The core equation outlined in the standard represents a precise thermal equilibrium. It relates the root-mean-square (RMS) short-circuit current to the specific thermal limitations of the conductor material and its insulation type:
This guide breaks down the standard's purpose, methodology, and application so you can apply the calculations without needing to decipher the technical jargon of the original document immediately.
"Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects" iec 949 pdf
: Allowing for slightly smaller (and less expensive) conductors or screens where heat dissipation is significant. Safety Compliance
Unlike standard adiabatic calculations—which assume all heat remains within the conductor—this standard accounts for , meaning it factors in the heat that dissipates into surrounding materials (like insulation and sheaths) during a fault. Key Features of the Standard The core equation outlined in the standard represents
The original document, IEC 949 (1988) – "Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects" – was officially renumbered as IEC 60949 in 1997.
Before diving into the technical details, it is important to clarify the naming. The International Electrotechnical Commission (IEC) updates its numbering system periodically. The original standard was filed under a numeric code that engineers colloquially shortened to "949." Today, the full designation is . heat escapes into the insulation.
Because manual non-adiabatic calculations are highly iterative and complex, power system engineers rarely calculate them by hand. The formulas outlined in the IEC 949 PDF are natively integrated into industry-standard electrical simulation software, including: (Cable Thermal Analysis modules) DigSILENT PowerFactory CYME Power Engineering Software Summary for Power Engineers
Validating bare conductors, grounding wires, and structural busbars against thermal degradation. Adiabatic vs. Non-Adiabatic Heating
Because the conductor is large (300 $mm^2$) and the duration is 1 second, heat escapes into the insulation. Let's say the calculation yields $\epsilon = 1.12$.
In standard cable calculations, engineers often assume an . This means the short circuit happens so fast (under 1 second) that 100% of the heat stays trapped inside the metal conductor. Zero heat escapes into the surrounding insulation or air.