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 (100 % load factor) just sufficient to produce asymptotically the maximum conductor temperature, the surrounding ambient conditions being assumed constant.
This document provides formulae for thermal resistance.
The formulae given are essentially literal and designedly leave open the selection of certain important parameters. These can be divided into three groups:
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— 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;
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— parameters related to the surrounding conditions which can vary widely, the selection of which depends on the country in which the cables are used or will be used;
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— 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 document 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 can be used. Guidance on the use of finite element methods for calculating cable current ratings is given in IEC TR 62095.
A Commented Version (CMV) of this standard is available here. Please note, the CMV of the standard is not adopted, and therefore doesn’t contain an additional national foreword, or national annex that might be contained in the BS adoption of the standard. However the IEC text that forms the content of the standard is identical to the BS adoption of the standard.
Commented Versions (CMVs) are specialized publications featuring detailed commentary from technical experts on the major changes between the new edition of a standard and the previous one. Comments may include an explanation for the changes as well as information on the impact of these changes on the application and usability of the standard.