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

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

25、n factor for a group of cables, the calculation is made for the hottest cable in the group using: The current-carrying capacity of the hottest cable is then given by: The surface temperature rise factor k lis calculated from: NOTEThe quantities W and T 4lare available from the calculation used for l

26、 tand it is convenient to calculate k l at the same time as l t . The term (T 4g /T 4l ) shall be calculated from the ratio (h l /h g ) by the use of the iterative relationship: starting with (T 4g /T 4l ) 1= (h l /h g ). NOTEEquation (4) converges quickly, one evaluation with (T 4g /T 4l ) 1= (h l

27、/h g ) is usually sufficient. Alternatively, when (h l /h g ) is less than 1,4, it is sufficient to substitute (h l /h g ) for (T 4g /T 4l ) in equation (1). Values for the ratio (h l /h g ) are given in Table 1 and in Figure 3 to Figure 5 for groups of multicore cables and for groups of single-core

28、 cables in trefoil formation. NOTEValues for other arrangements of cables should be determined by experiment. 4.2 Where there are previously calculated ratings The current-carrying capacity of the hottest cable in a group shall be calculated using the formulae given in part 2, section 1 for cables i

29、n free air, but with h gsubstituted for the heat emission coefficient h given in part 2, section 1. For the group configurations covered in Table 1 and in Figure 3 to Figure 5, values of the heat emission coefficient h gare derived, from: where the parameter h is given in part 2, section 1, for one

30、multicore cable or for one cable mounted in a trefoil group, assumed to be isolated, and the ratio (h l /h g ) is obtained from Table 1 or Figure 3 to Figure 5 of this standard. 4.3 Groups of cables installed in more than one plane Factors and current-carrying capacities for the hottest cable in a g

31、roup where cables are arranged in both the horizontal and vertical directions shall be evaluated by using the appropriate value of (h l /h g ) for the vertical clearance and ensuring that the horizontal clearance between cables, e, is not less than the appropriate value given in Table 1 for neglecti

32、ng side by side thermal proximity effect. 5 Values of clearance to avoid a reduction in current-carrying capacity Minimum clearances between the outside surfaces of neighbouring cables, necessary to avoid a reduction in current-carrying capacity from the value for one cable or circuit, assumed to be

33、 isolated, are given in column 2 of Table 1 for various arrangements of cables. The minima have been selected taking into account that it is not practicable to maintain small clearances precisely. Care shall be taken to provide supports adequate to ensure the desired spacing. If a clearance not less

34、 than the appropriate minimum value given in column 2 of Table 1 cannot be maintained over the entire length of the cable, one of the procedures given in clause 6 shall be applied. (1) (2) (3) (4) (5)BS 7769-2.2:1997 BSI 11-1998 3 6 Procedures to derive the reduction coefficient for grouped cables I

35、f a clearance not less than the appropriate value given in column 2 of Table 1 cannot be maintained with confidence throughout the length of the cable, the reduction coefficient shall be determined as follows: For horizontal clearances it shall be assumed that the cables are touching each other or t

36、he vertical surface. Appropriate values of (h l /h g ) are given in column 4 of Table 1 for calculating the reduction coefficient using one of the methods given in clause 4. For vertical clearances the reduction coefficient due to grouping shall be derived according to the value of the expected clea

37、rance: a) where the clearance is less than the appropriate value given in column 2 of Table 1, but can be maintained at a value not less than the minimum given in column 3, a reduction coefficient shall be derived using one of the methods of clause 4 with an appropriate value of (h l /h g ) obtained

38、 either from the formula in column 4 of Table 1 or from one of the curves in Figure 3 to Figure 5; b) where the clearance cannot be maintained at a value not less than the minimum given in column 3 of Table 1, is shall be assumed that the cables are touching each other. Suitable values of (h l /h g

39、) are provided in column 4 of Table 1 for calculating the reduction coefficient using a method in clause 4. NOTEThe formulae in Table 1 and the curves in Figure 3 to Figure 5 are valid only for the range of clearances indicated in the footnotes to the table and must not be extrapolated. Table 1 Data

40、 for calculating reduction coefficients for grouped cables Thermal proximity effect is negligible if e/D eis greater than or equal to: Thermal proximity effect is not negligible Arrangement of cables If e/D eis less than: Average value of h l /h g a,b 1 2 3 4 Side by side 2 multicore 0,5 0,5 1,41 3

41、multicore 0,75 0,75 1,65 2 trefoils 1,0 1,0 1,2 3 trefoils 1,5 1,5 1,25 One above the other 2 multicore 2 2 or 0,5 1,085 (e/D e )0,128or1,35 3 multicore 4 4 or 0,5 1,19 (e/D e )0,135or1,57 2 trefoils 4 4 or 0,5 1,106 (e/D e )0,078or1,39 Near to a vertical surface or to a horizontal surface below the

42、 cable 0,5 0,5 1,23 a The formulae for (h l /h g ) given in column 4 of this table and the curves of Figure 3 to Figure 5, shall not be used for values of (e/D e ) less than 0,5 or greater than the appropriate values given in column 2. b Average values for cables having diameters from 13 mm to 76 mm

43、. More precise values of (h l /h g ) for multicore cables may be evaluated for a specific cable diameter, both inside and outside this range, by consulting Table 2 of part 2, section 1.BS 7769-2.2:1997 4 BSI 11-1998 Figure 1 Typical groups of multicore cable a), b), c), d), e); e) is the largest siz

44、e of group covered by data Figure 2 Typical groups of trefoil circuits a), b), c); c) is the largest size of group coveredbydata Figure 3 Values of (h l /h g ) for two cables in a vertical planeBS 7769-2.2:1997 BSI 11-1998 5 Figure 4 Values of (h l /h g ) for three cables in a vertical plane Figure

45、5 Values of (h l /h g ) for two trefoil groups in a vertical plane6 blankBS 7769-2.2:1997 BSI 11-1998 List of references See national foreword.BSI 389 Chiswick High Road London W4 4AL | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |

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