IEC 60853-3:2002 pdf download

IEC 60853-3:2002 pdf download.Calculation of the cyclic and emergencycurrent rating of cables – Part 3: yclic rating factor for cables of all voltages,with partial drying of the soil.
4 Description of method
4.1 General descrIption
This method is an extension of the techniques used in IEC 60853-1 and IEC 60853-2 for calculating cyclic rating factors for a cable in uniform soil.
The method relies on a knowledge of the soil critical temperature; this is the temperature at which drying out of the soil takes place rapidly.
NOTE In the absence 01 more precise information, this temperature may be taken as 50 °C.
The size of the dry zone changes substantially with critical temperature. For a given conductor temperature the size of the dry zone will have a significant effect on the steady- state rating but the cyclic factor multiplying that steady-state rating is largely unaffected.
It is assumed initially that the soil critical temperature is equal to the peak cyclic value of the cable surface temperature, i.e. a dry zone is on the point of occurring, so that the soil surrounding the cable has uniform properties appropriate to its wet, on-site, state. The cyclic rating factor for these conditions is derived by the methods already given in IEC 60853-1 or
IEC 60853-2.
This factor is then adjusted so that it applies to the steady-state rating for the same assumed value of critical temperature, when there will be a dried-out zone. This adjustment is effected using formulae for the cable external thermal resistance appropriate to the steady-state procedure.
Such a factor can then be used to multiply the steady-state rating for any other soil critical temperature in order to obtain the appropriate permissible peak current.
4.2 Principles
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 and IEC 60853.2 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, IEC 60853-1 and IEC 60853-2, 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.
The computation of a cyclic rating factor, for any critical temperature, is effected as follows:
— the thermal response of the cable, including the effect of its internal thermal capacitance, is obtained by the use of appropriate formulae from 4.2.1, 4.2.2 and 4.2.3 of IEC 60853-2:
— the thermal response of the soil, and the effect of adjacent cables in a group (if any), are obtained by the use of formulae from clauses 5 to 7 of IEC 60853-2:
— the effect of drying out is obtained by the use of 4.3.2.
4.3 Formulae
4.3.1 Without drying out
There are optional formulae for a cyclic rating factor in clause 5 of IEC 60853-2 but the general one is as follows.