# 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.

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:

$I=\epsilon \text{}{I}_{AD}$

*where I* - permissible short circuit current, A (or kA)

* I _{AD}* - 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*.