How to measure prospective fault current (PFC)

Helpful video reference. GSH Electrical's video "Measuring Prospective Fault Current (PFC) at the Origin of a Single Phase Supply (Live Test)" shows the actual test procedure at a real UK domestic supply, with commentary on what the readings mean. GSH Electrical (@gsh_electrical) run a popular channel covering UK domestic and commercial electrical installation practice.

1. Understand what PFC actually measures

Prospective fault current (PFC) is the theoretical maximum current that would flow if a negligible-impedance short circuit occurred at the point of measurement. At the origin of a single-phase domestic installation, this means at the busbars or the incoming terminals of the main switch.

There are two components to PFC: PSCC (Prospective Short-Circuit Current, measured line to neutral) and PEFC (Prospective Earth Fault Current, measured line to earth). The higher of the two values is recorded as the PFC for the installation and compared against the breaking capacities of all protective devices.

In most UK homes fed from a local distribution transformer via a standard service cable, PFC values at the origin are typically between 0.8 kA and 3 kA. Homes very close to a substation can see values up to 6 kA or beyond in commercial streets.

2. Confirm the tester is suitable and calibrated

You need a multifunction installation tester with a PFC or prospective fault current function. Popular UK instruments from Megger, Fluke, Metrel and Robin (Amprobe) all include this function. The test leads must be rated for the test — use the leads supplied with the instrument or manufacturer-approved equivalents.

Check that the tester has a valid calibration certificate. BS 7671 and the Electrical Safety Council both require test instruments used for formal verification work to be calibrated at appropriate intervals. A sticker or certificate should confirm the last calibration date.

3. Identify and access the test points

In a domestic property, the test is carried out at the incoming live side of the main switch in the consumer unit. This means the meter tails side, not the circuit side. The supply cannot be switched off for this test.

Before opening the consumer unit cover, confirm with the occupant that the supply should remain on. Isolate individual circuits where possible to reduce load, but the incoming supply must remain energised. Wear insulated gloves and safety glasses whenever working near or within a live consumer unit.

The test points are the line (L) terminal, neutral (N) terminal and earth (E) terminal on the supply side — often accessible at the busbars or at the incoming terminals of the 100 A main switch. In some installations with sealed service heads, you may need to test at the load side of the main switch instead, accepting a small deduction in the measured value from the main switch impedance.

4. Measure PSCC (line to neutral)

Connect the red (line) test lead to the L terminal and the black lead to the N terminal. Select the PFC or PSCC function on the instrument. The tester applies a brief test current — typically lasting a fraction of a second — and calculates the prospective short-circuit current from the measured impedance.

Record the reading in kA. A typical domestic value at the origin on a TN-C-S supply is 0.8 to 2 kA. A value above 3 kA warrants closer attention to device ratings. Some instruments display the result as a resistance (Ze or Zs) from which PFC is calculated; check whether your instrument is displaying current directly or impedance.

5. Measure PEFC (line to earth)

Move the black test lead from N to the earth (E) terminal. Repeat the measurement with the same PFC function selected. This gives the Prospective Earth Fault Current — the current that would flow in a line-to-earth fault at the origin.

On a TN-C-S (PME) supply the PEFC is usually similar to the PSCC, because the PEN conductor provides a low-impedance path. On a TT earthing system, where the earth return path passes through soil and an earth electrode, the PEFC may be substantially lower than the PSCC — sometimes only a few hundred amps.

6. Record PFC and verify equipment ratings

Take the higher of the PSCC and PEFC readings as the PFC for the installation. Record it on the Schedule of Test Results on the Electrical Installation Certificate or Electrical Installation Condition Report.

BS 7671 Regulation 434.5.1 requires that every protective device — including the main switch, each MCB and each RCBO — has a rated breaking capacity (Icu or Icn) not less than the PFC at the point where it is installed. For most domestic consumer units, the main switch is rated at 6 kA or 10 kA, and standard MCBs and RCBOs carry a 6 kA rating, which is sufficient for the vast majority of UK domestic supplies.

If the measured PFC exceeds any device's rated breaking capacity, that must be recorded as a defect requiring remediation — typically by upgrading the device to one with a higher breaking capacity or adding upstream current-limiting protection.

When to call us

PFC is measured as part of every initial verification of a new installation and every periodic EICR. If you have recently had consumer unit work done and have not received a Schedule of Test Results showing the PFC reading, the documentation is incomplete. Richard carries out full initial verifications and EICRs across Sandwich and east Kent, including PFC measurement at the origin on every visit.