Non-adiabatic effects


As mentioned, the adiabatic equation assumes no heat is dissipated from the cable during a fault.  While putting the calculation on the safe side, in some situations, particularly for longer fault duration there is the potential to be able to get away with a smaller cross section.  In these instances, it is possible to do a more accurate calculation.

Considering non-adiabatic effects is more complex.  Unless there is some driver, using the adiabatic equations is just easier.  Software is available to consider non adiabatic effects, however, there is a cost, time and complexity associated with this.

The IEC also publish a standard which deals with non-adiabatic equations:

  • IEC 60949 "Calculation of thermally permissible short-circuit current, taking into account non-adiabatic heating effects".

 

The method adopted by IEC 60949 is to use the adiabatic equation and apply a factor to cater for the non-adiabatic effects:

  I=ε I AD

where I - permissible short circuit current, A (or kA)
IAD - adiabatic calculated permissible short circuit current, A (or kA)
ε - factor to allow for heat dissipation from cable

The bulk of the IEC 60949 standard is concerned with the calculation of ε.

Comments

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Shadi Abd Al Azim Feb 06, 2021 8:05 PM
Regarding copper wire screen, which factor i have to use 0.5 or 0.7?
which is better for short circuit rating copper wire screen and followed by tapes or covered by PVC?
what do IEC mean in sub clause 5.3.1 "the screen wires are fully surrounded by non—metallic materials."?
Steven McFadyen Feb 07, 2021 1:03 PM
Good point. The IEC clause is not overly clear.

I would interpret 5.3.1 as each screen wire individually surrounded by insulation.  Whereas in 5.3.2, the between-wire insolation is achieved by an air-gap.

Choose 0.7 or 0.5 depending on the situation you have in your cable.