Electrical

What Is Prospective Fault Current (PFC)?

Every certificate has a box for prospective fault current and every breaker is stamped 6000 or 10000 — those numbers have to agree, because the device must be able to safely interrupt the biggest fault the supply can deliver.

Updated 20 Aug 20268 min read

Every electrical certificate has a box for prospective fault current, and every consumer unit has breakers stamped 6000 or 10000. Those two numbers have to agree — the device must be able to safely interrupt the biggest fault the supply can deliver.

This guide covers what PFC actually is, the difference between PSCC and PEFC, how to measure it at the origin, how to calculate it from loop impedance, and what the kA ratings on your devices really mean.

Key takeaways

PFC is the current that would flow in a fault of negligible impedance — the worst case waiting to happen.
PSCC is a fault between live conductors; PEFC is a fault from line to earth. Record the higher at the origin.
Calculate it with Ipf = U0 / Z — 230V divided by the loop impedance of the fault path.
A device's rated breaking capacity (kA) must be at least the PFC where it's installed.
16kA is the standard worst-case assumption for a UK domestic supply near the transformer; most homes read far lower.

What prospective fault current is

Prospective fault current is the current that would flow if a fault of negligible impedance occurred at the point being considered — a dead short, with nothing limiting the current except the impedance of the supply and conductors up to that point. "Prospective" because it's the worst case waiting to happen, not something flowing now.

PFC is an umbrella term covering two distinct faults. PSCC (prospective short-circuit current) is a fault between live conductors — line to neutral on single phase, or line to line on three phase. PEFC (prospective earth fault current) is a fault from line to earth, flowing round the earth fault loop. The value recorded on the certificate is the higher of the two at the origin. On TN-C-S the two are often close; on TN-S and especially TT they can differ substantially, which is why you measure both.

Why it matters

Two reasons, both in BS 7671. First, breaking capacity: the prospective fault current must be determined at every relevant point, and protective devices must be capable of interrupting it. An MCB asked to break more current than its rating can fail destructively — welded contacts, a sustained arc, a burning board. Second, energy let-through: the fault level feeds the adiabatic check that proves conductors survive the fault until disconnection. Higher fault current means more energy per unit time through every cable in the path.

PFC is highest where impedance is lowest — at the origin, right after the cut-out — and falls as you move out through the installation, because every metre of conductor adds impedance. That's why the measurement is taken at the origin: it captures the worst case for everything downstream.

How to measure PFC

At the main incoming terminals, with a loop/PFC tester: test line to neutral (this gives the PSCC), test line to earth (this gives the PEFC), and record the higher of the two as the installation's PFC.

Most modern multifunction testers do this directly in a PFC mode and display the answer in kA. Under the hood it's the same loop impedance measurement as a Ze test, converted by Ohm's law. Take the measurement with a firm connection — probe uncertainty matters when the impedance is a fraction of an ohm — and treat marginal results near a device's rated capacity with caution rather than optimism.

Same loop, two readings

The PFC measurement is the same loop test as Ze — measure the loop once and read it both ways: Zs for disconnection, PFC for breaking capacity.

Calculating PFC from loop impedance

The arithmetic is Ohm's law: Ipf = U0 / Z, where U0 is the nominal voltage (230V single phase) and Z is the loop impedance of the fault path — L-N impedance for PSCC, L-E loop impedance (Ze or Zs) for PEFC.

Worked example, earth fault at the origin: measured Ze = 0.35Ω on TN-C-S gives PEFC = 230 / 0.35 ≈ 657A (0.66kA). Worked example, short circuit: measured L-N impedance of 0.28Ω gives PSCC = 230 / 0.28 ≈ 821A (0.82kA). The higher figure — 821A — goes on the certificate. Further into the installation a circuit with Zs = 1.2Ω sees only 230 / 1.2 ≈ 192A: enough to trip a B32 instantly, far below any breaking-capacity concern.

Typical values and three-phase supplies

Typical UK PFC values: a property close to the distribution transformer, up to 16kA; a typical urban domestic supply, 1–6kA; the end of a long overhead rural supply, well under 1kA. The 16kA figure is the standard worst-case assumption for a UK single-phase domestic supply near the transformer — most homes measure far lower. Distance is the dominant variable: every metre of service cable pulls the fault level down.

On a three-phase board, the line-to-line fault path sees 400V across roughly the impedance a single-phase L-N test measures, so the PSCC between lines can approach twice the single-phase reading. Where you can only test L-N and L-E with a single-phase tester at a three-phase origin, it's common practice to double the highest single-phase reading to estimate the three-phase fault level and check device ratings against that.

Device ratings: Icn, Ics and PFC vs Zs

Domestic MCBs and RCBOs to BS EN 60898/61009 are commonly rated 6kA (marked 6000) or 10kA (marked 10000). The comparison is simple: rated breaking capacity must be at least the PFC at the point of installation. Two ratings hide behind the number — Icn, the rated (ultimate) breaking capacity the device can interrupt safely once but may not be serviceable afterwards; and Ics, the service breaking capacity it can interrupt and remain fit for continued use. Where measured PFC genuinely approaches 6kA, the options are higher-capacity devices or manufacturer-declared backup protection from the upstream cut-out fuse.

It's easy to treat PFC and Zs as separate boxes, but they're two views of the same measurement. The max Zs check proves the fault current is high enough to trip the device within the disconnection time; the PFC check proves the device can survive the fault being that high. A loop impedance that's comfortably low for disconnection is, by definition, delivering a large fault current — so the better your Zs looks, the more the breaking capacity question matters.

Guidance only

Values here are for guidance. Always verify against the current edition of BS 7671, the IET On-Site Guide and manufacturer data for the actual installation.

Frequently asked questions

What's the difference between PSCC and PEFC?

PSCC (prospective short-circuit current) is a fault between live conductors — line to neutral or line to line. PEFC (prospective earth fault current) is a fault from line to earth round the earth fault loop. PFC is the umbrella term; you record the higher of the two at the origin.

How do I calculate PFC from loop impedance?

Ipf = U0 / Z: divide 230V by the loop impedance of the fault path. For example, Ze of 0.35Ω gives 230 / 0.35 ≈ 657A. Use L-N impedance for PSCC and L-E impedance for PEFC.

What's a typical PFC for a UK house?

Most urban domestic supplies measure 1–6kA. 16kA is the standard worst-case assumption for a property right next to the transformer, and long rural overhead supplies can be well under 1kA. Distance from the transformer is the dominant factor.

From guidance to action

PFC calculatorConvert any measured loop impedance to fault current in amps and kA.Max Zs calculatorCheck the same loop against disconnection-time limits.