During a fault, a conductor must carry the fault current for as long as the protection takes to clear it without its insulation exceeding the permitted short-circuit temperature. The adiabatic method provides a quick check by assuming the fault clears too fast for any heat to leave the conductor, which makes the result conservative and safe to design against.
The relationship is I²t = k²S², or rearranged, the withstand current for a given time is I = k × S ÷ √t, where S is the conductor cross-section in mm², t is the fault duration in seconds, and k is a constant set by the conductor metal and the insulation. The standard k factors (per IEC) are:
| Conductor and insulation | k |
|---|---|
| Copper, XLPE | 143 |
| Copper, PVC | 115 |
| Aluminium, XLPE | 94 |
| Aluminium, PVC | 76 |
XLPE outperforms PVC because it tolerates a higher short-circuit temperature (250°C against PVC’s lower limit), so the same conductor withstands more fault energy. The one-second withstand current for common copper and aluminium XLPE sizes, taken directly from the formula, is:
| Size (mm²) | Copper XLPE, 1 s | Aluminium XLPE, 1 s |
|---|---|---|
| 16 | 2.3 kA | 1.5 kA |
| 25 | 3.6 kA | 2.4 kA |
| 50 | 7.2 kA | 4.7 kA |
| 95 | 13.6 kA | 8.9 kA |
| 185 | 26.5 kA | 17.4 kA |
| 300 | 42.9 kA | 28.2 kA |
For a different clearing time, divide by the square root of the time: a 0.2 second clearance permits a larger current (divide by √0.2, about 0.45, so roughly 2.2 times the one-second figure). Worked example: 95 mm² copper XLPE at one second is 143 × 95 = 13,585 A, close to 13.6 kA.
This check confirms that the cable, and not only the breaker, can withstand the prospective fault level at its point of installation. The screen and armour have their own withstand, calculated the same way with their own k. When sizing a feeder such as LV XLPE power cable or a control cable, the fault rating is as real a constraint as the load current.