IEC 60949 (previously numbered IEC 949), titled "Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects," provides the methodology for determining the maximum short-circuit current a cable can withstand without sustaining permanent damage. This standard is essential for protection engineers who must ensure that protective devices (circuit breakers and fuses) operate faster than the time it takes for the cable to reach its temperature limit.
The IEC 60949 approach follows a structured three-step process to determine the final permissible current: iec 949 pdf work
The adiabatic formula used in the IEC 60949 Standard is defined as: IEC 60949 (previously numbered IEC 949), titled "Calculation
: Calculate a factor that represents the non-adiabatic (heat-dissipating) properties of the specific cable environment. Final Calculation : Multiply the two together ( ) to get the true thermally permissible current. IEC Webstore Key Technical Parameters Calculations within the IEC 60949 framework rely on several variables: Final Calculation : Multiply the two together (
The core principle of IEC 949 is the . A short-circuit event typically occurs over a very short duration (milliseconds to a few seconds). During this brief period, the heat generated within the conductor ($I^2R$ losses) does not have sufficient time to dissipate into the insulation or surrounding environment. Therefore, the standard assumes all heat generated is retained within the conductor, causing an instantaneous temperature rise. This provides a conservative "worst-case" scenario for the conductor temperature.