Adiabatic Equation

During a short circuit, the enormous fault current generates heat in the conductor so rapidly that there is no time for the heat to dissipate to the surrounding insulation and environment — the process is therefore adiabatic (no heat loss). The conductor temperature rises until the protective device clears the fault. The equation k²S² ≥ I²t ensures that the conductor's thermal capacity (left side) exceeds the thermal energy delivered by the fault (right side). The k value is a material constant that depends on the conductor material (copper or aluminium) and the insulation type (PVC, XLPE, or mineral), representing the conductor's ability to absorb heat without the insulation exceeding its limiting temperature. For example, k = 115 for PVC-insulated copper conductors. If the check fails — meaning the fault energy exceeds the conductor's withstand — the engineer must either increase the conductor size or reduce the disconnection time by selecting a faster protective device. This verification is especially critical for protective conductors (earth wires), which may be smaller than the associated phase conductors.