The thermal withstand capability of a power cable during short-circuit conditions is one of the most important design considerations. During a fault, an extremely high current flows through the conductor and metallic sheath of the cable for a short duration. This fault current produces intense heat. If the temperature exceeds the permissible thermal limit, permanent damage may occur to conductor, insulation or sheath of the cable.

To prevent such damage, IEC 60949 provides a standard method for calculating the permissible short-circuit current of cables.

Principle of Calculation IEC 60949

During traditional short-circuit calculations we assume that all heat generated during the fault remains inside the conductor. In other words no heat transfer occurs to insulation or surrounding materials. We refer this assumption as the adiabatic condition.

However, during an actual short circuit some heat flows into adjacent insulation and other layers. This heat transfer cools the conductor little bit faster. Therefor, this cooling effect slightly increases the permissible short-circuit current. Thus, the standard IEC 60949 introduces a non-adiabatic correction factor.

Steps of the IEC 60949 calculation procedure

First we calculate the adiabatic short-circuit current of the conducting portion of the cable. Obviously, the conducting portion means the central conductor and metallic sheath.

Second, we can calculate the non-adiabatic correction factor. Although, many manufacturers consider it as optional. Then, we multiply non-adiabatic correction factor with adiabatic short-circuit current to obtain the more accurate short-circuit current

The standard equation is $I=\varepsilon I_{AD}$

Where, $I$ is permissible short-circuit current, $\varepsilon$ is non-adiabatic correction factor and $I_{AD}$ is the adiabatic short-circuit current.

Adiabatic Short-Circuit Current Formula

IEC 60949 gives the fundamental thermal equation as,

$I_{AD}^2 t=K^2S^2\ln\left(\frac{\theta_f+\beta}{\theta_i+\beta}\right)$

Where,

• I_AD​ = Adiabatic short-circuit current (A)
• $t$ = Fault duration (s)
• $K$ = Material constant
• $S$ = Cross-sectional area (mm²)
• $\theta_i$​ = Initial conductor temperature (°C)
• $\theta_f$​ = Final permissible temperature (°C)
• $\beta$ = Reciprocal of temperature coefficient

Material Constants from IEC 60949

IEC 60949 provides material constants for conductors and metallic sheaths.

Material	K	β
Copper	226	234.5
Aluminium	148	228
Lead	41	230
Steel	78	202

Simplified Adiabatic Short-Circuit Current Formula and K Values from IEC 60364

For fast engineering calculations, we can also use a simplified equation with below relevant K values. The value comes out from this equation matches the actual detailed IEC equation result,

$$I=\frac{KS}{\sqrt{t}}$$

Where, for conductors,

Insulation Type	K Value for Copper	K Value for Aluminium
PVC	115	76
XLPE / EPR	143	94

For metallic sheaths

Metallic Sheath Material	K Value
Copper	128
Aluminium	85
Lead	41
Steel	52
Bronze	180

However, we can use both the equations for calculating short -circuit current of conductor and sheath portion. We always recommend to use the first detailed equation. That is the equation

$I_{AD}^2 t=K^2S^2\ln\left(\frac{\theta_f+\beta}{\theta_i+\beta}\right)$

Non-Adiabatic Correction Factor

IEC 60949 provides the following simplified equation for conductors:

$\varepsilon=\sqrt{1+X\sqrt{\frac{t}{S}}+Y\left(\frac{t}{S}\right)}$

Where, $X$ and $Y$ depend on insulation type, $S$ is conductor area, and $t$ is the fault duration. For XLPE insulated conductors,

Conductor Material	X	Y
Copper	0.41	0.12
Aluminium	0.57	0.16