#Prospective Fault Current Explained: What PFC Is and How to Measure It

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.

#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. A fault between live conductors: line to neutral on single phase, or line to line / line to line to line on three phase.
• PEFC — prospective earth fault current. 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 of the installation. On a TN-C-S supply the two are often close (the neutral and earth share a path back to the transformer); on TN-S and especially TT they can differ substantially, which is why you measure both rather than assuming.

#Why it matters

Two reasons, both in BS 7671:

1. Breaking capacity. Regulation 434.1 requires the prospective fault current to be determined at every relevant point of the installation, 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. The kA rating on the device exists precisely for this comparison.
2. 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:

1. Test line to neutral — this gives the PSCC.
2. Test line to earth — this gives the PEFC.
3. 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. Practical points: 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.

#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 a TN-C-S supply:

PEFC = 230 / 0.35 ≈ 657A (0.66kA)

Worked example — short circuit. Measured L-N loop impedance of 0.28Ω:

PSCC = 230 / 0.28 ≈ 821A (0.82kA)

The higher figure — 821A here — goes on the certificate as the PFC. Further into the installation, a circuit with Zs = 1.2Ω at its furthest point sees an earth fault current of only 230 / 1.2 ≈ 192A: high enough to trip a B32 instantly, far below any breaking-capacity concern. The PFC calculator does the conversion to amps and kA from any measured impedance.

#Typical values in UK installations

The 16kA figure is the standard worst-case assumption for a UK single-phase domestic supply near the transformer — most homes measure far lower, but the number explains the way consumer unit standards are framed. Distance is the dominant variable: every metre of service cable and main adds impedance and pulls the fault level down.

#Three-phase supplies

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. This is the doubling convention: 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 to check device ratings against that. Three-phase boards also tend to sit on beefier supplies to start with, so this is where 10kA+ ratings earn their keep.

#Device ratings: Icn, Ics and the numbers on the breaker

Domestic MCBs and RCBOs to BS EN 60898/61009 are commonly rated 6kA (marked 6000) or 10kA (marked 10000). The comparison is simple: the rated breaking capacity must be at least the PFC at the point the device is installed.

Two ratings hide behind that number:

• Icn — rated (ultimate) breaking capacity. The maximum fault the device can interrupt safely once. It may not be serviceable afterwards.
• Ics — service breaking capacity. The fault level the device can interrupt and remain fit for continued use.

For domestic fault levels the distinction rarely bites — a 6kA device on a 1.5kA supply has margin either way. Where measured PFC genuinely approaches 6kA, options are higher-capacity devices or relying on manufacturer-declared backup protection from the upstream cut-out fuse — a conditional arrangement that must come from the manufacturer's data, not assumption.

#PFC and Zs: two sides of the same loop

It's easy to treat PFC and Zs as separate boxes on the certificate, 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 required disconnection time; the PFC check proves the device can survive the fault being that high. A loop impedance that's comfortably low for disconnection purposes is, by definition, delivering a large fault current — so the better your Zs looks, the more the breaking capacity question matters. A complete test sequence needs both, which is why they sit next to each other on the schedule and in the test routine: measure the loop once, and read it both ways.

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

#Do it faster with TradePlanr

The free prospective fault current calculator converts any measured loop impedance to fault current in amps and kA — test L-N and L-E, enter the readings, record the higher. It sits alongside the max Zs and other BS 7671 tools, all free with no sign-up. TradePlanr is a job management app built for electricians — quotes, job scheduling, invoices and certificates — for a flat £9.99 a month.