Prospective Fault Current (PFC) Calculator

PSCC (Prospective Short-Circuit Current) is the current that would flow in a short circuit between live conductors — line-to-neutral on a single-phase supply — calculated as Uo divided by the line-neutral loop impedance Zpn. PEFC (Prospective Earth Fault Current) is the current that would flow in a line-to-earth fault — calculated as Uo divided by the line-earth loop impedance Zs. PSCC is normally higher than PEFC because the line-to-neutral return path has lower impedance than the line-to-earth path, so it carries the larger fault current. The higher of the two readings is recorded as the prospective fault current and checked against the breaking capacity of protective devices. See also our Prospective Fault Current Calculator for a more detailed analysis from cable resistances.

BS 7671 Regulation 434.5.1 requires that every protective device has a rated short-circuit breaking capacity not less than the prospective fault current at its point of installation. If the fault current exceeds the breaking capacity, the device may not be able to safely interrupt the fault — potentially causing an arc flash, fire, or explosion. This is a critical safety requirement verified during initial verification.

UK domestic installations typically see PSCC values between 1kA and 6kA at the origin. Supplies fed from older overhead lines may be as low as 0.8kA, while modern underground supplies in urban areas can reach 16kA or more. Most domestic MCBs and consumer units are rated at 6kA breaking capacity. If PSCC exceeds 6kA, you need devices rated to 10kA or higher, or an upstream device providing back-up protection.

For PSCC enter the line-to-neutral loop impedance (Zpn) — most multifunction testers measure this directly when you select the PSC or L-N loop test. For PEFC enter the line-to-earth loop impedance (Zs), which is the L-E loop test, or Ze + (R1+R2) at the point of interest. Do not use Ze (the external earth loop measured with the earthing conductor disconnected from the MET) for PSCC — Ze is part of the earth-fault path, not the short-circuit (line-neutral) path. Typical origin values: line-neutral loop is often a few tenths of an ohm; Ze for TN-S (cable sheath earth) = 0.35-0.8Ω, TN-C-S (PME) = 0.2-0.35Ω, TT = can be very high (1-200Ω or more).

Yes — modern multifunction testers like the Megger MFT or Metrel measure PFC directly in kA. The tester calculates it from the measured loop impedance using Ohm's law (I = Uo / Z) with the nominal voltage (230V for a 230V supply) and no correction factor — for example the Megger MFT manual shows 230V ÷ 0.13Ω = 1.77kA. This calculator follows the same convention, so the figures here match what your tester reports from the same impedance. Take the line-neutral reading for PSCC and the line-earth reading for PEFC; the higher of the two is the recorded prospective fault current.