The standard uses a two-step approach to find the permissible current ( ): Assume no heat escape. Apply a Non-Adiabatic Factor ( ): A modifying factor that accounts for heat loss.
For engineers performing these calculations, the standard defines several critical variables: Initial Temperature ( theta sub i The temperature of the conductor before the fault (e.g., 90 raised to the composed with power C for XLPE). Final Temperature ( theta sub f
ϵ=1+XtS+Y(tS)epsilon equals the square root of 1 plus cap X the square root of the fraction with numerator t and denominator cap S end-fraction end-root plus cap Y open paren the fraction with numerator t and denominator cap S end-fraction close paren end-root Where variables and iec 949 pdf work
While a powerful tool, IEC 60949 is not without its challenges, which you should be aware of in your work:
IEC 60949 introduces the . It accounts for the fact that even during a brief short-circuit, a portion of the heat flows outward from the conductor into the surrounding materials (like the insulation, sheath, or armour). The standard uses a two-step approach to find
The standard follows a structured approach to calculate the permissible short-circuit current ( Ikcap I sub k A. Calculate Adiabatic Short-Circuit Current ( Iadcap I sub a d end-sub First, determine the current assuming all heat is retained:
Before calculating heat loss, you must establish the baseline adiabatic current rating ( IADcap I sub cap A cap D end-sub Final Temperature ( theta sub f ϵ=1+XtS+Y(tS)epsilon equals
By understanding the mathematics and application of this document, power systems engineers can avoid cable failure and optimize conductor sizing. 📘 Core Principles of IEC 60949
[ \epsilon = 1 + \frac\alpha\beta \cdot \left(1 - e^-\beta \cdot t\right) ]
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