BASEC News

Choosing the Right Cable for the Environment

06/09/10
The British Approvals Service for Cables (BASEC) is a recognised sign of assurance of independent cable testing and approval. BASEC is non-profit making Government-nominated body and has for more than 30 years been a mark of reassurance to those specifying cable. Dr Jeremy Hodge, BASEC chief executive, answers some questions from PHAM News about the organisation, its role and advises on specifying the right cable for its environment.

What should you look for when buying electric cable for installations?

A: To ensure safe use look for the BASEC mark. It is a common misunderstanding that a cable is compliant with standards just because the supplier claims that it has been produced to a particular standard. Cable marked with only a standard number or less should be treated with caution, and completely unmarked cable should avoided.  It is probable that nobody independent of the manufacturer has examined such cables, and the claims made may be unreliable.  Only cable marked with the ‘BASEC’ name is BASEC approved, by demonstrating its compliance to the required standards.  For flexible harmonised cables, BASEC is a member of the European HAR scheme, and cables marked <HAR> may be used in the same way as a BASEC marked cable.

BASEC rigorously tests cable to meet necessary and appropriate British, European and International standards, through regular detailed examination of manufacturers' production processes and controls.  One of the main features of BASEC’s approval regime is the regular retesting of cable.

Cable standards not only specify the dimensions and materials of a cable, they also require that a range of specific tests are undertaken to prove the construction and performance in terms of temperature, external influences, thermal constraints, shock protection and voltage drop.  Many non-approved cables have not been subject to the required tests which could result in a short circuit or an electric shock.

Non-approved or counterfeit cable identified by BASEC during mystery shopping in the UK market is often light on copper, with ambiguous marking, and in many cases the materials used fail performance tests.  In more severe cases, cable may disintegrate after a short period in use, or even fail simple fire tests.

In line with Part P of the Building Regulations, how can I ensure safe cable use?

A: Plumbers installing electrical systems such as boiler circuits or power showers under Part P of the Building Regulations in England and Wales need to have in mind the requirements of the IEE Wiring Regulations 17th Edition.  Appropriate sizes of cable need to be used for the designed electrical load, and the cable construction and materials need to be appropriate for the environment and likely usage.  In most cases a limited number of regular cable types may be used, but in some circumstances specialist advice on cable selection may be needed.

What are the most common cable types used in plumbing situations?

A: Regular power circuits (rings, radials and spurs) are usually cabled with PVC sheathed flat twin and earth, for which the specification is BS 6004.  If a low smoke halogen free cable is required, the equivalent specification is BS 7211.  The same two standards also cover single core conduit wires and also the familiar green / yellow earthing cables.  Cable traders often use catalogue codes such as “6242Y” or “6242B” for these types of cable, but these are not specifications and the relevant BS number should always be quoted.

The main British Standard for flexible cables is BS 6500, though these types are increasingly becoming harmonised across Europe.  BS 6500 includes regular PVC and rubber sheathed cables, and also some heat resistant types, in up to 5 cores.  European harmonised cables are often referred to by their harmonised code designation, such as H05VV-F (ordinary duty PVC sheathed).  Heat resisting types are available in either PVC (e.g., H05V2V2-F) or as an EPR rubber type (e.g., H05BB-F).  Low smoke halogen free equivalents are also available.  For assurance of quality manufacture, look for the BASEC or HAR marks.

What are “external influences”?

A: The IEE Wiring Regulations set out a range of conditions which the installer needs to assess, in order to specify the correct type and size of cable for the conditions.  These include: presence of water or humidity, water tanks; solid foreign bodies such as dust; corrosive or polluting substances such as acids or oils; threat of impact which might damage the cable; vibration and other mechanical stresses such as stretching; mould and vermin such as mice, rats, birds; solar and UV radiation; and a number of others.  The Wiring Regulations take a cautious approach, and if there is significant risk then an enhanced cable type is usually recommended.  Once external influences have been assessed, the usability of various cable types can be checked against BS 7540 “Electric Cables: Guide to Use”.

What are typical temperature ranges?

A: Cables need to be kept within an allowable range of temperature, otherwise the insulation or sheathing material may decay and crack, compromising safety.  Each cable type is rated at a specific maximum conductor temperature, based on the materials used.  Ordinary PVC and similar materials are usually rated at 70°C.  Rubber materials, enhanced temperature PVC, XLPE and low smoke zero halogen materials are usually rated at 90°C.  Higher temperatures are available for special heat resisting types – some silicone rubber cables may operate up to 250°C, and mineral insulated cables well above that.  At the other extreme, only cables intended for use outdoors in freezing conditions should be used below 0°C.  Flexible cables in particular should be carefully specified for cold conditions – look for a -40°C test rating if in doubt.

What are low smoke halogen free cables?

A: Buildings are increasingly being specified with higher specification cables for fire and smoke performance, particularly in the public sector.  To gain the “LSHF” description cables must pass two specific tests on smoke emission and acid gas emission in a fire, which are part of the relevant British or European Standard specification.  Cables made with PVC cannot pass these tests.

What about thermal insulation?

A: With increasing emphasis on reducing carbon emissions, the amount of thermal insulation used in buildings is increasing.  Cables always run slightly warm, because of the internal resistance in the copper conductors.  This heat can build up unless it is lost through convention or conduction.  If a cable is mounted in thermal insulation, against a wooden joist, or in the ground, the thermal environment will be different and the cable will reach a different maximum temperature.  In some circumstances this might result in overheating, so a larger sized cable needs to be specified to keep it cool enough.  It is important to assess the mounting conditions as described in the IEE Wiring Regulations and to use appropriate de-rating factors, so that the right size of cable can be chosen and safety is not compromised.

What about shock protection?

A: Most installations by plumbers will be in single-phase domestic situations, but in larger premises three phase circuits may be present, and in these circumstances a full scope qualified electrician should be consulted.  The cable voltage rating is important in these circumstances, and the normal rating of 300/500V, may need upgrading to 600/1000V.

What is voltage drop?

A: For small installations with short runs of cable it is usual to base cable sizes on the maximum current load (including fault current) that can be anticipated.  However, when designing and installing long runs of cable, such as to a distant outbuilding it is important to consider the drop in voltage in the cable over that distance and to install a larger cable to minimise this drop.  A cable of several standard sizes larger than that calculated from the load alone may be needed.  In such circumstances the advice of a full scope qualified electrician should be obtained.

Further information about BASEC is available at www.basec.org.uk or contact BASEC directly on 01908 267300

 

 

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