of copper or aluminium busbars mounted on insulators and enclosed in standard lengths of steel trunking, which are arranged to be fitted together thus forming a continuous busbar along the entire length of the distribution route. It is sometimes more economical to use busbars in place of long runs of sub-main cables. At intervals, typically every 1m, a tap-off point is provided. At these points tap-off units may be fitted; these can comprise unfused lighting units or power units protected with HRC fuses or circuit breakers. The units are provided with contact fingers which are designed to fit onto the busbars. Connections from these tap-off units to individual accessories, motors or other electrical equipment can be made by flexible connections, PVC sheathed cables, or conduit. The advantages of this system are that the trunking and busbars can be erected before the installation of the machinery, and the latter can be connected up and set to work as soon as they are installed. By bringing the heavy main feeders near to the actual loads, the circuit wiring is reduced to a minimum and voltage drop is lower than would otherwise be the case. Subsequent additions and alterations to plant layout can be easily accomplished, and where busbar sections have to be removed they can be used again in other positions. If a large number of small machines are to be fed it is usual to fit a distribution board near the trunking system and to protect this with a tap-off box fitted with HRC fuses of suitable capacity. Circuit wiring from the distribution board is usually carried out in heavy gauge screwed conduit. The system is comparatively expensive in first cost and, therefore, is best employed where heavy loads and a large number of machines have to be provided for. However, whilst material costs may be higher, installation is 255
256 PART j II Practical Work quicker and overall there may be cost benefits from using the system. Once installed there is very little depreciation or need for maintenance, and it has a high recovery value should it be necessary to dismantle and install it elsewhere. Several well-known manufacturers supply this trunking in various sizes, together with all necessary tees, bends, tap-off boxes and other accessories. Earth continuity is usually provided by an external copper earth link which ensures good continuity. Lighting trunking is also available whereby the trunking is fitted with integral busbars and the lighting fittings are simply clipped in place. Where there are long runs of busbar trunking it is necessary to provide expansion joints to take up any variations in length due to changes in temperature. These expansion joints usually take the form of a short length of trunking enclosing flexible copper braided conductors instead of solid busbars. It is advisable to provide one of these in every 30m run of trunking. Busbars are available with additional auxiliary conductors and these may be used for emergency lighting or self-test systems. Busbars are available with seven conductors as standard and some have smaller auxiliary conductors. No other conductors of any kind may be installed inside trunking containing bare copper busbars. All lids and covers must always be kept in position as a protection against vermin and also to avoid accidental contact with live busbars. The trunking should be marked prominently on the outside at intervals with details of the voltage of the busbars and the word DANGER. The conductors shall be installed so that they are not accessible to unauthorised persons. FIGURE 11.1 This type of lighting trunking has integral busbars carrying conductors for the supply to the luminaires. The 54-W twin fittings are available with either wide angle or semispecular narrow beam characteristics, enabling effective lighting from different heights. The trunking can be suspended from the building frame and, with the narrow beam reflectors, is suitable for high-bay illumination. The clip-in fittings can be arranged to pick up from different phases and the whole assembly can be carried out without the use of tools, a particular advantage when working at heights (W.T. Parker Ltd).
Chapter j 11 Busbar and Modular Wiring Systems 257 FIGURE 11.2 The busbar lighting units installed in a high-bay storage building. The lighting units have been installed and the installation is ready for the lids to be clipped on (W.T. Parker Ltd). Insulators shall be spaced to prevent conductors coming in contact with each other, with earthed metal or other objects. The conductors shall be free to expand or contract during changes of temperature without detriment to themselves or other parts of the installation. In damp situations the supports and fixings shall be of non-rusting material. If conductors are to be installed where exposed to flammable or explosive dust, vapour or gas, or where explosive materials are handled or stored, additional screens, caps and accessories should be fitted. These are able to improve the IP rating of the equipment. Where the trunking passes through walls or floors no space shall be left round the conductors where fire might spread. Fire barriers should be provided at these points inside the trunking. All runs of overhead busbar trunking must be capable of isolation in case of emergency or maintenance by means of an isolating switch fixed in a readily accessible position (Electricity at Work Regulations 1989 Regulation 12). Busbar System for Rising Mains A similar busbar system is frequently used for vertical rising mains for multistorey buildings. This usually consists of copper or aluminium busbars of capacities of 100 2500A with two, three or four conductors. These are usually metalclad and are made in 4m sections, although all-insulated rising busbar systems are also obtainable. Tap-off boxes with fuselinks or fuseswitches can be provided for distribution to each floor where distribution boards can be fitted near the tap-off units. For these vertical runs it is very important that fire-resisting barriers be fitted inside the trunking at the level of each floor. These fire barriers can be purchased with the trunking, and the manufacturers will fit these in the required positions if provided with the necessary details. Where the trunking passes through floors,
a b (a) The pre-formed wiring units with plugs attached are conveniently stored and carried on this wheeled trolley preparatory to installation. All units are numbered to ensure correct installation. (b) Modular wiring installed ready for the distribution board to be secured and plugged in. c d (c) Distribut ion board in position with modular wiring looms fitted into the distribution cupboard, carried on a cable basket route. (d) Installing modular cable units on cable basket behind a suspended ceiling. e f (e) The junction box and cable basket enable a neat installation to be made and one where fault finding and alterations are easily accommodated at a later date. (f) Modular units installed in a wall partition. Sufficient spare cable is arranged to enable the fitting of switch and socket units and to carry out changes should these be needed at a later date (all M.W. Cripwell Ltd). FIGURE 11.3 Modular wiring.
Chapter j 11 Busbar and Modular Wiring Systems 259 whether in a specially formed riser cupboard, or run on the surface of a wall, it is necessary to ensure that the floor is made good by non-combustible material round the outside of the trunking to prevent the spread of fire. The supply for these runs of busbars is usually effected by a feeder box with provision for whatever type of cable is used. The manufacturers should be consulted as to the correct size of busbars to use, and IEE Regulations recommend that the maximum operating temperature should not exceed 90 C. Where rubber or PVC cables are connected to busbars operating at comparatively high temperatures the insulation and sheath must be removed for a distance of 150mm from the connection and replaced by suitable heat resisting insulation. 11.2 MODULAR WIRING SYSTEM Modular wiring is encountered in large sites where multiple installations of identical or similar design are required, or where speed of on-site installation is paramount in progressing the installation work. Modular wiring comprises factory assembled units with cables bunched together inside flexible conduit, and with end plugs pre-wired ready for assembly to matching sockets fitted to the equipment onsite. The units are made up in pre-arranged lengths with the cores of appropriate cross-section connected to the correct pins in the plugs. Home runs are used where the cross-sectional area of the cables is arranged to take the load of several circuits and these feed junction units from which modular final circuits are run. Whilst the initial cost of the equipment may well be greater than with traditional wiring, one main advantage of the system is that all the connections are made up in controlled conditions in a factory where the working environment is more stable than on a building site. A second advantage is that installation on-site is quick and easy. Once the cable containment is complete (typically using cable tray, basket or ladder), it is a matter only of selecting the correct pre-assembled modular cable, running it on the correct route and plugging in the end fittings to the on-site equipment plugs. It is, of course, important to select the correct loom for the individual application, these being identified by numbering during manufacture. Routing in rooms is usually inside the wall void and large diameter holes (100mm) allow the end plugs to be connected to individual switches, outlets or accessories. Any surplus length of the modular unit is fed back into the wall void once the switch or socket is fitted. This can then easily be recovered if any extensions or alterations are later required.