How do you resolve a problem with an electrical cable install? Ask an expert. Dr Jeremy Hodge discusses electrical contractors’ top five problems and offers advice on how to prevent problems:
1) Cable damaged during installation - Armoured cables can be damaged during installation. The highest risk is during the pulling process, particularly over complex routes. The external cable sheath can be scored by sharp edges such as a cable tray end or even a screw or bolt, making a groove in the sheath that may split later. Installers should check for sharp edges before pulling cable, and take care not to twist or kink cable as it comes from the drum, particularly as it passes around rollers. Installers should take notice of the minimum bend radius recommended by the manufacturer, both in installation and in cable tray design. Un-terminated ends of cables should be protected by end caps until they are terminated.
2) Conductor resistance – Independent tests have shown that cables can have excessive conductor resistance – some cables have been up to two standard sizes smaller than marked, which could cause overloading and overheating. Installers should check purchase documentation, design specifications and test their installation for acceptable applied loading and earthing (test results should be documented). Look for the BASEC mark on the cable sheath and the drum/reel, which proves it has been independently tested.
3) Failing smoke for fire test – To describe a cable as ‘LSHF’ (Low Smoke Halogen Free), it must pass two tests – a smoke cube test, which sets a maximum accumulated smoke density produced when burning the cable with a standard fire, and a corrosive/acid gas test where the cable materials are roasted and must give off less than 0.5% acid gas. These tests are specified in cable standards, which include the description ‘having low emission of smoke and corrosive gases when affected by fire’ – cables made from PVC or similar are unlikely to pass these tests.
The electrical market is moving towards 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 (BS 7846 armoured cable or mineral cable BS EN (IEC) 60702-1) are needed.
4) ‘Strippability’ – Manufacturing faults can render a cable unstrippable. Installers should check this before first fix installation. If cable has been laid and the installer cannot strip the cable at second fix this costs time and money to fix. Check by feeling the free end of the cable with a fingernail to see if the sheath and bedding can be lifted away from the cores – if so, return the cable to your supplier.
5) Markings – Cable problems often are not found until after installation – too late to avoid the enormous cost of rectifying the situation. Contractors should ensure that the cable is correctly specified and check markings on the cable sheath – not just the packaging. Look for a recognisable manufacturer’s identification, so if there is a problem the supplier can be traced – never accept cable without an origin marking. Look for third-party approved cable from BASEC, HAR or equivalent, and specify the British or European standard number required to suppliers.
