De-rating Cables Challenges Electricians

Published: 11th May 2010
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An increasing number of electricians may need to take 17th edition courses due to the testing task of de-rating cables, according to an industry authority.

Electrical expert Chris Atkin told the Professional Electrician and Installer magazine that more electricians are being faced with cable selection problems since the growth in popularity of thermal insulation.

Most thermal insulation procedures take the form of 'in fill' within the cavity wall or roof space thermal blanketing which aims to make a building more thermally and energy efficient.

The problem with thermal insulation is that an inability to dissipate heat, with the consequence of an increase of resistance, could become critical and affect the performance of cables.

Electricians and designers have to take this thermal effect into account and in essence 'strip out' the thermal effect, a process known as de-rating.

A cable should preferably not be installed in a location where it is likely to be covered by thermal insulation, according to the 17th edition of the IEE wiring regulations; however, this is not always possible and necessitates de-rating.

Electricians are being encouraged to take 17th edition courses in order to understand the new procedures because there are 57 methods for installing thermal insulation included in the new revisions.

Mr Atkin warned that installation numbers in the 17th edition are significantly different from older tables so electricians would benefit from taking 17th edition courses in order to modernise their working practice.

The revisions also take into account new technology such as Combined Heat and Power (CHP), photo-voltaic cells (solar panels) small scale generators and more.

De-rating of cables in thermally insulated walls has changed and impacts the use of 2.5mm cable for use on ring mains where the cable is within an insulated wall or partition including laying up against the wall skin.

Selecting the correct cable for the intended application is imperative as electricians must ensure a reasonable lifespan of conductors and insulation, said Mr Atkin.

Conductors and insulation are subjected to the thermal effects of carrying electric current so it is important to meet the necessary requirements, according to Mr Atkin.

The first edition of BS7671 was published in 1992 when the British Standards Institution (BSI) adopted the IEE Wiring Regulations, 16th Edition and made it a National Standard.

The British Electrotechnical Committee (BEC) is part of the BSI and is the UK member of the International Electrotechnical Commission (IEC) and the European Committee for Electrotechnical Standardisation (CENELEC).


Electrical courses and 17th edition courses are available at Able Skills.

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