[ Spotlight: Cable safety ]
stripping of the cable should be attempted. If it is difficult to strip without damaging the insulation material, affected cable should be returned to the supplier.
Most common problems Conductor resistance When tested, some electric cable can indicate excessive conductor resistance. Conductors have been found to be up to two standard sizes smaller than that marked on the cables, which could result in overloading and overheating in use. End users are encouraged to check purchase documentation and design specifications, and test their installation to ensure that the applied loading and earthing are acceptable. Findings should then be recorded in documentation.
Failing smoke for fire test To describe a cable as ‘LSHF’ (Low Smoke Halogen Free), it must pass two tests: (a) a smoke cube test, which sets a maximum accumulated smoke density produced when burning the cable with a standard fire; and (b) a corrosive and acid gas test, where the cable materials are roasted and must give off a maximum of 0.5 per cent acid gas. These tests are both specified in cable standards that include the description ‘having low emission of smoke and corrosive gases when affected by fire’. Cables made from PVC or similar halogenated materials are unlikely to pass these tests. In terms of market practice, more people are specifying and using LSHF cables. However, this does not mean that these cables are fire-resistant. For example, for fire-fighting lifts and smoke extraction systems, robust fire-resistant cables are needed to continue to work in a fire. For this you need special fire-resistant cables, such as BS 7846 armoured cable or mineral cable to BS EN (IEC) 60702-1.
Strippability
Sometimes due to a manufacturing fault, cables are found to be unstrippable. Installers should check their cable is strippable before first fix installation. If cable has been laid and plastering or decoration has been
completed, when the installer comes to fit accessories at second fix and the cable cannot be stripped easily, there can be serious difficulties which will cost the installer both time and money. Strippability can be readily checked by feeling the free end of each reel of cable with a fingernail to check if the sheath and bedding can be lifted away from the cores. If it will not come away from the cores easily, then normal
Cable damaged during installation Armoured cables are intended for use in tough environments, such as burial, but they can be damaged during installation. The highest risk is during the process of pulling cable into buildings, particularly over complex routes. The external cable sheath can be scored by a sharp edge, such as a cable tray end or even a screw or bolt, and leave a longitudinal groove that can result in later splitting of the sheath. Installers should check for sharp edges before pulling cable. Pulling in cable from a central point, in two directions, can sometimes result in twists or kinks in the cable, and this can result in bulging or displaced armour wires. Installers should take care not to introduce twists or kinks into cable as it comes from the drum, particularly as it passes around rollers. Installers should take notice of the minimum bend radius of cable recommended by the manufacturer, both in the installation process and in cable tray design and layout. Finally, the un-terminated ends of armoured cables should be protected by end caps until terminated. If BASEC discovers cable with serious faults, a public
warning will be issued. The warnings will be printed in trade publications, electrical web portals, on the BASEC website and through trade bodies and inspectorates. BASEC will normally identify the cable through a brand name that is printed on the cable sheath, or provide a description of the markings on the cable. The faults will also be explained in detail and the potential consequences, such as overheating in use or electric shock.
Know your cable markings To help safeguard against the risk of installing cable that is substandard, contractors should ensure that the cable supplied by the distributor is the correctly specified cable, and check the markings on insulation or the cable sheath – not just the packaging.
Look for: n Independent third party approval – such as BASEC; n Name of cable manufacturer – its identification mark; n Standard number – the cable should be made to BS or IEC;
n Purchase records – keep them!; n Environment – ensure the cable is compliant for its intended use;
Any supplier can put a BS number on a cable – it does not mean it is has been independently tested
n CE marking on packaging; and n Traceability information – to track cable through supply chain.
Make sure checks are made on delivery to ensure it complies before installation. If a company installs unsafe cable, not only do they risk costs that could put them out of business, but also they will have contravened health and safety regulations. If you see only an old CMA code (for example, 6491X)
printed on a cable, then it should be treated with caution, as it is probable that nobody independent of the manufacturer
March 2012 ECA Today 33
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