Prospective Fault Current (Ipf)

Compute Ipf in kA from nominal voltage and earth fault loop impedance. Used to verify OCPD breaking capacity per BS 7671 Section 434.

Nominal voltage (V)

Zs (Ω)

Enter voltage and Zs to calculate Ipf.

Formula

Ipf (kA) = (V ÷ Zs) ÷ 1000. For UK single-phase, nominal voltage defaults to 230 V.

Why it matters

The protective device must have a rated breaking capacity equal to or greater than the prospective fault current at its terminals. A 6 kA MCB on a circuit with 8 kA Ipf can fail catastrophically under fault. BS 7671 Section 434 requires this check.

Used in

EICR — recorded at origin and per DB.
EIC — recorded on new installations.

Reference

BS 7671:2018+A2:2022, Section 434

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