Cables installed outside, in the gardens and yards of domestic premises, are exposed to an increased risk of damage. Therefore, careful consideration should be given to the selection and erection of cables routed externally, including those supplying accessories, equipment or outbuildings such as garages, sheds or greenhouses.
External influences
To ensure an electrical installation is suitable for the environmental conditions, the designer should make an assessment of the influences that may affect the safe operation of the planned installation, and select cables and associated equipment that are adequately constructed, or otherwise protected (Regulations 512.2.1 and 512.2.2 refer).
Section 522 (Selection and erection of wiring systems in relation to external influences) of BS 7671 contains a number of external influences that the designer should consider when selecting wiring systems and associated equipment. Regulation 522.6.1 requires wiring systems to be selected and erected to minimise damage that may arise from mechanical stress caused by impact during installation, use or maintenance.
Protection against impact may be provided by:
- selecting a wiring system that provides appropriate mechanical protection, such as using a steel wired armoured cable or enclosing cables in conduit, and
- routing cables so that the risk of impact is minimised.
Generally, where pvc insulated and sheathed (Twin & Earth) cable is routed externally, it should, where accessible, be protected against impact.
Wiring systems
Surface-mounted cables should not be installed where there is a risk of contact with corrosive or polluting substances (including water) or where they will be frequently splashed, immersed or submersed in water, unless cables are constructed for such use. Wiring systems that are surface-mounted should be of suitable construction for their installed position and securely fixed to rigid structures, such as walls constructed from bricks or blocks, to minimise exposure to the aforementioned detrimental effects. Fixing to non-rigid items, such as timber fences should be avoided.
It should be recognised that cables/ conduit fixed to some timber fences may by exposed to vibration and mechanical stress caused by wind loading to the fence; meaning cables could be pulled from their fixings/enclosure (see Fig 1).
Furthermore, most timber fences at some stage after erection will require treating with preservative which may have a detrimental effect on fixed cables. Consideration should be given to the foliage of certain plants, such as ivy, which may encircle cables leading to additional strain being placed on fixings.
Installing cables on rigid structures (walls) that may be exposed to excessive flora growth is not precluded; however, contractors should highlight the problems associated with flora to the client and choose appropriate cables and fixings to limit the effects of such growth (Regulation 132.5.1 refers).
Cables routed underground
Cables installed underground should be suitably identified and be buried at a suffcient depth in relation to the finished ground level. For example, where a cable is routed through ground that will be finished with concrete the risk of impact is lower than where a cable is routed through ground that will be subject to digging, such as a flowerbed (Regulation 522.8.10 refers).
Cables routed overhead
Cables installed overhead should be adequately supported (Regulation Groups 522.7 and 522.8 refer). This is of particular concern where the distance between two points is such that undue strain is placed on the cable. To overcome this issue, contractors could use catenary wire with insulated and sheathed cables secured to the wire.
Where it is necessary to install cables overhead they should be at a suitable height throughout their length. Guidance provided in HSE GS6 and ENA Technical specification 43-8 details required and recommended minimum heights of spans above ground.
Fixed equipment and accessories
As with wiring systems, fixed equipment and accessories (equipment for short) for a planned installation should be selected and erected taking due account of the external influences identified during the assessment. A classification of external influences is contained in Appendix 5 of BS 7671.
In general, enclosures should have appropriate (IP) ratings for their location and where practicable, equipment should be positioned so as to minimise the risk of impact. For example, in a location having moving vehicles, accessories should be mounted at a suitable height to avoid impact from vehicles whilst still providing accessibility. Equipment that has to be installed where a risk of impact exists should be of such construction to provide sufficient mechanical strength to resist damage should impact occur or be given additional protection, such as a metallic enclosure or metallic guard.
Special installations or locations
Whilst an outdoor area, in its own right, is not a special location, there may be instances where a designer may need to apply relevant sections of Part 7 of BS 7671. For example, if the installation includes outdoor lighting, a swimming pool or hot tub the general requirements of BS 7671 should be supplemented or modified by the particular requirements contained in the appropriate Section of Part 7.
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