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1、BRITISH STANDARD BS IEC 60287-2-1:1994 +A2:2006 Electric cables Calculation of the current rating Part 2: Thermal resistance Section 2.1 Calculation of thermal resistance ICS 29.060.20 ? Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boein
2、g Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- BS IEC 60287-2-1:1994+A2:2006 This British Standard was published under the authority of the Standards Board and comes into effect on 15 February 1997 BSI 2008 ISBN 978 0 580
3、 62555 8 National foreword This British Standard is the UK implementation of IEC 60287-2-1:1994+A2:2006. It supersedes BS 7769-2-2.1:1997 (IEC 60287-2-1:1994+A1:2001), which is withdrawn. NOTE On implementation of IEC amendment 2:2006 to IEC 60287-2-1:1994, the British Standard was renumbered to BS
4、IEC 60287-2-1:1994+A2:2006. It had previously been dual numbered as BS 7769-2-2.1:1997. 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 amendm
5、ent 1 is indicated by !“. The UK participation in its preparation was entrusted to Technical Committee GEL/20, Electric cables. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions
6、 of a contract. Users are responsible for its correct application. Compliance with a British Standard cannot confer immunity from legal obligations. Amendments/corrigenda issued since publication Amd. No.DateComments 1366524 July 2002Implementation of IEC amendment 1:2001 30 June 2008Implementation
7、of IEC amendment 2:2006 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- BS IEC 60287-2-1:1994+A2:2006 BSI 200
8、8i Contents Page Inside front coverNational foreword Introduction1 1General1 1.1Scope1 1.2Symbols2 2Calculation of thermal resistances3 2.1Thermal resistance of the constituent parts of a cable, T1, T2 and T34 2.2External thermal resistance T49 3Digital calculation of quantities given graphically14
9、3.1General16 3.2Calculation of ?s by means of a diagram (Figure 8)19 Figure 1 Diagram showing a group of q cables and their reflection in the ground-air surface22 Figure 2 Geometric factor G for two-core belted cables with circular conductors23 Figure 3 Geometric factor G for three-core belted cable
10、s with circular conductors24 Figure 4 Thermal resistance of three-core screened cables with circular conductors compared to that of a corresponding unscreened cable25 Figure 5 Thermal resistance of three-core screened cables with sector-shaped conductors compared with that of a corresponding unscree
11、ned cable26 Figure 6 Geometric factor for obtaining the thermal resistances of the filling material between the sheaths and armour of SL and SA type cables27 Figure 7 Heat dissipation coefficient for black surfaces of cables in free air28 Figure 8 Graph for the calculation of external thermal resist
12、ance of cables in air31 Table 1 Thermal resistivities of materials20 Table 2 Values for constants Z, E and g for black surfaces of cables in free air 21 Table 3 Absorption coefficient of solar radiation for cable surfaces 22 Table 4 Values of constants U, V and Y 22 G Copyright British Standards Ins
13、titution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- blank Copyright British Standards Institution Provided by IHS under license with BSI - Uncon
14、trolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- 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 mor
15、e conveniently. Each part is divided into sections which are published as separate standards. Part 1: Formulae of ratings and power losses; Part 2: Formulae for thermal resistance; Part 3: Sections on operating conditions. This section of IEC 60287-2 contains methods for calculating the internal the
16、rmal resistance of cables and the external thermal resistance for cables laid in free air, ducts and buried. The formulae in this standard contain quantities which vary with cable design and materials used. The values given in the tables are either internationally agreed, for example, electrical res
17、istivities and resistance temperature coefficients, or are those which are generally accepted in practice, for example, thermal resistivities and permittivities of materials. In this latter category, some of the values given are not characteristic of the quality of new cables but are considered to a
18、pply to cables after a long period of use. In order that uniform and comparable results may be obtained, the current ratings should be calculated with 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
19、 these may be used, and the corresponding current rating declared in addition, provided that the different values are quoted. Quantities related to the operating conditions of cables are liable to vary considerably from one country to another. For instance, with respect to the ambient temperature an
20、d soil thermal resistivity, the values are governed in various countries by different considerations. Superficial comparisons between the values used in the various countries may lead to erroneous conclusions if they are not based on common criteria: for example, there may be different expectations
21、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 are 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 va
22、ry significantly with time, depending on the soil type, the topographical and meteorological conditions, and the cable loading. The following procedure for choosing the values for the various parameters should, therefore, be adopted: Numerical values should preferably be based on results of suitable
23、 measurements. Often such results are already included in national specifications as recommended values, so that the calculation may be based on these 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
24、 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 conditions 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 tr
25、oughs 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 a continuous constant current (100 % load factor) just sufficient to produce asymptotically the maximum conductor temperature, the surrounding ambient
26、conditions being assumed constant. This section provides formulae for thermal resistance. The formulae given are essentially literal and designedly leave open the selection of certain important parameters. These may be divided into three groups: parameters related to construction of a cable (for exa
27、mple, thermal resistivity of insulating material) for which representative values have been selected based on published work; parameters related to 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; paramete
28、rs which result from an agreement between manufacturer and user and which involve a margin for security of service (for example, maximum conductor temperature). BS IEC 60287-2-1:1994+A2:2006 BSI 2008Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Li
29、censee=Boeing Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- 2 1.2 Symbols The symbols used in this standard and the quantities which they represent are given in the following list: D?aexternal diameter of armourmm Ddintern
30、al diameter of ducmm Deexternal diameter of cable, or equivalent diameter of a group of cores in pipe-type cable mm D*eexternal diameter of cable (used in 2.2.1)m Doexternal diameter of ductmm Dsexternal diameter of metal sheathmm Docthe diameter of the imaginary coaxial cylinder which just touches
31、the crests of a corrugated sheath mm Dotthe diameter of the imaginary coaxial cylinder which would just touch the outside surface of the troughs of a corrugated sheath = Dit + 2ts mm Dicthe diameter of the imaginary cylinder which would just touch the inside surface of the crests of a corrugated she
32、ath = Doc 2ts mm Ditthe diameter of the imaginary cylinder which just touches the inside surface of the troughs of a corrugated sheath mm Econstant used in 2.2.1.1 F1coefficient for belted cables defined in 2.1.1.2.2 F2coefficient for belted cables defined in 2.1.1.2.5 Ggeometric factor for belted c
33、ables geometric factor for SL and SA type cables Hintensity of solar radiation (see 2.2.1.2)W/m2 Kscreening factor for the thermal resistance of screened cables KAcoefficient used in 2.2.1 Ldepth of laying, to cable axis or centre of trefoilmm LGdistance from the soil surface to the centre of a duct
34、 bankmm Nnumber of loaded cables in a duct bank (see 2.2.7.3) T1thermal resistance per core between conductor and sheathK?m/W T2thermal resistance between sheath and armourK?m/W T3thermal resistance of external servingK?m/W T4thermal resistance of surrounding medium (ratio of cable surface temperatu
35、re rise above ambient to the losses per unit length) K?m/W T*4external thermal resistance in free air, adjusted for solar radiationK?m/W T?4thermal resistance between cable and duct (or pipe)K?m/W T?4thermal resistance of the duct (or pipe)K?m/W T?4thermal resistance of the medium surrounding the du
36、ct (or pipe)K?m/W U V constants used in 2.2.7.1 Wddielectric losses per unit length per phaseW/m Wklosses dissipated by cable kW/m WTOTtotal power dissipated in the trough per unit lengthW/m Ycoefficient used in 2.2.7.1 Zcoefficient used in 2.2.1.1 G ? ? ? BS IEC 60287-2-1:1994+A2:2006 BSI 2008 Copy
37、right British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- 3 daexternal diameter of belt insulationmm dcexternal diameter of
38、 conductormm dcmminor diameter of an oval conductormm dcMmajor diameter of an oval conductormm dMmajor diameter of screen or sheath of an oval conductormm dmminor diameter of screen or sheath of an oval conductormm dxdiameter of an equivalent circular conductor having the same cross-sectional area a
39、nd degree of compactness as the shaped one mm gcoefficient used in 2.2.1.1 hheat dissipation coefficientW/m2K5/4 lnnatural logarithm (logarithm to base e) nnumber of conductors in a cable pthe part of the perimeter of the cable trough which is effective for heat dissipation (see 2.2.6.2) m r1circums
40、cribing radius of two or three-sector shaped conductorsmm s1axial separation of two adjacent cables in a horizontal group of three, not touchingmm tinsulation thickness between conductorsmm t1insulation thickness between conductors and sheathmm t2thickness of the beddingmm t3thickness of the serving
41、mm tithickness of core insulation, including screening tapes plus half the thickness of any non-metallic tapes over the laid up cores mm tsthickness of the sheathmm u in 2.2.2 uin 2.2.7.3 x, ysides of duct bank (y x) (see 2.2.7.3)mm ?mean temperature of medium between a cable and duct or pipe?C ?per
42、missible temperature rise of conductor above ambient temperatureK ?factor to account for dielectric loss for calculating T4 for cables in free airK ?factor to account for both dielectric loss and direct solar radiation for calculatingfor cables in free air using Figure 8 K ?difference between the me
43、an temperature of air in a duct and ambient temperatureK ?difference between the surface temperature of a cable in air and ambient temperature K ?temperature rise of the air in a cable troughK ?ratio of the total losses in metallic sheaths and armour respectively to the total conductor losses (or lo
44、sses in one sheath or armour to the losses in one conductor) ?1mloss factor for the middle cable Three cables in flat formation without transposition, with sheaths bonded at both ends ?11loss factor for the outer cable with the greater losses ?12loss factor for the outer cable with the least losses
45、?thermal resistivity of earth surrounding a duct bankK?m/W ?thermal resistivity of concrete used for a duct bankK?m/W 2L De - - L G r b - - T4 * ? ? ? ? ? BS IEC 60287-2-1:1994+A2:2006 BSI 2008Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee
46、=Boeing Co/5910770001 Not for Resale, 08/14/2008 21:56:18 MDTNo reproduction or networking permitted without license from IHS -,-,- 4 2 Calculation of thermal resistances 2.1 Thermal resistance of the constituent parts of a cable, T1, T2 and T3 This clause gives the formulae for calculating the ther
47、mal resistances per unit length of the different parts of the cable T1, T2 and T3 (see 1.4 of part 1). The thermal resistivities of materials used for insulation and for protective coverings are given in Table 1. Where screening layers are present, for thermal calculations metallic tapes are conside
48、red to be part of the conductor or sheath while semi-conducting layers (including metallized carbon paper tapes) are considered as part of the insulation. The appropriate component dimensions must be modified accordingly. 2.1.1 Thermal resistance between one conductor and sheath T1 2.1.1.1 Single-core cables The thermal resistance between one conductor and the sheath T1 is given by: NOTEFor corru