BS IEC 60287-2-1 pdf download.Electric cables – Calculation of the current rating – Part 2-1: Thermal resistance – Calculation of thermal resistance
1 Scope
This part 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 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 a continuous constant current (1 00 % load factor) just sufficient to produce asymptotically the maximum conductor temperature, the surrounding ambient conditions being assumed constant. This part of IEC 60287 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 example, 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; – parameters which result from an agreement between manufacturer and user and which involve a margin for security of service (for example, maximum conductor temperature). Equations given in this part of IEC 60287 for calculating the external thermal resistance of a cable buried directly in the ground or in a buried duct are for a limited number of installation conditions. Where analytical methods are not available for calculation of external thermal resistance finite element methods may be used. Guidance on the use of finite element methods for calculating cable current ratings is given in IEC TR 62095.
4 Calculation of thermal resistances
4.1 Thermal resistance of the constituent parts of a cable, T 1 , T 2 and T 3 4.1 .1 General Clause 4 gives the formulae for calculating the thermal resistances per unit length of the different parts of the cable T 1 , T 2 and T 3 (see 1 .4 of IEC 60287-1 -1 :2006 and IEC 60287-1 -1 :2006/AMD1 :201 4). 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 considered 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 shall be modified accordingly. 4.1 .2 Thermal resistance between one conductor and sheath T 1 NOTE For paper-insulated cables ρ f = ρ i and, hence, the second term on the right hand side of the above equation can be ignored. For cables with extruded insulation, the thermal resistivity of the filler material is likely to be between 6 K∙m/W and 1 3 K∙m/W, depending on the filler material and its compaction. A value of 1 0 K∙m/W is suggested for fibrous polypropylene fillers. The above equation is applicable to cables with extruded insulation where each core has an individual screen of spaced wires and to cables with a common metallic screen over all three cores. For unarmoured cables of this design t 1 is taken to be the thickness of the material between the conductors and outer covering (serving).
4.1 .5 Pipe-type cables For these three-core cables, we have: a) The thermal resistance T 1 of the insulation of each core between the conductor and the screen. This is calculated by the method set out in 4.1 .2 for single-core cables. b) The thermal resistance T 2 is made up of two parts: 1 ) The thermal resistance of any serving over the screen or sheath of each core. The value to be substituted for part of T 2 in the rating equation of 1 .4 of IEC 60287-1 -1 :2006 and IEC 60287-1 -1 :2006/AMD1 :201 4 is the value per cable, i.e. the value for a three-core cable is one-third the value of a single core. The value per core is calculated by the method given in 4.1 .3 for the bedding of single- core cables. For oval cores, the geometric mean of the major and minor diameter d ⋅ shall be used in place of the diameter for a circular core assembly. M d m 2) The thermal resistance of the gas or oil between the surface of the cores and the pipe. This resistance is calculated in the same way as that part of T 4 which is between a cable and the internal surface of a duct, as given in 4.2.7.2. The value calculated will be per cable and should be added to the quantity calculated in 4.1 .5 b)1 ) above, before substituting for T 2 in the rating equation of 1 .4 of IEC 60287-1 -1 :2006 and IEC 60287-1 -1 :2006/AMD1 :201 4. c) The thermal resistance T 3 of any external covering on the pipe is dealt with as in 4.1 .4. The thermal resistance of the metallic pipe itself is negligible.
BS IEC 60287-2-1 pdf download
