Q & A of the Day – Guidance Note 1 - are there de-rating factors regarding cable?

02.04.2008

Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about Guidance Note 1 - selection and erection, is answered by SELECT:

Question: In Guidance note 1 - Selection & Erection - Section 7 'Other influences' it is stated that: 'The use of high temperature cable requires careful selection of termination systems and accessories and switchgear which are appropriate to the operating temperature of the cable. Alternatively, where high temperature cable is to be used, it should be de-rated, so as not to allow the conductor temperature to exceed the maximum operating temperature of the equipment to which it is to be connected'.

Most manufactures of miniature circuit breakers (MCBs) state a utilisation temperature of up to 60oC. Are any MCBs rated at higher operating temperatures in order to avoid de-rating of conductor current carrying capacity? What is the benefit of using an XLPE insulated conductor as it should be de-rated? Are there any de-rating factors from cable manufacturers to consider?

Answer: As you indicate, high temperatures can cause problems with both cables and equipment. XLPE cables are more suitable when high temperatures are present. Temperature de-rating factors can de identified from Table 4C1 (BS7671). With regards to electrical equipment, the manufacturer may be the best source of information.

To see many more Q & A in Voltimum UK's Experts Area, please click on the link:

http://www.voltimum.co.uk/consult.php?universe=consult.index.questions

An MCB - what is the rated operating temp?

XLPE cable - are there de-rating factors?

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SELECT
The Walled Garden, Bush Estate,
Midlothian, Scotland EH26 0SB
Tel: 0131 445 5577
Fax: 0131 445 5548
Email: admin@select.org.uk
Web: www.select.org.uk

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Source: SELECT & James Hunt

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