Wiring systems should be selected and installed to avoid damage to the cable jacket or insulation and terminations during installation, use and maintenance. Jake Green, Head of Technical Engagement at Scolmore Group, takes a look at suitable means of supporting cables for a variety of conditions in addition to the requirements of BS 7671.
All cables and connectors must be supported in such a way that the level of naturally existing mechanical stress will not cause the cable/conductor or its terminations to be at risk (522.8.5).
Furthermore, all cable supports and sheaths must not have sharp edges that could damage the wiring system, and cables/connectors must not be damaged by installation (522.8.11 and 522.8.12). Therefore, it is important to be careful when selecting the appropriate usage supports for cables/connectors.
Types of support
There are a wide range of options available to the contractor when selecting cable supports. These include, among others:
• Cable ties
• Cable ties
• Cable clips
• Cable tie accessories
• Cable glands
The type of installation depends on the nature of the surface on which the support is installed. For example, cable ties will be suitable for securing mounted cables to a basket tray or cable tray, while cable clips will be suitable for wooden surfaces.
Likewise, the type of support depends on the type of cable being installed. For example, cable cleats will be suitable for steel wire armored cables.
guidance
The Site Guide (OSG) guidelines published by IET are useful in specifying the nature of cable support and the recommended spacing between sections. Appendix D covers cabling in general, specific applications such as caravans and the like, overhead wiring as well as support and conduits.
Table D1 details the spacing of cable supports in accessible positions. Figure 1 gives an example of the recommended spacing between clamps for a cable no larger than 9 mm in diameter, based on Table D1. As the cable diameter increases, the distance between the clamps is allowed to grow.
Although support distances are important, the purpose of cable supports is to ensure that there is no undue stress on the cable or its ends.
Likewise, when cables are bent, it is important that there is not unnecessary stress on the internal conductors that occurs when the bend is too tight (522.8.3). Figure 2 shows an example of the recommended bending radius for 1.5 mm² insulated and “double and earth” sheathed (Table D5 OSG).
Cable Entry Two issues must be addressed when cables enter an accessory: the risk of damage to the sheath/insulation at the point of entry, and any potential stress on the connections.
When a cable enters a metal accessory box that has no protection from sharp edges, there is a risk that the insulation surrounding the connectors will be damaged, resulting in an arc and shock hazard. When cables enter a metal accessory box, it is important to provide protection, such as rubber grommets or the like.
Even when cables entering an accessory box or other metal enclosure are protected from damage, the connection ends may still be affected by fatigue as the cables are left unsupported.
In such conditions, it is necessary to provide adequate support and provide pressure glands to ensure the electrical and mechanical strength of the terminations (522.8.5 and note).
conclusion
All cables and connectors must be adequately supported to ensure that there is no undue stress on the cable or terminations (522.8), so care must be taken to select appropriate support (clamps, glands, etc.) for the cables. Unicrimp has a range of products that can assist a contractor in carrying out their duties.
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