Exploring the benefits of prefabricated wiring

The profile of prefabricated wiring solutions for electrical services was highlighted at a recent seminar launching the new guide to electricity in buildings by the Chartered Institution of Building Services Engineers.When you consider the amount of work involved in even a very simple commercial electrical installation, the benefits of prefabricated wiring systems become very apparent. Howard Porter, an associate with Arup Research & Development, highlighted the problem at a seminar to launch a new guide from the Chartered Institution of Building Services ‘Electricity in buildings, CIBSE Guide K’. Traditional approach His simple installation (Fig. 1) involved a small power circuit with just three socket outlet backboxes and a lighting circuit with three lighting points and a backbox for the light switch. Such an installation is normally carried out using single-core cables installed in steel trunking and conduit. For this small project, cables from a distribution board would initially be run out in trunking. Conduit would branch off the trunking to the three lighting points and the light-switch backbox. That section alone would require four lengths of conduit. Four bends would be involved in running conduit between the socket-outlet backboxes. Installing the power aspect of the system with its three socket-outlet boxes would require a further four lengths of conduit. All eight lengths of conduit would have to be cut to length and threaded by the contractor. The next stage is to support the conduit and trunking on the building fabric or inside the ceiling void. And all that work is necessary before the pulling and terminating of wires can begin. Prefabricated solution An equivalent installation using prefabricated wiring dispenses with the conduit and trunking. For the power part of the system described above, a precut length of cable is run from the distribution board to a connection box, where it is simply plugged in using a prewired connector. Similar lengths of cable are run out to each of the socket backboxes. The lighting part of the system is similar, with the further benefit that luminaires can be supplied with plug-in external connections to obviate the need for wired connections. Howard Porter says, ‘Prefabricated wiring systems are becoming increasingly popular in modern commercial installations because of the speed of installation and ease of modification when users’ requirements change.’ Their principal areas of application are for small power and lighting. Their use on high-power circuits is still not common. A deterrent to the use of prefabricated wiring systems is that they are not covered by any British Standard or included in the Wiring Regulations. ‘This in itself,’ says Howard Porter, ‘does not mean that the systems are not safe or properly constructed. Some manufacturers indicate where certain components of their systems do meet recognised standards; this is often limited to the connectors.’ There is an argument that the bulk of a prefabricated system is not covered by the Wiring Regulations, since it is plugged in and thus not fixed wiring. The basic components of a traditional system are cheap and readily available, made to recognised standards and interchangeable between manufacturers. In sharp contrast, there is practically no compatibility between prefabricated products from different manufacturers. Once a particular manufacturer’s system has been chosen for an installation, it is not possible to change. If the supplier should run into production difficulties or, even, cease trading, the consequences for the project could be serious.
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Similarly, despite the much vaunted ability of prefabricated systems to adapt to future changes in layout, churn, if extra parts are not available that benefit is jeopardised. General principles Although the components of prefabricated systems vary between manufacturers, the general principles are the same. A central connection box is supplied via plug-in cable from the distribution board. Further plug-in cables connect from this box to the final socket outlets, luminaires and light switches. Some manufacturers can incorporate switching modules for luminaires in the distribution boxes. The cables are available with connections on both ends or on just one, with the other end having wire tails. There are systems for single- and 3-phase circuits and with a range of different conductors to supply normal and essential power circuits. Interconnecting cables can be armoured or unarmoured multicore cables. Many manufacturers use metal-clad cable, which is effectively a flexible steel conduit with single-core cables installed during manufacture. Some cables have BASEC approval, although there is no British Standard for finished products. All components are inspected and tested at the factory, and require testing on site according to the Wiring Regulations. Four tests are usually required. • Continuity of conductor protective circuits (earth). • Insulation resistance. • Polarity. • Phase-earth-loop impedance. To fully exploit the benefits of prefabricated systems, an installation has to be designed in detail to allow for any non-standard components or cable lengths. Once the design has been fixed, Howard Porter warns that it may be impossible to modify the installation to accommodate design changes during construction. Larger market share Despite the problems associated with prefabricated wiring systems, Howard Porter expects them to take a larger share of the market. Their main advantage is that components are made in a factory under controlled conditions and simply plugged together on site, making them well suited to an installation with many identical areas or rooms, such as hotel bedrooms or a block of flats.



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