= Initial operating temperature of the conductor before the fault ( ∘Craised to the composed with power C θftheta sub f = Maximum permitted final temperature of the conductor ( ∘Craised to the composed with power C = Conductor material constant = Material-dependent reciprocal temperature coefficient Step 2: The Non-Adiabatic Modifying Factor ( IEC 60949:1988
While IEC 949 focuses on the adiabatic method, it acknowledges that for longer fault durations or specific installations, some heat dissipation does occur.
Iad=k⋅S⋅1tcap I sub a d end-sub equals k center dot cap S center dot the square root of 1 over t end-fraction end-root
The standard follows a three-step approach to determine the permissible current: : This assumes all heat is retained in the conductor.
The duration of the short circuit, typically capped at 5 seconds for these formulas. Liban Cables Resources for IEC 60949 Official Standard: You can purchase the current edition from the IEC Webstore Technical Summaries: Sites like CableDatasheet
) and volumetric heat capacities for common materials like copper, aluminum, lead, and steel. Permissible Limits:
Unlike simpler methods that assume no heat escapes the conductor (adiabatic), this standard provides a method to account for , meaning it considers heat transfer to surrounding materials like insulation or armor. Core Calculation Principle
Reduce cost by accurately calculating that a smaller, cheaper cable is adequate, rather than over-sizing based on purely adiabatic assumptions.
If the cross-sectional area of these components is too small, the temperature will exceed the thermal threshold of the insulation material (such as XLPE or PVC). For example, can safely handle a peak short-circuit temperature of 250°C , whereas Polyvinyl Chloride (PVC) degrades if it surpasses 140°C to 160°C . 2. Adiabatic vs. Non-Adiabatic Calculations
): This is the baseline calculation assuming no heat escapes the conductor. The formula typically used for this is:
): Use the standard thermodynamic constants for the conductor metal to find the base adiabatic rating. Compute the Non-Adiabatic Factor (
When a short circuit takes place, the current spikes way past the cable’s nominal operating limit. This electrical surge creates instantaneous heat inside the cable components, including: The phase conductors (Copper or Aluminum)
These limits ensure that the insulation does not degrade, ignite, or lose its dielectric properties during the fault.
Note: While the prompt mentions "IEC 949," the correct designation for the current standard regarding the calculation of thermally permissible short-circuit currents, specifically regarding the adiabatic method, is . (The older reference "IEC 949" is largely obsolete and has been superseded by the 60949 series). This essay addresses the practical and theoretical work surrounding that standard and its PDF documentation.
In electrical power systems, the design of cable infrastructure must account for both steady-state load conditions and transient fault conditions. While cables are sized based on continuous current ratings (ampacity) to prevent overheating during normal operations, they must also possess sufficient thermal capacity to withstand the immense energy dissipated during a short-circuit event.