# WG 19 - TC 20/WG 19

## TC 20/WG 19

### General Information

IEC 60287-1-1:2023 is 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, 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 current ratings and losses. The formulae given are essentially literal and designedly leave open the selection of certain important parameters. These can 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 can vary widely, the selection of which depends on the country in which the cables are used or will 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).

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IEC 60287-1-3:2023 provides a method for calculating the phase currents and circulating current losses in single-core cables arranged in parallel. The method described in this document can be used for any number of cables per phase in parallel in any physical layout. The phase currents can be calculated for any arrangement of sheath bonding. For the calculation of sheath losses, it is assumed that the sheaths are bonded at both ends. A method for calculating sheath eddy current losses in two circuits in flat formation is given in IEC 60287-1-2.

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IEC 60287-2-1:2023 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.

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IEC 60287-1-2:2023 provides a method for calculating the eddy current losses in the metallic sheaths of single-core cables arranged as a three-phase double circuit in flat formation. The sheaths are bonded at one point or are cross-bonded so that there are no significant sheath circulating currents. Where metallic sheaths are bonded at both ends there are significant circulating currents which result in a lower current-carrying capacity. A method of calculating circulating current losses for double circuits is provided in IEC 60287-1-3.

The method descibed in this document provides coefficients which are applied as corrections to the loss factors for the sheaths of one isolated three-phase circuit. These corrections are negligible for cables where the parameter m is less than approximately 0,1 (m = ω/(Rs · 107)), which corresponds to a sheath longitudinal resistance higher than 314 µΩ/m at 50 Hz.

Consequently, the method is used for most sizes of aluminium-sheathed cables, but is not required for lead-sheathed cables unless they are unusually large.

The coefficients are provided in tabular form and have been computed from fundamental formulae for sheath losses, the evaluation of which calls for expertise in computer programming which will possibly not be readily available in general commercial situations. The development of simplified formulae for some of the tabulated coefficients is under consideration.

Losses for cables in a single circuit is covered in IEC 60287-1-1.

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IEC 60287-3-1:2017 is available as IEC 60287-3-1:2017 RLV which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.

IEC 60287-3-1:2017 is applicable to the conditions of steady-state operation of cables at all voltages, buried directly in the ground, in ducts, 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 defines site reference conditions, however the general values are superseded by specific national requirements. This edition includes the following significant technical changes with respect to the previous edition:

- the updated list of national laying conditions is now covered in Annex A;

- Clause 5 about the information required from the purchaser for the selection of the appropriate type of cable has been removed.

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IEC 60287-2-3:2017 describes a method for calculating the continuous current rating factor for cables of all voltages installed in ventilated tunnels. The method is applicable to any type of cable.

The method applies to natural as well as forced ventilation.

Longitudinal heat transfer within the cables and the surroundings of the tunnel is assumed to be negligible.

All cables are assumed to be identical within the tunnel and it is assumed that the tunnel cross-section does not change with distance along the tunnel.

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IEC 60287-2-1:2015 is available as IEC 60287-2-1:2015 RLV which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.

IEC 60287-2-1:2015 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 part of IEC 60287 provides formulae for thermal resistance. This second edition of IEC 60287-2-1 cancels and replaces the first edition, published in 1994, Amendment 1:2001, Amendment 2:2006 and Corrigendum 1:2008. This edition includes the following significant technical changes with respect to the previous edition:

a) inclusion of a reference to the use of finite element methods where analytical methods are not available for the calculation of external thermal resistance;

b) explanation about SL and SA type cables;

c) calculation method for T3 for unarmoured three-core cables with extruded insulation and individual copper tape screens on each core;

d) change of condition for X in 5.4;

e) inclusion of constants or installation conditions for water filled ducts in Table 4.

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IEC 60287-3-2:2012 sets out a method for the selection of a cable size taking into account the initial investments and the future costs of energy losses during the anticipated operational life of the cable. Matters such as maintenance, energy losses in forced cooling systems and time of day energy costs have not been included in this standard. Two examples of the application of the method to hypothetical supply systems are given in Annex A.

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IEC/TR 62602:2009, which is a technical report, specifies the nominal cross-sectional areas, in the range 20 AWG to 2 000 kcmil (0,52 mm2 to 1 010 mm2), for conductors in electric power cables and cords for a wide range of types. Requirements for numbers and sizes of wires and resistance values are also included. These conductors include solid and stranded copper, aluminium and aluminium alloy conductors in cables for fixedinstallations and flexible copper conductors. This technical report is not intended to apply to conductors designed for use in cables intended for telecommunication or data transmission, winding wires or similar products. Unless indicated to the contrary in a particular clause, this technical report relates to conductors in finished cables and not to conductors made or supplied for inclusion into a cable. The annexes give supplementary information covering measurement of resistance (Annex A), temperature correction factors for resistance measurement (Annex B) and dimensional limits of circular conductors (Annex C).

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Gives guidance on the short-circuit maximum temperature limits of electric cables having rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV), with regard to the following: - insulating materials; - oversheath and bedding materials; - conductor and metallic sheath materials and methods of connection. The design of accessories and the influence of the installation conditions on the temperature limits are taken into consideration.

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Gives guidance on the short-circuit maximum temperature limits of electric cables having rated voltages above 30 kV (Um = 36 kV) up to the voltage limits given in IEC 60141 and IEC 60840, with regard to the following: - insulating materials; - oversheath and bedding materials; - conductor and metallic sheath materials and methods of connection. The design of accessories and the influence of the installation conditions on the temperature limits are taken into consideration. The calculation of the permissible short-circuit current in the current-carrying components of the cable should be carried out in accordance with IEC 60949.

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Gives guidance on the short-circuit maximum temperature limits of electric cables having rated voltages of 1kV (Um = 1,2 kV) and 3 kV (Um = 3,6 kV), with regard to the following: - insulating materials; - oversheath and bedding materials; - conductor and metallic sheath materials and methods of connection. The design and accessories and the influence of the installation conditions on the temperature limits are taken into consideration.

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Describes a method for calculating the continuous current rating factor for cables of all voltages where crossings of external heat sources are involved. The method is applicable to any type of cable. The method assumes that the entire region surrounding a cable, or cables, has uniform thermal characteristics and that the principle of superposition applies. The principle of superposition does not strictly apply to touching cables and hence the calculation method set out in this standard will produce an optimistic result if applied to touching cables

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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, 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. Provides formulae for current ratings and losses. 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).

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Specifies the nominal cross-sectional areas, in the range 0,5 mm2 to 2 500 mm2, for conductors in electric power cables and cords of a wide range of types. Requirements for numbers and sizes of wires and resistance values are also included. These conductors include solid and stranded copper, aluminium and aluminium alloy conductors in cables for fixed installations and flexible copper conductors. The standard does not apply to conductors for telecommunication purposes. The applicability of this standard to a particular type of cable is as specified in the standard for the type of cable. Unless indicated to the contrary in a particular clause, this standard relates to the conductors in the finished cable and not to the conductor as made or supplied for inclusion into a cable. Informative annexes are included giving supplementary information covering temperature correction factors for resistance measurement (Annex B) and dimensional limits of circular conductors (Annex C). The principal changes with respect to the previous edition are as follows: a) the consolidation of material from IEC 60228A; b) addition of a definition for nominal cross-sectional area; c) an increase in the range of conductor sizes in Tables 1 and 2; d) addition of a note that solid aluminum alloy conductors, having the same dimensions as aluminum conductors, will have a higher resistance; e) strengthening of the recommendations for dimensional limits of compacted stranded copper conductors.

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The most important tasks in cable current rating calculations are the determination of the conductor temperature for a given current loading or, conversely, the determination of the tolerable load current for a given conductor temperature. In order to perform these tasks the heat generated within the cable and the rate of its dissipation away from the conductor, for a given conductor material and given load, must be calculated. The ability of the surrounding medium to dissipate heat plays a very important role in these determinations and varies widely because of factors such as soil composition, moisture content, ambient temperature and wind conditions. The heat is transferred through the cable and its surroundings in several ways. For underground installations the heat is transferred by conduction from the conductor, insulation, screens and other metallic parts. It is possible to quantify the heat transfer processes in terms of the appropriate heat transfer equation as shown in Annex A (equation A.1). Current rating calculations for power cables require a solution of the heat transfer equations which define a functional relationship between the conductor current and the temperature within the cable and its surroundings. The challenge in solving these equations analytically often stems from the difficulty of computing the temperature distribution in the soil surrounding the cable. An analytical solution can be obtained when a cable is represented as a line source placed in an infinite homogenous surrounding medium. Since this is not a practical assumption for cable installations, another assumption is often used; namely, that the earth surface is an isotherm. In practical cases, the depth of burial of the cables is in the order of ten times their external diameter, and for the usual temperature range reached by such cables, the assumption of an isothermal earth surface is a reasonable one. In cases where this hypothesis does not hold; namely, for large cable diameters and cables located close to the ground surface, a correction to the solution has to be used or numerical methods should be applied. With the isothermal surface boundary, the steady-state heat conduction equations can be solved assuming that the cable is located in a uniform semi-infinite medium. Methods of solving the heat conduction equations are described in IEC 60287 (steady-state conditions) and IEC 60853 (cyclic conditions), for most practical applications. When these methods cannot be applied, the heat conduction equations can be solved using numerical approaches. One such approach, particularly suitable for the analysis of underground cables, is the finite element method presented in this document.

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Provides a method for calculating the phase currents and circulating current losses in single-core cables arranged in parallel. The method described in this standard can be used for any number of cables per phase in parallel in any physical layout. The phase currents can be calculated for any arrangement of sheath bonding. For the calculation of sheath losses, it is assumed that the sheaths are bonded at both ends. A method for calculating sheath eddy current losses in two circuits in flat formation is given in IEC 60287-1-2.

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Gives a method for calculating the cyclic rating factor, for cables of all voltages, where partial drying out of the surrounding soil is anticipated. The method is based on one of the three methods published in a CIGRE document. The method is applicable to any type of cable, but it is recommended that it should be applied only to installations of one multi-core cable or to three single-core cables or to groups of circuits where the intercircuit spacing is sufficient to permit free vertical movement of soil moisture between the zones of dry soil associated with each circuit. This Standard does not preclude the use of other methods of calculation where full details of the load cycle are not known. The method assumes that the entire region surrounding a cable or cables has uniform thermal characteristics prior to drying out; the only non-uniformity being that caused by drying. As a consequence the method should not be applied, without further consideration, to installations where special backfills, having properties different from the site soil, are used.

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Establishes suitable procedures for commissioning, operating and maintaining hydraulic turbines and associated equipment. Applies to impulse and reaction turbines of all types, and especially to large turbines directly coupled to electric generators. Also applies to pump-turbines when operating as turbines, and water conduits, gates, valves, drainage pumps, cooling-water equipment, generators, etc., where they cannot be separated from the turbine and its equipment.

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