New technologies & applications
Investment leads to sound joints1 May 2007
Successfully laying underground cabling calls for efficient jointing and termination methods. Adherence to good installation practices can eliminate or reduce many of the problems that inevitably arise.
Underground cabling systems, with their hundreds of joints and terminations, are a vital part of any power distribution network. Yet all too often, the quality of their installation is compromised by short cuts, lack of knowledge and lack of time. This can lead to faults, ultimately causing power outages that can take considerable time to address. It should also be borne in mind that even a single failed joint can cost thousands of pounds to install or repair.
Of course, the utilities, contractors and installers responsible for power cabling face many challenges. In the UK for example, one may find beneath the ground a tangle of many different types of cable, some of which date back to the 19th century, and have long since been superseded. Imagine an installer opening up a joint bay, only to find an old cable type that he does not recognise, but which he still somehow has to join to a 21st century cable. However desirable it may be to do so, it is frequently impossible to replace old cable, owing to its being inaccessibly located, for example, beneath a busy road.
Nor does time stand still. There are many networks where paper cabling is gradually being phased-out, but will need to co-exist with modern polymeric cabling for years to come. Changes to cable manufacturers' specifications – notably the shrinking of primary insulation diameters – impose added pressure, requiring electrical engineers at the sharp end to be aware of all these developments, which will vary from supplier to supplier.
Additionally, contractors and their installers have to work in increasingly difficult environments. Legislation, such as the UK’s Traffic Management Act, also means that installers must spend less time on 'streetworks' in order to avoid large fines. Furthermore, utilities and their contractors often have to share service trenches with a number of other utilities, so there is a high risk of inadvertent damage to other cables or pipes. All this must be faced in an industry where demand for power is on the increase, a fact likely to be exacerbated in London by the 2012 Olympics, which will demand a considerable amount of extra power, not only in the Olympic village, but across all the other Olympic sites and associated buildings, such as new hotels, rail networks and roads.
Applying best practice
While some of these issues are inescapable facts of life, there is much that can be done to ensure that the installation of cables, joints and terminations runs smoothly, so minimising the chances of failed joints in the future. There is a range of 'best practices' that can be adopted, and while the value of some of them may be obvious, their implementation is far from universal.
Training is a key example of something that should not be taken for granted, although it often is. Investing in training can mark the difference between producing a high quality cable network and one that is prone to joint failures. For instance, a jointer may be trained in using a particular joint, such as a single core polymeric type, yet when confronted with an unfamiliar three-core joint, nonetheless attempts to repair it. As a result, the joint fails, incurring additional time and cost to remedy. Similarly, the jointer may be trained in using a particular manufacturer's accessories, but not those of another. When using a new brand for the first time he assumes wrongly that it has a similar specification to the familiar brand, and so the work later needs to be repeated.
Accessing comprehensive training
While utilities and larger contractors often have formal training programmes, smaller contractors, installers and owners of private networks are unlikely to have these resources. Instead, they can attend third party courses, particularly those run by manufacturers of cable accessories, such as 3M. Unfortunately, take-up of these courses is by no means standard in the industry.
Although problems are less frequent among the staff of power utilities and larger contractors, when workloads are high smaller organisations may be subcontracted, and this may lead to problems. On industrial sites, resident electricians may try to carry out work on low or medium voltage network joints. Given that these are infrequently installed, electricians are often reliant on guesswork. It may be argued in mitigation that training is difficult to justify when it is not chargeable time, but conversely, training can help a smaller contractor or installer to be more competitive, by establishing a reputation for high quality work.
It is also important to invest in refresher courses, to carry out re-assessment of skills at regular intervals: and while working in the field, to read installation instructions thoroughly and to check measurements at every stage. This is because there is an inevitable delay between training and its implementation. For example, a jointer may be trained on a particular kind of joint, but does not come across that particular arrangement for a couple of years, by which time there may well have been changes or revisions to the joint or installation. This will have been addressed in the
instructions accompanying the joint kit, but the installer may not have noted these details.
Such instances can have significant consequences. Take for instance 3M 11kV transition joints, instructions for which have been amended because a build-up kit is now required. This is due to cable manufacturers’ having reduced the thickness of the insulation layer used in the polymeric cable, so although the cable core may still have a 95 mm2 cross sectional area, the cable itself is in fact several mm smaller in diameter than equivalent cables manufactured five to ten years ago. Unless instructions are followed properly, problems may arise when installing the joint.
It also makes sense to explain more fully why a particular installation procedure needs to be conducted in a certain way. One should not be surprised if installers without this background information are tempted to cut corners.
Using the correct tools
It is necessary to stress that installation jobs should only be given to installers who have the proper set of tools, since otherwise it is impossible to guarantee in many cases the correct completion of joints.
So what is needed? A kit costing £1500 includes six or seven tools that together with standard hand tools enable the installer to tackle most eventualities. The very minimum requirement for working on polymeric cables is a tool to remove the cable jacket or sheath, a tool for bonded screen polymeric cable, another for peel-able screen polymeric, and a further tool to remove the primary insulation. The latter tool alone costs £350. and the sums involved may be significant to an installer. However, using a knife for cable stripping – still a common practice – is simply not good enough to guarantee that the work is completed precisely and correctly. If an installer intends to carry out power joint work over a period of years, then it pays to invest.
The right techniques
There are also installation techniques that actively reduce the amount of equipment needed on-site, as well as simplifying jointing and terminating as a whole. In medium voltage environments, cold-applied techniques – which are used to 'shrink' joint splice bodies and terminations onto the cable in a water and airtight fit – are now the preferred method for creating cable joints and terminations. Since heat is not required, there is no need for gas bottles to be carried on site. Ranges that give the installer an all-in-one unit, such as 3M Cold Shrink, include everything required to complete the joint or termination. This allows installations to be carried out at a consistent and high level, reducing human error, which in turn reduces joint failures and power outage. Furthermore, joints and terminations can be performed far more rapidly; for example a medium voltage transition joint can be completed in just two hours, as opposed to half a day with other methods.
There are many challenges to face when carrying out cable jointing and termination, but they can be overcome through the well judged investment of time and resources. The return is improved network reliability, as well as reduced maintenance and repair costs.