Power Cables - Powering ahead with change
The cables sector which provides the main electrical supply is undergoing a series of
changes with revised standards, new installation approaches and even new elements in the make-up of the cables themselves. Here, Dr Jeremy Hodge, Chief Executive of the British Approvals Service for Cables, assesses the shifting issues
The power cables market - not traditionally the most high profile of the various sectors of the industry - has been developing steadily while market conditions have changed, new specifications and installation methods are adopted, and new and revised British and international standards are introduced.
The most significant drivers in the power cable market have been the setting of higher specifications by building designers - especially to address fire safety concerns - volatile commodity prices affecting the cable supply chain, and increasingly global trading in cable products.
The rise in the price of copper, which is the major cost component of power cables, has led many buyers to review their purchasing patterns and to look further afield, but some of these cables have demonstrated quality problems.
In some cases where physical size and flexibility are not a problem and where permitted in the standards, aluminium cables may prove to be a good option, although there remain some performance and usability concerns, such as with crimped terminations.
Two of the most commonly-used types of armoured power cable, PVC-sheathed to BS 5467 and low smoke to BS 6724, have recently been updated to incorporate the latest material specifications. The opportunity has also been taken to make the requirements on core and sheath colours more flexible to reflect the wider range of geographies and jurisdictions in which these cables are now being used.
The cables both use cross-linked polyethylene (XLPE) as the insulation material around the conductors, which offers several performance advantages. Armoured cables using PVC or other traditional materials, such as to BS 6346, are becoming less and less specified.
Fire Safety
With increasing expectations from fire safety legislation and concerns over liability, engineers and designers have been changing the cables they specify. Increasingly, the move is away from traditional cable sheathing materials like PVC towards Low Smoke Halogen Free (LSHF) construction.
Standards for these types of cable include requirements for testing smoke emission and the production of acid gases, and specifiers should check that these tests have been done if the cable does not carry appropriate approvals.
Fire resistant cables are also developing, with the recent publication of a new fire test for larger cross-section fire-resistant cables. For example, BS 8491 specifies an additional fire test that simultaneously applies flames at 842 ºC, direct mechanical impact and direct application of a water jet, simulating fire fighting conditions. Three ratings are possible for fire survival times of 30, 60 or 120 minutes - the latter being particularly suitable in cases where fire fighting services such as smoke extract systems will be used for an extended period. Cables which may pass this test include various special designs based on BS 7846 and other standards.
Mineral insulated cable, however, remains probably the best all-round performer for fire resistance, and is available in a wide range of sizes starting at the very small. It is used for fire alarms, detection equipment and emergency lighting where there can be no compromise, especially in large public buildings such as hospitals, schools, shopping malls and airports where large numbers of people may move about.
The risk of anything going wrong is greatly reduced because of its key features, namely its ability to withstand high temperatures up to the melting point of copper at over 1080 ºC, its ability to continue power and data transmission even after significant damage, and its reduced need for maintenance.
However, many fire alarm and emergency lighting installations have changed to use soft skin fire performance cable to BS 5839-1 and BS 7629-1, because of the relative ease of installation and the overall cost of the installation.
Installation
The installation of power cables on many projects is changing in that the installer is increasingly being expected to take care of both the medium voltage (e.g., 11kV to BS 6622 or BS 7835) cable from the utility supply and the low voltage power distribution cables around the building or site. In essence, this is quite a major step.
It requires the contractor to acquire further knowledge of this type of installation, a specialist area in its own right. Indeed, further training on medium voltage working and jointing may be required and is advisable to ensure best practice is applied. Even though jointing equipment is similar to low voltage, higher-rated products must be used and closer co-operation with the utility is needed.
Although standards for many types of cable have been harmonized at the European or international level, those for power supply cables remain closely linked to national regulations, guidance and practice.
Harmonics
Another issue which has an implication for cable choice is power harmonics. As more complex electrical loads are being used in buildings, for example computers and UPS systems, more installations are becoming prone to the effects of harmonics, which can result in overheating and other complex effects.
These problems are often difficult to investigate; they can occur in fully compliant cables and often mimic other faults such as low conductivity.
Power system designers need to be aware of the potential and if necessary apply alternative design approaches such as larger sized neutrals in three phase systems, in order to minimize or avoid the problem. Harmonic filter systems are available should the problem not be addressed at the design stage.
Approvals
One of the most serious problems for any project manager or installer is a batch of faulty cable. Project delays and the cost of replacement are unwelcome.
It is unfortunate that some buyers still take at face value the claims of compliance made by some manufacturers or traders of cable, without checking that compliance has been demonstrated.
Many non-approved cables have not been subject to the required tests and one of the early signs to recognising potential problematic cable is a product with few markings or marked with only a standard number. This should be treated with caution. It is possible that nobody independent of the manufacturer has examined that cable and the claims made may be unreliable.
Faulty cables have always been an issue but never to the extent that they are today. The source of much of it appears to be the rise in copper and other material prices which may have tempted some cable manufacturers to cut corners and use less copper or cheaper polymer materials in the manufacturing process. Drawing down the diameter of the copper wire too much has the effect of reducing conductivity, and cables with insufficient conductivity may overheat and cause fire or offer a reduced level of safety against electric shock. Cable standards specify the minimum conductivity required.
More recently we have also seen examples of materials other than pure copper, such as steel wire, copper-coated aluminium or badly recycled copper in cables that should be made from pure copper.
Poor quality insulation or sheathing material may result in insulation resistance problems on testing an installation, or they may have unacceptable fire or smoke performance.
It cannot be stressed enough that there is a need to move towards greater compliance in the purchase and use of approved products for now and the future.
For the power cables market, doing the simple things well in these changing times would ensure safe installations using quality products that are fit for purpose in an area of high importance for the short and longer term.
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