1、BRITISH STANDARD BS 7769-2-2.1: 1997 IEC 60287-2-1: 1994 Incorporating Amendment No. 1 Electric cables Calculation of the current rating Part 2: Thermal resistance Section 2.1 Calculation of thermal resistance ICS 29.060.20 Licensed Copy: London South Bank University, London South Bank University, T
2、ue Mar 13 14:37:38 GMT+00:00 2007, Uncontrolled Copy, (c) BSIBS 7769-2-2.1:1997 This British Standard, 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 24 July 2002 Th
3、e following BSI references relate to the work on this standard: Committee reference GEL/20 Draft announced in BSI Update May 1995 ISBN 0 580 26922 1 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee GEL/20, Electric cables,
4、 upon which the following bodies were represented: Association of Consulting Engineers Association of Manufacturers of Domestic Electrical Appliances BEAMA (Electrical Cable and Conductor Accessory Manufacturers Association) British Approvals Service for Cables British Cable Makers Confederation Bri
5、tish Iron and Steel Producers Association British Plastics Federation Department of Trade and Industry (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: E
6、RA Technology Ltd. Institution of Incorporated Executive Engineers London Underground Ltd. Amendments issued since publication Amd. No. Date Comments 13665 24 July 2002 See national foreword Licensed Copy: London South Bank University, London South Bank University, Tue Mar 13 14:37:38 GMT+00:00 2007
7、, Uncontrolled Copy, (c) BSIBS 7769-2-2.1:1997 BSI 24 July 2002 i Contents Page Committees responsible Inside front cover National foreword ii Introduction 1 1G e n e r a l 1 1.1 Scope 1 1.2 Symbols 2 2 Calculation of thermal resistances 3 2.1 Thermal resistance of the constituent parts of a cable,
8、T 1 , T 2and T 3 4 2.2 External thermal resistance T 4 9 3 Digital calculation of quantities given graphically 14 3.1 General 16 3.2 Calculation of sby means of a diagram (Figure 8) 19 Figure 1 Diagram showing a group of q cables and their reflection in the ground-air surface 22 Figure 2 Geometric f
9、actor G for two-core belted cables with circular conductors 23 Figure 3 Geometric factor G for three-core belted cables with circular conductors 24 Figure 4 Thermal resistance of three-core screened cables with circular conductors compared to that of a corresponding unscreened cable 25 Figure 5 Ther
10、mal resistance of three-core screened cables with sector-shaped conductors compared with that of a corresponding unscreened cable 26 Figure 6 Geometric factor for obtaining the thermal resistances of the filling material between the sheaths and armour of SL and SA type cables 27 Figure 7 Heat dissip
11、ation coefficient for black surfaces of cables in free air 28 Figure 8 Graph for the calculation of external thermal resistance of cables in air 31 Table 1 Thermal resistivities of materials 20 Table 2 Values for constants Z, E and g for black surfaces of cables in free air 21 Table 3 Absorption coe
12、fficient of solar radiation for cable surfaces 22 Table 4 Values of constants U, V and Y 22 G Licensed Copy: London South Bank University, London South Bank University, Tue Mar 13 14:37:38 GMT+00:00 2007, Uncontrolled Copy, (c) BSIBS 7769-2-2.1:1997 ii BSI 24 July 2002 National foreword This Section
13、 of BS 7769 has been prepared by Technical Committee GEL/20. It is identical with IEC 60287-2-1:1994, Electric cables Calculation of the current rating Part 2: Thermal resistance Section 1: Calculation of thermal resistance, including amendment 1:2001, published by the International Electrotechnical
14、 Commission (IEC). The start and finish of text introduced or altered by amendment is indicated in the text by tags . Tags indicating changes to IEC text carry the number of the IEC amendment. For example, text altered by IEC amendment 1 is indicated by . From 1 January 1997, all IEC publications ha
15、ve the number 60000 added to the old number. For instance, IEC 27-1 has been renumbered as IEC 60027-1. For a period of time during the change over from one numbering system to the other, publications may contain identifiers from both systems. Cross-references The British Standards which implement i
16、nternational or European publications referred to in this document may be found in the BSI Catalogue under the section entitled “International Standards Correspondence Index”, or by using the “Search” facility of the BSI Electronic Catalogue or of British Standards Online. This publication does not
17、purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and i
18、i, pages 1 to 31 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Licensed Copy: London South Bank University, London South Bank University, Tue Mar 13 14:37:38 GMT+00:00 2007, Uncontrolled Copy, (c) BSIBS 7769-2-2.1:1997 BSI 24 July
19、2002 1 Introduction IEC 60287 has been divided into three parts and sections so that revisions of, and additions to, the document can be carried out more conveniently. Each part is divided into sections which are published as separate standards. Part 1: Formulae of ratings and power losses; Part 2:
20、Formulae for thermal resistance; Part 3: Sections on operating conditions. This section of IEC 60287-2 contains methods for calculating the internal thermal resistance of cables and the external thermal resistance for cables laid in free air, ducts and buried. The formulae in this standard contain q
21、uantities which vary with cable design and materials used. The values given in the tables are either internationally agreed, for example, electrical resistivities and resistance temperature coefficients, or are those which are generally accepted in practice, for example, thermal resistivities and pe
22、rmittivities of materials. In this latter category, some of the values given are not characteristic of the quality of new cables but are considered to apply to cables after a long period of use. In order that uniform and comparable results may be obtained, the current ratings should be calculated wi
23、th the values given in this standard. However, where it is known with certainty that other values are more appropriate to the materials and design, then these may be used, and the corresponding current rating declared in addition, provided that the different values are quoted. Quantities related to
24、the operating conditions of cables are liable to vary considerably from one country to another. For instance, with respect to the ambient temperature and soil thermal resistivity, the values are governed in various countries by different considerations. Superficial comparisons between the values use
25、d in the various countries may lead to erroneous conclusions if they are not based on common criteria: for example, there may be different expectations for the life of the cables, and in some countries design is based on maximum values of soil thermal resistivity, whereas in others average values ar
26、e used. Particularly, in the case of soil thermal resistivity, it is well known that this quantity is very sensitive to soil moisture content and may vary significantly with time, depending on the soil type, the topographical and meteorological conditions, and the cable loading. The following proced
27、ure for choosing the values for the various parameters should, therefore, be adopted: Numerical values should preferably be based on results of suitable measurements. Often such results are already included in national specifications as recommended values, so that the calculation may be based on the
28、se values generally used in the country in question; a survey of such values is given in Part 3, Section 1. A suggested list of the information required to select the appropriate type of cable is given in Part 3, Section 1. 1 General 1.1 Scope This section of IEC 60287 is solely applicable to the co
29、nditions of steady-state operation of cables at all alternating voltages, and direct voltages up to 5 kV, buried directly in the ground, in ducts, in troughs or in steel pipes, both with and without partial drying-out of the soil, as well as cables in air. The term “steady state” is intended to mean
30、 a continuous constant current (100 % load factor) just sufficient to produce asymptotically the maximum conductor temperature, the surrounding ambient conditions being assumed constant. This section provides formulae for thermal resistance. The formulae given are essentially literal and designedly
31、leave open the selection of certain important parameters. These may be divided into three groups: parameters related to construction of a cable (for example, thermal resistivity of insulating material) for which representative values have been selected based on published work; parameters related to
32、the surrounding conditions which may vary widely, the selection of which depends on the country in which the cables are used or are to be used; parameters which result from an agreement between manufacturer and user and which involve a margin for security of service (for example, maximum conductor t
33、emperature). Licensed Copy: London South Bank University, London South Bank University, Tue Mar 13 14:37:38 GMT+00:00 2007, Uncontrolled Copy, (c) BSIBS 7769-2-2.1:1997 2 BSI 24 July 2002 1.2 Symbols The symbols used in this standard and the quantities which they represent are given in the following
34、 list: D a external diameter of armour mm D d internal diameter of duc mm D e external diameter of cable, or equivalent diameter of a group of cores in pipe-type cable mm D* e external diameter of cable (used in 2.2.1)m D o external diameter of duct mm D s external diameter of metal sheath mm D oc t
35、he diameter of the imaginary coaxial cylinder which just touches the crests of a corrugated sheath mm D ot the diameter of the imaginary coaxial cylinder which would just touch the outside surface of the troughs of a corrugated sheath = D it+ 2t s mm D ic the diameter of the imaginary cylinder which
36、 would just touch the inside surface of the crests of a corrugated sheath = D oc 2t s mm D it the diameter of the imaginary cylinder which just touches the inside surface of the troughs of a corrugated sheath mm E constant used in 2.2.1.1 F 1 coefficient for belted cables defined in 2.1.1.2.2 F 2 co
37、efficient for belted cables defined in 2.1.1.2.5 G geometric factor for belted cables geometric factor for SL and SA type cables H intensity of solar radiation (see 2.2.1.2) W/m 2 K screening factor for the thermal resistance of screened cables K A coefficient used in 2.2.1 L depth of laying, to cab
38、le axis or centre of trefoil mm L G distance from the soil surface to the centre of a duct bank mm N number of loaded cables in a duct bank (see 2.2.7.3) T 1 thermal resistance per core between conductor and sheath K m/W T 2 thermal resistance between sheath and armour K m/W T 3 thermal resistance o
39、f external serving K m/W T 4 thermal resistance of surrounding medium (ratio of cable surface temperature rise above ambient to the losses per unit length) K m/W T* 4 external thermal resistance in free air, adjusted for solar radiation K m/W T 4 thermal resistance between cable and duct (or pipe) K
40、 m/W T 4 thermal resistance of the duct (or pipe) K m/W T 4 thermal resistance of the medium surrounding the duct (or pipe) K m/W U V constants used in 2.2.7.1 W d dielectric losses per unit length per phase W/m W k losses dissipated by cable k W/m W TOT total power dissipated in the trough per unit
41、 length W/m Y coefficient used in 2.2.7.1 Z coefficient used in 2.2.1.1 G Licensed Copy: London South Bank University, London South Bank University, Tue Mar 13 14:37:38 GMT+00:00 2007, Uncontrolled Copy, (c) BSIBS 7769-2-2.1:1997 BSI 24 July 2002 3 d a external diameter of belt insulation mm d c ext
42、ernal diameter of conductor mm d cm minor diameter of an oval conductor mm d cM major diameter of an oval conductor mm d M major diameter of screen or sheath of an oval conductor mm d m minor diameter of screen or sheath of an oval conductor mm d x diameter of an equivalent circular conductor having
43、 the same cross-sectional area and degree of compactness as the shaped one mm g coefficient used in 2.2.1.1 h heat dissipation coefficient W/m 2 K 5/4 ln natural logarithm (logarithm to base e) n number of conductors in a cable p the part of the perimeter of the cable trough which is effective for h
44、eat dissipation (see 2.2.6.2) m r 1 circumscribing radius of two or three-sector shaped conductors mm s 1 axial separation of two adjacent cables in a horizontal group of three, not touching mm t insulation thickness between conductors mm t 1 insulation thickness between conductors and sheath mm t 2
45、 thickness of the bedding mm t 3 thickness of the serving mm t i thickness of core insulation, including screening tapes plus half the thickness of any non-metallic tapes over the laid up cores mm t s thickness of the sheath mm uin 2.2.2 u in 2.2.7.3 x, y sides of duct bank (y x) (see 2.2.7.3)m m me
46、an temperature of medium between a cable and duct or pipe C permissible temperature rise of conductor above ambient temperature K factor to account for dielectric loss for calculating T 4for cables in free air K factor to account for both dielectric loss and direct solar radiation for calculating fo
47、r cables in free air using Figure 8 K difference between the mean temperature of air in a duct and ambient temperature K difference between the surface temperature of a cable in air and ambient temperature K temperature rise of the air in a cable trough K ratio of the total losses in metallic sheath
48、s and armour respectively to the total conductor losses (or losses in one sheath or armour to the losses in one conductor) 1m loss factor for the middle cable Three cables in flat formation without transposition, with sheaths bonded at both ends 11 loss factor for the outer cable with the greater lo
49、sses 12 loss factor for the outer cable with the least losses thermal resistivity of earth surrounding a duct bank K m/W thermal resistivity of concrete used for a duct bank K m/W 2L D e - LG rb - T 4* Licensed Copy: London South Bank University, London South Bank University, Tue Mar 13 14:37:38 GMT+00:00 2007, Uncontrolled Copy, (c)
