WIRING
circuits for point of use etc. We also see cable systems inside trunking to reduce the need for unnecessary containment, where the circuits wired in a cable system are limited. It’s usual to see the odd SWA, MICC or other such cables in trunking alongside 6491X singles, especially where modifications and alterations to an installation have been made. Things can get trickier with conduits, as the friction from pulling a sheathed cable system through them can damage them and any other 6491X singles in the same conduit. In both cases, the designer needs to take into account any thermal constraints, as a wiring system may have its current capacity impaired by the enclosed effects of being encased in a conduit or trunking. Likewise, the thermal constraints of the 6491X conductors may be affected where a sheathed wiring system is introduced to the same conduit or trunking. We can’t mix and match 6491X or similar singles by placing them on either a cable tray or ladder, as they would not have mechanical protection. So, from a mix-and-match perspective, wiring systems can use trunking and conduit as a containment method if thermal constraints allow, but singles must be in either conduit or trunking. Is twin and earth only for domestic? Often seen as a domestic-only wiring system, PVC/PVC flat twin and earth, is actually far from it. Twin and earth and similar variations are wiring systems in their own right, and can be used in commercial and industrial installations, where the designer sees fit. The only problem you will find is that they are less flexible, given their solid conductors, below 4mm2. Other than that, they can be used anywhere a similarly specified cable wiring system can be used, providing they meet the requirements for low smoke and fumes.
Fig 4
Examples of plastic (left) and metal (right) conduit protection
Fig 5
Fig 6
An example of a metal trunking installation
An example of a plastic trunking installation
in its use for connecting solid metallic conduit between expansion joints within buildings and similar. One thing to remember is that flexible metallic conduits are unsuitable for use as a cpc; therefore, any links between fixed metallic conduits would require earthing links to ensure the earth continuity of the containment system. What about the cpc? With metallic conduit and trunking, the structure itself takes on the role of the cpc, so only the line and neutral of the circuit are carried internally, with a connection made at the accessory to either the conduit or the trunking. Plastic conduit and trunking are obviously non-conductive, so the cpc for each circuit must be taken inside with the line and neutral conductors to each accessory served. Which one do I use? A designer can use whichever they choose to, but will generally be driven by the following: • Client needs or requirements • Usage
• External influences • Durability • Accessibility for maintenance, repairs or alterations Whichever method is used, the installer must possess the correct skills needed. Complex wiring systems need high levels of competence and experience to ensure compliance with BS 7671. Both plastic conduit and trunking arguably require less complex equipment and training to construct. However, they are not easy to install correctly. Metal conduits generally require specialist equipment to install, and metallic conduits will likely involve powered cutters or tooling. When dealing with containment, the same applies, although generally speaking, power tools will need to be used to install most cable tray and ladder products. Can you mix and match? Mixed systems are often seen and indeed necessary, for example, when going from a trunking system to a conduit system to disseminate final
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