AMENDMENT 1 Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects
|Publication Date:||1 September 2008|
|ICS Code (Cables):||29.060.20|
|ICS Code (Electrical engineering in general):||29.020|
The method of calculating the short-circuit rating of any current carrying component of a cable has generally been based on the assumption that the heat is retained inside the current carrying component for the duration of the short-circuit (i.e. adiabatic heating). However, there is some heat transfer into the adjacent materials during the short-circuit and advantage can be taken of this. This standard gives a simple method for incorporating this non-adiabatic heating effect when calculating short-circuit ratings so that the same short-circuit ratings are obtained by different designers. It is recognized that more sophisticated computer methods are available but these do not significantly affect the accuracy and are considered too complex to be standardized.
The formulae contain quantities which vary with the materials used in the cables. Values are given in the tables ; these values are either internationally standardized, for example electric resistivities and resistance temperature coefficients, or those which are generally accepted in practice, for example specific heats.
In order that uniform and comparable results may be obtained, the short-circuit ratings shall be calculated using the method and the values given in this standard. However, where it is known with certainty that other values of material constants are more appropriate then these may be used, and the corresponding short-circuit rating declared in addition, provided that the different values are quoted.
In this standard the worst case conditions have been assumed and the short-circuit ratings will be pessimistic.
The non-adiabatic method is valid for all short-circuit durations. When compared to the adiabatic method it will provide significant increases of the permissible short-circuit currents in the case of screens, sheaths, and possibly small conductors of <10mm2 (especially when used as screen wires). For the usual range of power cable conductors 5% is the minimum increase in short-circuit current that would be useful in practice, so that for ratios of short-circuit duration to conductor cross-sectional area of < 0.1 s/mm2 the improvement in current is negligible and the adiabatic method may be used. This covers the majority of practical situations.
The approach set out in this standard is to:
a) calculate the adiabatic short-circuit current,
b) calculate a modifying factor that takes account of the non-adiabatic heating effect,
c) multiply a) and b) to obtain the permissible short-circuit current.