BS 7769-2 2-1997 Electric cables - Calculation of the current rating - Thermal resistance - A method for calculating reduction factors for groups of cables in free air protected fr.pdf
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1、BRITISH STANDARD BS 7769-2.2: 1997 IEC 287-2-2: 1995 Electric cables Calculation of the current rating Part 2: Thermal resistance Section 2.2 A method for calculating reduction factors for groups of cables in free air, protected from solar radiation ICS 29.060.20BS 7769-2.2:1997 This British Standar
2、d, having been prepared under the direction of the Electrotechnical Sector Board, was published under the authority of the Standards Board and comes into effect on 15 February 1997 BSI 11-1998 The following BSI references relate to the work on this standard: Committee reference GEL/20 Draft announce
3、d in BSI Update November 1996 ISBN 0 580 26923 X Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee GEL/20, Electric cables, upon which the following bodies were represented: Association of Consulting Engineers Association o
4、f Manufacturers of Domestic Electrical Appliances BEAMA Electrical Cable and Conductor Accessory Manufacturers Association British Approvals Service for Cables British Cable Makers Confederation British Iron and Steel Producers Association British Plastics Federation Department of Trade and Industry
5、 (Consumer Safety Unit, CA division) Electricity Association London Regional Transport The following bodies were also represented in the drafting of the standard, through subcommittees and panels: ERA Technology Ltd. Institution of Incorporated Executive Engineers London Underground Ltd. Amendments
6、issued since publication Amd. No. Date CommentsBS 7769-2.2:1997 BSI 11-1998 i Contents Page Committees responsible Inside front cover National foreword ii Introduction 1 1 Scope 1 2 Normative references 1 3 List of symbols 1 4 Method 2 5 Values of clearance to avoid a reduction in current-carrying c
7、apacity 2 6 Procedures to derive the reduction coefficient for grouped cables 3 Figure 1 Typical groups of multicore cable a), b), c), d), e); e) is the largest size of group covered by data 4 Figure 2 Typical groups of trefoil circuits a), b), c); c) is the largest size of group covered by data 4 F
8、igure 3 Values of (h l /h g ) for two cables in a vertical plane 4 Figure 4 Values of (h l /h g ) for three cables in a vertical plane 5 Figure 5 Values of (h l /h g ) for two trefoil groups in a vertical plane 5 Table 1 Data for calculating reduction coefficients for grouped cables 3 List of refere
9、nces Inside back coverBS 7769-2.2:1997 ii BSI 11-1998 National foreword This Section of BS 7769 has been prepared by Technical Committee GEL/20. It is identical with IEC 287-2-2:1995 Electric cables Calculation of the current rating Part 2: Thermal resistance Section 2: A method for calculating redu
10、ction factors for groups of cables in free air, protected from solar radiation, published by the International Electrotechnical Commission (IEC). A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct ap
11、plication. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-references Publication referred to Corresponding British Standard BS 7769 Electric cables. Calculation of the current rating. IEC 287-1-1:1994 Part 1 Current rating equations (100 % load fa
12、ctor) and calculation of losses Section 1.1:1996 General (Identical) IEC 287-2-1:1994 Part 2 Thermal resistance Section 2.1:1996 Calculation of thermal resistance (Identical) Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 6, an inside back co
13、ver and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on theinside front cover.BS 7769-2.2:1997 BSI 11-1998 1 Introduction This section provides a method and data for calculating group reducti
14、on factors for cables in groups running horizontally in free air. Dielectric losses are neglected. It should be read in conjunction with part 2, section 1. 1 Scope The method described in this International Standard is applicable to any type of cable and group running horizontally, provided that the
15、 cables are of equal diameter and emit equal losses. Information is provided on the reduction in permissible current when cables are mounted adjacent to each other. It is limited to the following cases: a) a maximum of nine cables in a square formation, see Figure 1, and b) a maximum of six circuits
16、 each comprised of three cables mounted in trefoil, with up to three circuits placed side by side or two circuits placed one above the other, see Figure 2. Caution is advised where air flow around the cables may be restricted by proximity to neighbouring objects. NOTEFurther work is to be done to ex
17、tend and refine the data and to include the effect of dielectric loss. Information is provided for the following situations: Where a rating for one cable or circuit assumed to be isolated exists, group reduction factors can be derived for the same type of cable, see 4.1. Where previously calculated
18、ratings are not available, the data provided can be used to calculate permissible currents for groups of cables, using the formulae in parts 1 and 2, see4.2. Where adequate clearances can be provided between cables to avoid a reduction in permissible current, see clause 5. 2 Normative references IEC
19、 287-1-1:1994, Electric cables Calculation of the current rating Part 1: Current rating equations (100 % load factor) and calculation of losses Section 1: General. IEC 287-2-1:1994, Electric cables Calculation of the current rating Part 2: Thermal resistance Section 1: Calculation of thermal resista
20、nce. 3 List of symbols D e= external diameter of a multi-core cable or of one single-core cable mounted in trefoil mm F g= group reduction factor l g= rating of the hottest cable in a group A l t= rating for one cable or circuit, assumed to be isolated A T 4l= external thermal resistance of one cabl
21、e, assumed to be isolated, used for calculating l t K.m/W T 4g= external thermal resistance of the hottest cable in a group K.m/W W = power loss from one multi-core cable or one single-core cable mounted in trefoil, assumed to be isolated, when carrying the current l t W/m e = clearance between adja
22、cent cables in a group (note, this is measured between cable surfaces, not between cable axes as in IEC287-1-1 and IEC 287-2-1 mm h l= heat dissipation coefficient of one multicore cable or of one single-core cable mounted in trefoil, assumed to be isolated, in free air W/m 2 .K 5/4 h g= heat dissip
23、ation coefficient of the hottest cable in a group W/m 2 .K 5/4 k l= cable surface temperature rise factor of one multicore cable or of one single-core cable mounted in trefoil, assumed to be isolated, in free air = q a= ambient temperature used for calculating l t C q c= conductor temperature used f
24、or calculating l t Ccable surface temperature riseconductor temperature rise -BS 7769-2.2:1997 2 BSI 11-1998 4 Method 4.1 When group reduction factors can be applied to existing ratings When the permissible current for an isolated cable or circuit is known and it is desired to calculate the reductio
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