Top Dos and Don’ts - Cable Delivery, Pulling and Installation
Although present everywhere, cables seldom impact their users; they are usually hidden from view, and, if they are fulfilling their role correctly, require no attention. They should perform reliably for many decades if they have been correctly specified, conform to relevant standards and specifications, carry BASEC or another recognised approval, and have been carefully maintained. Protection from environmental and mechanical stress is also essential.
However, even cables that are sourced responsibly are at risk during initial delivery and installation. During this phase, they will be subjected to movements, stresses and strains - and if these exceed the cable design limits, damage becomes a strong possibility. Even simple issues like snagging on sharp edges can have serious consequences.
Installation damage can lead to customer dissatisfaction, significant delays in project completion, extra resources being needed, and in particular, additional costs imposed on installers for removing and replacing damaged cables. These problems can be exacerbated if the effects of any damage do not become apparent, potentially affecting the supply of power, control or communications, until after the installation has gone into service.
Below are some top tips for a clean, trouble-free installation:
Cable delivery and cutting to length: Safe handling of cable starts with the supplier, often a distributor or wholesaler. Manufacturers will deliver cables on an appropriately sized drum or reel, loaded under controlled factory conditions. Cutting the cable to length, and redrumming onto an often-smaller delivery drum can result in over-bending if the drum belly diameter is too small; additionally, the cable may suffer external scuffing damage.
End caps: If suitable end caps are not fitted onto the cut length, water may penetrate the cable during storage or on site.
Bend radius – armoured cables: Bend radius is a key consideration, particularly for armoured cables and for higher voltages of 11kV or more. Minimum bend radius is generally defined by cable standards, or is advisory; it should be communicated by manufacturers and passed on by suppliers to the installers. Sometimes two figures are used; a dynamic radius for the pull, and a possibly smaller final static radius for when the cable is no longer under tension.
Maximum cable tension and sidewall pressure limits can both cause damage if exceeded when pulling cables. To prevent this, cable installation routing should be closely assessed, and several rollers or guides used at any bend points. A single roller or guide’s external radius is always likely to be much lower than the cable’s minimum bend radius. As a simple rule of thumb, for a four-core armoured cable larger than 40mm OD, a full-sized bicycle wheel should fit within the bend.
Bend radius – screened cables: With screened cables such as BS 7629-1- rated fire alarm types, the minimum bend radius must be maintained to ensure fire integrity. Kinks in the screening or over-tight entry into accessories may compromise fire performance. All such cables will have been fire tested at a specified bend radius, which the manufacturer should communicate to their suppliers and users.
Bend radius monitoring: Monitoring the bend radius on site is not straightforward. Many installers use simple go / no-go templates, but these can sometimes miss the tight parts of bends. Purpose-designed radius gauges can measure extremely accurately, providing reassurance that a cable installation has been completed correctly and safely.
Cable pulling: Problems can occur where cables are pulled in two directions from a common mid-point. Some cable is initially pulled in one direction, then the remaining cable is laid out from the drum before being pulled in another direction. This approach can cause significant damage to a cable that cannot twist freely during installation, or where loops of cable introduce twists. Damage to armour can result in bird-caging and sheath splits.
Avoiding cable torsion when pulling is important, and is largely addressed by using rotating pulling heads and a suitable cable pulling sock. Pulling the cable through a rope will restrict rotation and can create problems. Tension built up around each bend will propagate back down the cable, causing issues nearer the drum.
Competent and experienced cable installation teams, using the correct equipment including tension-monitored winches, pulling socks, and a generous quantity of rollers and guides, should experience fewer problems while ensuring a trouble-free installation.