In general, the size of a dry zone where the boundary just achieves a certain critical temperature rise with cyclic loading is smaller than the zone which will form for the same critical temperature rise with steady-state loading. The situation where the cable surface temperature rise is just equal to the soil critical temperature, and a dry zone forms only with steady-state loading, is a particular case. However, a rating factor determined for this latter case is applicable to the steady-state rating with any other critical temperature rise and size of dry zone.
One further step is necessary in order to use a cyclic factor based on a critical temperature equal to the cable surface temperature. Because of the nature of the computation used in IEC 60853-1/2 to derive cyclic rating factors, the value of the factor obtained assumes that the cable external thermal resistance is the same for both cyclic and steady- state loading. The correct value of peak current for the load cycle is obtained when the cyclic factor multiplies the steady-state rating for this value of resistance.
While this equality of external thermal resistance applies for the uniform non-migration conditions assumed in IEC 60287-2-1 and IEC 60853-1/2, it is not so when drying can take place. The size of the dry zone, and hence the cable external thermal resistance, changes with the type of loading. In the latter case the rating factor has to be adjusted so that it can be used to multiply the rating for the higher external thermal resistance occurring with the steady-state. Such an adjustment can be made by using the ratio of the appropriate external thermal resistances.
