Prospective fault current (PFC) is the highest current that could flow at a point in an installation if a fault of negligible impedance occurred between live conductors (prospective short-circuit current, PSCC) or between a line conductor and earth (prospective earth fault current, PEFC), and it is measured in kA at the origin and recorded on every EIC and EICR. BS 7671 Chapter 43 requires each protective device to have a breaking capacity at least equal to the PFC at its point of installation, which for most domestic MCBs to BS EN 60898 is 6 kA.
- PFC is the larger of PSCC (line-to-neutral or line-to-line) and PEFC (line-to-earth) at the origin.
- It is measured live with the loop function of a multifunction tester at the same time as Ze.
- Every protective device must have a breaking capacity at or above the PFC where it is installed.
- UK DNOs can declare up to 16 kA at a single-phase cut-out; measured domestic values are usually 0.5 to 3 kA.
- PFC is recorded on the EIC and EICR supply characteristics page, not on a Minor Works Certificate.
What is prospective fault current in practice?
When a dead short occurs, the only thing limiting the current is the impedance of the supply and the conductors up to the fault. Close to a large transformer that impedance is tiny and the current can be thousands of amps. The protective device must interrupt it without failing, so the designer needs the worst case, which is at the origin.
In effect PFC ≈ U0 ÷ Z, where U0 is 230 V and Z is the loop impedance to the fault: a Ze of 0.1 Ω gives a PEFC of about 2.3 kA, and a line-to-neutral impedance of 0.05 Ω gives a PSCC of about 4.6 kA. Modern MFTs display PFC directly from the loop measurement.
When is PFC measured?
| Certificate | PFC recorded? | Where |
|---|---|---|
| Electrical Installation Certificate | Yes | Supply characteristics and earthing arrangements |
| EICR | Yes | Supply characteristics and earthing arrangements |
| Minor Works Certificate | No | Not on the model form |
PFC is measured for every new installation, consumer unit replacement and periodic inspection, and at any sub-board, because the breaking capacity of the devices there must suit the value at that point. It matters most in flats and commercial units near a substation, where PFC can exceed the 6 kA rating of standard MCBs; there, 10 kA devices or backup protection from the upstream fuse, as permitted by BS 7671 Regulation 434.5.1, are needed.
How is PFC measured?
The measurement is made with the loop or PFC function of the MFT at the origin, with the installation isolated:
- PEFC: test between line and earth, at the same time as Ze with the main earthing conductor disconnected.
- PSCC: test between line and neutral. On a single-phase supply this is normally the higher of the two.
- Three-phase: test each line to neutral, and line to line where the instrument supports it; otherwise GN3 accepts doubling the highest line-to-neutral value.
- Record the highest value as the PFC on the certificate.
The certificate allows the figure to be marked "measured" or "enquiry" (from the DNO's declared maximum). On site, the measured value is expected.
Breaking capacity to compare against
| Device | Standard | Typical rated short-circuit capacity |
|---|---|---|
| MCB | BS EN 60898 | 6 kA (some 10 kA) |
| RCBO | BS EN 61009 | 6 kA (some 10 kA) |
| Consumer unit assembly | BS EN 61439-3 | 16 kA conditional, relying on the DNO cut-out fuse |
| BS 88 fuse | BS 88-2 / BS 88-3 | 80 kA |
Common mistakes with PFC
- Recording only PEFC. The form asks for the higher of PEFC and PSCC, and on most supplies PSCC is the larger.
- Measuring at a socket-outlet. PFC falls with distance from the origin, so a reading at an accessory understates the value the board's devices must handle.
- Copying the DNO figure and marking it "measured". If it came from an enquiry, tick the enquiry box.
- Fitting 6 kA devices where PFC exceeds 6 kA without confirming backup protection from the upstream fuse.
- Forgetting three-phase line-to-line values, roughly double the line-to-neutral figure.
Related terms
- External earth fault loop impedance (Ze)
- Earth fault loop impedance (Zs)
- Multifunction tester (MFT)
- RCD and RCBO
- Electrical Installation Certificate (EIC)
- Schedule of test results
How Certio helps
Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.