The standard uses a three-step approach to determine the permissible current ( IEC Webstore Calculate the Adiabatic Short-Circuit Current ( cap I sub cap A cap D end-sub
The primary application of IEC 949 is to ensure coordination between the cable thermal capacity and the protective device (circuit breaker or fuse) characteristics. Engineers must verify that the "let-through energy" ($I^2t$) of the protective device is less than the "withstand energy" of the cable. iec 949 pdf work
The work does not end with the adiabatic equation. IEC 60949 explicitly states that the adiabatic assumption is only valid for very short fault durations (typically less than 0.1 seconds). For longer faults, heat dissipation becomes significant, and one must turn to the more complex (Non-adiabatic calculations). However, the adiabatic method remains the industry standard for LV (Low Voltage) installations because it provides a conservative, safe, and computationally simple result. The standard uses a three-step approach to determine
The non-adiabatic method assumes the conductor starts at its rated operating temperature (e.g., 90°C for XLPE). If the cable was pre-loaded at 105% before the fault, recalculate. IEC 60949 explicitly states that the adiabatic assumption
– Multiply the adiabatic result by the modifying factor to get the final permissible current. Key Technical Parameters