IEC 60724 pdf download – Short-circuit temperature limits of electric cables with rated voltages of 1 kV (U m = 1,2 kV) and 3 kV (U m = 3,6 kV)

IEC 60724 pdf download – Short-circuit temperature limits of electric cables with rated voltages of 1 kV (U m = 1,2 kV) and 3 kV (U m = 3,6 kV)

IEC 60724 pdf download – Short-circuit temperature limits of electric cables with rated voltages of 1 kV (U m = 1,2 kV) and 3 kV (U m = 3,6 kV)
1 scope
This International Standard gives guidance on the short-circuit maximum temperature limits ofelectric cables having rated voltages of 1 kV(Um = 1,2 kV) and 3 kv(um = 3,6 kV), with regardto 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 temperaturelimits are taken into consideration.
The calculation of the permissible short-circuit current in the current-carrying components ofthe cable should be carried out in accordance with lEC 60949.
2Normative references
The following referenced documents are indispensable for the application of this document.Fordated references, only the edition cited applies. For undated references, the latest edition ofthe referenced document (including any amendments) applies.
IEC 60055(all parts),Paper-insulated metal-sheathed cables for rated voltages up to 18/30 kV(with copper or aluminium conductors and excluding gas-pressure and oil-filled cables)
IEC 60502-1:1998,Power cables with extruded insulation and their accessories for ratedvoltages from 1 kV(um =1,2 kV) up to 30 kV(um = 36 kV) – Part 1: Cables for rated voltagesof 1 kv (um = 1,2 kv)and 3 kv (um = 3,6 kV)
IEC 60949:1988,Calculation of thermally permissible short-circuit current,taking into accountthe non-adiabatic heating effects
3 Factors governing the application of the temperature limits
The short-circuit temperatures given in clause 4 are the actual temperatures of the current-carrying component as limited by the adjacent material in the cable and are valid for short-circuit durations of up to 5 s. When calculating the allowable short-circuit current,thesetemperatures will be obtained if heat loss into the insulation during the short-circuit is takeninto account (non-adiabatic heating). lf heat loss during the short-circuit is neglected (adiabaticheating), the calculations give short-circuit currents that are on the safe side.
NOTE The temperature limits given in clause 4 should also not be exceeded with repeated short-circuits occurringin a short time.
The 5 s time period mentioned is the limit for the temperatures quoted to be valid and not forthe application of the adiabatic calculation method.The time limit for the use of the adiabaticmethod has a different definition,being a function of both the short-circuit duration and thecross-sectional area of the current-carrying component.This is dealt with in IEC 60949.
Caution may be needed when using the conductor temperatures specified when the cables aresheathed with a lower temperature material,especially for cables with conductor cross-sectional areas of 1 000 mm2 and above.This is because the high thermal time constant ofthese cables will cause the oversheath to attain high temperatures for longer times. In addition,the high mechanical forces could result in insulation deformation. Nevertheless, it should bestressed that for conductor cross-sectional areas above 1 000 mm2 the permissible short-circuit current is so high that it is not normally attained in common systems.
Where other temperature limits are known with certainty to be more appropriate for thematerials or the cable design, then these may be used.
3.2.1 Paper insulated cables (mass-impregnated cables according to lEc 60055)
The temperature limits for paper insulated cables impregnated with oil/resin or non-drainingcompound are imposed by the tendency to compound migration and void formations. All paperinsulated cables are also limited by thermal degradation of the cable components and bypossible tearing of paper tapes due to movement of the cores.
3.2.2Polymeric insulated cables (according to lEC 60502-1)
For thermoplastic insulating materials,the temperature limits should be applied with cautionwhen the cables are either directly buried or securely clamped when in air.Local pressure dueto clamping or the use of an installation radius less than that specified for the cable, especiallyfor cables that are rigidly restrained,can lead to high deforming forces under short-circuitconditions. Where these conditions cannot be avoided it is suggested that the limit be reducedby 10 c.
Attention should be given to the design and installation of joints and terminations if the short-circuit limits set out in this standard are to be safely used. The following aspects are notexclusive and are provided for guidance only. It is desirable that the performance of anaccessory be considered in the context of the particular installation.
a) Longitudinal thrust in cable conductors can be considerable, depending on the degree of
lateral restraint imposed on the cable.Conductor stresses as high as 50 N/mm2 can easilyoccur. These forces may cause buckling of conductors and other damage in a joint ortermination.
b) Longitudinal tension in cable conductors is also to be expected after a short-circuit. This
tension may exist for a very long period, particularly if the cable is only partly loaded afterthe short-circuit. A minimum conductor stress of 40 N/mm2 should be used for designpurposes.