09.07.2008
Iain Collings, Senior Design Engineer - Applications Engineering, from Prysmian Cables & Systems, writes about how the 17th Edition has affected cables in terms of Appendix 4:
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| Cables are covered in Appendix 4. |
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The
17th Edition of the Wiring Regulations brings BS7671 in line with IEC documentation. The UK cable industry has over the last 10 years worked closely with the international committees responsible for these documents and hence there is very little change from the traditional UK cable current ratings.
Prysmian has dedicated many man days in the preparation of
Appendix 4, and other parts of the regulations by providing data into the IEC system, and also by direct representation on the committees/panels.
The current ratings can still be found within Appendix 4 of the document. The new Edition brings the current ratings in line with the international IEC ratings. Most of the current ratings have not changed. Where there are changes, these have only been by the odd amp. Additional ratings have been added for multi-core armoured cables installed direct in ground and in underground ducts. The current ratings stated are based on conservative assumptions. If, however, more favourable installation parameters are known - e.g. thermal resistivity, depth of burial and ambient temperature, then higher ratings (and therefore - in some instances - smaller conductor sizes) can be used by referencing either the ERA 69-30 set of publications or contacting the cable manufacturer.
Of particular interest for domestic and office installations is the expansion of the twin flat and earth current ratings to reflect more fully the modern installation practices with particular emphasis on thermal insulation. Table 4D5 provides details.
Within Appendix 4, the tables of installation methods with pictorial examples and descriptions have been extensively revised and expanded to align with the IEC, from the previous 20 examples to 52 now. Current ratings still refer to tabulated Reference Methods. Each Installation Method - e.g. 52 indicates the relevant Reference Method (A to G) that should be used to establish the correct current ratings. Although the Reference Methods are now indicated by a letter rather than a number, the terms used in the current ratings tables are still familiar (e.g. free air, clipped direct, enclosed in conduit).
The Rating Correction Factors for ambient temperature have been simplified such that the type of protection e.g. MCB, BS3036 re-wireable fuse does not affect these factors anymore. Additional correction factors have been provided, not only for standard groupings of cables but also for multiple trays or ladder systems. Correction factors for direct buried and ducted ratings are included.
The mV/A/m stated in the regulations are unchanged from those previously stated, but the regulations themselves have changed with regard the allowed Voltage Drop tolerances (Appendix 12 refers).
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Related contacts
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Iain Collings B.Eng (Hons) C.Eng MIMechE
Senior Design Engineer, Applications Engineering
Prysmian Cables & Systems Ltd.,
Tel: +44 23 80608614
Email: Iain.collings@prysmian.com
Web: www.prysmian.co.uk |
Iain Collings
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Source: Prysmian & James Hunt |
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