The full text, available in the or Scribd versions , provides essential data tables and formulas:
: This method assumes no heat is lost to the surrounding insulation during the short circuit. It uses a simplified formula for quick estimations: : Permissible short-circuit current (A). : Cross-sectional area of the conductor ( mm2m m squared : Duration of the short circuit (s). : Constant depending on the material's thermal properties.
IEC 60949, titled "Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects" , defines how much current a cable can withstand during a short circuit before the insulation fails due to excess heat. iec 949 pdf
: Factor for the conductor material (e.g., 143 for copper/XLPE, 94 for aluminum/XLPE)
The standard uses a specific formula to calculate the permissible adiabatic short-circuit current ( cap I sub cap A cap D end-sub The full text, available in the or Scribd
: Tables containing specific heat capacities and resistivities for conductors (copper, aluminum) and sheaths (lead, steel, bronze).
IEC 949 remains a cornerstone standard for electrical power system design. By moving away from purely conservative adiabatic assumptions, it allows engineers to optimize cable selection, reducing material costs while maintaining rigid safety profiles. : Constant depending on the material's thermal properties
A: The standard is primarily intended for AC systems (50/60 Hz). For DC traction systems or battery banks, refer to IEC 61660-1.
The IEC 949 PDF provides formulas and factors (such as the ε factor) to adjust short-circuit current ratings based on real heat dissipation. This allows engineers to use slightly smaller, more cost-effective cables without sacrificing safety, provided the fault duration is long enough for heat to leave the conductor.
In a standard adiabatic calculation, engineers assume that the short circuit happens so rapidly (typically under 5 seconds) that . It assumes zero heat escapes into the surrounding insulation, screens, armour, or ambient air. While safe and conservative, this method often overestimates the final temperature rise, leading engineers to specify overly thick, expensive cables. 2. The Non-Adiabatic Reality (IEC 949)