IEC 60724 pdf – 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)
The 5 s time period mentioned is the limit for the temperatures quoted to be valid and not for the application of the adiabatic calculation method. The time limit for the use of the adiabatic method has a different definition, being a function of both the short-circuit duration and the cross-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 are sheathed with a lower temperature material, especially for cables with conductor cross- sectional areas of 1 000 mm 2 and above. This is because the high thermal time constant of these 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 be stressed that for conductor cross-sectional areas above 1 000 mm 2 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 the materials or the cable design, then these may be used.
3.2.1 Paper insulated cables (mass-impregnated cables according to IEC 60055) The temperature limits for paper insulated cables impregnated with oil/resin or non-draining compound are imposed by the tendency to compound migration and void formations. All paper insulated cables are also limited by thermal degradation of the cable components and by possible tearing of paper tapes due to movement of the cores. 3.2.2 Polymeric insulated cables (according to IEC 60502-1) For thermoplastic insulating materials, the temperature limits should be applied with caution when the cables are either directly buried or securely clamped when in air. Local pressure due to clamping or the use of an installation radius less than that specified for the cable, especially for cables that are rigidly restrained, can lead to high deforming forces under short-circuit conditions. Where these conditions cannot be avoided it is suggested that the limit be reduced by 1 0 °C.
3.3 Accessories 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 not exclusive and are provided for guidance only. It is desirable that the performance of an accessory 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/mm 2 can easily occur. These forces may cause buckling of conductors and other damage in a joint or termination.
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 after the short-circuit. A minimum conductor stress of 40 N/mm 2 should be used for design purposes.
c) With impregnated paper cables, compound expansion can give rise to considerable fluid pressure. If compound leaks out at joints and terminations, it could cause softening of the bitumen filling. Moisture may also be drawn back into the accessory and cable in a sufficient quantity to affect the performance of the insulation.
d) The use of a temperature limit only implies that any combination of current and time which produces temperatures not exceeding that limit is permissible. For short-circuit currents this is not sufficient. An additional limit should be set for the peak value of the current in order to avoid excessive electromagnetic forces. These forces are of particular importance at terminations and proper support is necessary to avoid undesirable movement and damage.