Prospective fault current is the highest current that could flow at a point in an installation if a fault of negligible impedance occurred there: between live conductors it is the prospective short-circuit current (PSCC), and between a line conductor and earth it is the prospective earth fault current (PEFC). BS 7671:2018+A4:2026 Regulation 643.7.3.201 requires it to be determined at the origin and at other relevant points, and the greater of PSCC and PEFC is recorded in kA as Ipf on the EIC and EICR. Every protective device must then have a breaking capacity at least equal to the Ipf where it is fitted (Regulation 434.5.1), which for most domestic MCBs is 6 kA.
- Ipf on the certificate is the greater of PSCC (line to neutral, or line to line on three-phase) and PEFC (line to earth), taken at the origin.
- It is a live test on the incoming supply, made with the loop or PFC function of the instrument, with the earthing conductor and bonding left connected.
- Regulation 434.5.1 requires each device's breaking capacity to be not less than the Ipf at its point of installation, unless backup protection applies under 536.4.201.
- UK DNOs declare a worst case of 16 kA single-phase and 25 kA three-phase; measured domestic values are usually 0.5 to 3 kA.
- The A4 model forms ask for Ipf twice: in the supply characteristics section and in the distribution board details block on every schedule of circuit details.
- A Minor Works Certificate has no Ipf box; an EIC and an EICR both do.
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 point of the fault. Next to a large transformer that impedance is tiny and the current can run to thousands of amps for the fraction of a second before the device clears it. The protective device has to interrupt that current without welding its contacts or blowing apart, so the designer needs the worst case, and the worst case is always at the origin where the conductors are shortest.
The arithmetic is simple. Ipf is approximately U0 divided by the loop impedance to the fault, so with 230 V and a line-to-neutral impedance of 0.05 Ω the PSCC is about 4.6 kA, and with a Ze of 0.12 Ω the PEFC is about 1.9 kA. BS 7671 Appendix 14 sets out the calculation method, and a modern loop instrument such as the Megger MFT-X1 does the division for you and displays the fault current directly from the loop measurement.
PSCC and PEFC are rarely equal. On a TN-C-S supply the neutral and the earth are the same conductor back to the cut-out, so the two figures are close, with PEFC sometimes the higher because of parallel paths through the bonding. On a TN-S supply the earth path runs through the cable sheath or a separate conductor, so PEFC is usually lower than PSCC. On TT the earth path is through the electrode and the general mass of earth, so PEFC is tiny, often tens of amps, while PSCC is unchanged. That is why the form asks for the greater of the two: whichever one it is, the device has to break it. For the earthing arrangements themselves, see Earthing Arrangements Explained: TN-S, TN-C-S and TT.
Where does PFC go on the certificate?
The BS 7671:2018+A4:2026 model forms, which the IET published on 15 April 2026 and which have applied on their own since Amendment 2 was withdrawn on 15 October 2026, record Ipf in two places. Section F of the EIC and Section I of the EICR ("Supply characteristics and earthing arrangements") have the box "Prospective fault current, Ipf (2) ... kA" next to "External earth fault loop impedance, Ze (2) ... Ω", with note (2) stating that both may be obtained by enquiry or by measurement. Then the distribution board details block at the top of every schedule of circuit details has its own "Zdb ... Ω" and "Ipf ... kA", so a sub-board gets the figures that apply at that board, not the ones from the origin.
| Certificate | Ipf recorded? | Where on the A4 form |
|---|---|---|
| Electrical Installation Certificate | Yes | Section F supply characteristics, plus the DB details block on each schedule |
| EICR | Yes | Section I supply characteristics, plus the DB details block on each schedule |
| Minor Electrical Installation Works Certificate | No | Not on the model form (Sections A to E) |
The schedule of circuit details also carries a "Breaking capacity (kA)" box for the overcurrent device on each circuit, column 11, sitting between "Rating (A)" in column 10 and "Maximum permitted Zs (Ω)" in column 12. That column is the one you check against the board's Ipf. For the full layout of the schedule, see How to Fill In a Schedule of Test Results, Column by Column.
Guidance note 18 on the EICR, and the matching note on the EIC, states that the Ipf recorded is to be the greater of the short-circuit and earth fault currents. Recording only the PEFC because it came off the instrument with the Ze reading is one of the most common form errors, and on a TN-S or TT supply it can understate the value by a wide margin.
How do you measure prospective fault current?
PFC is a live test on the energised incoming supply, made at the origin with the loop or PFC function of the instrument, and it carries the usual live-working precautions: GS38 leads and probes, and a plan for what happens if the instrument or a lead fails. A PSCC test is in effect a controlled short across the incoming supply, and IET Guidance Note 3 is clear that fused leads are there to protect the instrument, not to make the test safe. NICEIC's technical guidance adds a point that catches a lot of people out: the earthing conductor, main protective bonding conductors and circuit protective conductors should all be connected as in normal operation while you measure PFC, because parallel earth paths lower the loop impedance and raise the fault current, and it is the highest value you are after.
That is the opposite of the Ze test, where you disconnect the main earthing conductor to shut out those parallel paths. The practical sequence at a single-phase origin is:
- With the main earthing conductor disconnected, measure Ze line to earth and note it. Reconnect the earthing conductor.
- With everything connected, measure line to earth on the PFC range and note the PEFC.
- Measure line to neutral on the PFC range and note the PSCC.
- Record the greater of steps 2 and 3 as Ipf. Note on the form whether it was measured or obtained by enquiry.
If the instrument has no fault current range, divide 230 V by the measured loop impedance for each test and record the higher result, which is the method BS 7671 Appendix 14 describes. The Ze test itself is covered in What is Ze? External earth fault loop impedance.
Three-phase supplies
On a three-phase supply the maximum fault current is between line conductors at 400 V, not between a line and neutral. Measure line to neutral and line to earth on each phase, then line to line if the instrument is rated for 400 V on its loop range. Where it is not, the rule of thumb published by NICEIC and accepted in Guidance Note 3 is to take twice the highest single-phase value as the line-to-line PSCC: a 4.6 kA line-to-neutral reading becomes 9.2 kA. It is a conservative estimate, which is the right way to err when the figure sets the breaking capacity you need. More on the rest of three-phase verification is in Three-Phase Testing Basics for Domestic Electricians.
Enquiry rather than measurement
Note (2) on the form allows Ipf to be obtained by enquiry, and Guidance Note 3 (section 2.6.16 in recent editions) says that for many 100 A single-phase domestic installations with a consumer unit to BS EN 61439-3 carrying a 16 kA conditional rating, measurement at the origin is not strictly necessary, because the DNO's declared maximum of 16 kA already proves the board is adequate. In practice most inspectors measure anyway: the instrument gives the figure in the same operation as Ze, and a measured value is what scheme assessors expect to see. What you must not do is write the DNO's 16 kA in the box and describe it as measured. If it came from an enquiry, say so.
What breaking capacity do the devices need?
Regulation 434.5.1 says the breaking capacity of each fault-current protective device shall be not less than the prospective fault current at its point of installation, with a lower breaking capacity permitted where another device upstream has the necessary capacity and the energy let-through of both devices together does not exceed what the downstream device and conductors can withstand. Regulation 536.4.201 covers that backup protection in consumer units and similar switchgear. The ratings you will meet are:
| Device | Standard | Typical rated short-circuit capacity | Where the figure is |
|---|---|---|---|
| MCB | BS EN 60898-1 | 6 kA (some 10 kA) | Icn in a rectangle on the front, e.g. 6000 |
| RCBO | BS EN 61009-1 | 6 kA (some 10 kA) | Icn in a rectangle on the front |
| Moulded-case breaker | BS EN 60947-2 | 10 kA to 36 kA and above | Icu and Ics on the label; Ics may be as low as 25% of Icu |
| Consumer unit assembly | BS EN 61439-3 (Annex ZB) | 16 kA conditional | Relies on a 100 A BS 88 cut-out fuse upstream |
| BS 88 fuse | BS 88-2 / BS 88-3 | 80 kA | Fuse body marking |
The Icn figure on an MCB to BS EN 60898-1 is the rated short-circuit capacity, and the standard also defines a service short-circuit capacity, Ics, that the device must break and still be fit to carry on in service: Ics equals Icn for devices up to 6 kA, and is 75% of Icn (not less than 6 kA) for 10 kA devices. Devices to BS EN 60947-2 are a different animal: their Ics is set by the manufacturer as a percentage of Icu, typically between 25% and 100%, and the device may need inspection before it is put back into service after a full short-circuit break. If you are selecting devices for a board fed close to a substation, read the label for both figures.
The 16 kA conditional rating on a domestic consumer unit is the thing that makes 6 kA MCBs acceptable on a supply the DNO declares at 16 kA. The assembly has been type-tested at 16 kA with a 100 A BS 88-2 fuse upstream, and that fuse limits the let-through energy the breakers see. The rating is conditional on that upstream fuse. Feed the same board through a 100 A switch-fuse with a different fuse type, or through a breaker, and the conditional rating no longer applies and you are back to Regulation 434.5.1 on its own.
When is PFC likely to exceed 6 kA?
A single-phase domestic origin with 25 mm² tails and a service cable of ordinary length will usually measure between about 0.5 kA and 3 kA. The figure climbs where the service is short and the transformer is close: ground-floor flats with the substation in the basement, shops on a parade with the transformer behind them, industrial units with their own 11 kV/400 V transformer. A 25 mm² service cable at roughly 0.06 Ω keeps the figure under 4 kA; a 95 mm² or 185 mm² supply off a 1,000 kVA transformer can measure 10 kA and more at the main switch.
| Supply | Typical maximum Ze (TN) | PEFC at that Ze | PSCC at the same board |
|---|---|---|---|
| TN-C-S (PME) single-phase | 0.35 Ω | about 0.66 kA | similar, often slightly higher |
| TN-S single-phase | 0.8 Ω | about 0.29 kA | higher, set by L-N impedance |
| TT | electrode-dependent, often 21 Ω or more | about 11 A at 21 Ω | unchanged, set by L-N impedance |
| Three-phase near a substation | 0.05 Ω or less | 4.6 kA and above | around twice the single-phase figure line to line |
Where the measured Ipf at a board is above the Icn of the devices in it, the options are 10 kA devices (BS EN 60898-1 allows ratings up to 25 kA), a BS 88 switch-fuse upstream to provide backup protection under 536.4.201, or a board whose declared conditional rating covers the figure with the upstream device actually installed. On an existing installation it goes on the EICR as an observation against Regulation 434.5.1. Electrical Safety First's Best Practice Guide 4 (Issue 7.4, August 2026) has no specific line for it, so the code comes from the guide's general test: a device that cannot be relied on to clear a fault at its point of installation is "potentially dangerous" if the fault occurs, which is a C2 in most inspectors' hands, and the observation should say what the measured Ipf and the device's Icn were. On an EIC it is a design fault that should never reach the certificate.
What are the common mistakes with PFC?
The errors that scheme assessors and third-party reviewers pick up most often are, in rough order of frequency, these.
Recording only the PEFC. The instrument gives a fault current alongside the Ze reading, and it goes straight in the box. On TN-S and TT that is the lower of the two figures. Do the line-to-neutral test as well.
Measuring with the earthing conductor disconnected. Carried over from the Ze test. It understates the PEFC because the parallel paths through the bonding are removed. Reconnect before the PFC tests.
Measuring at a socket-outlet. PFC falls with every metre of cable from the origin, so a reading at an accessory tells you what a fault at that accessory would draw, not what the devices at the board must handle. The figure for the certificate is taken at the origin, or at the board for the DB details block.
Writing the DNO figure as measured. Enquiry is allowed, and the form says so, but it is a different thing from measurement and the note is there to be used.
Forgetting line-to-line on three-phase. The highest fault current on a three-phase supply is between lines at 400 V. If the instrument cannot measure it, double the highest single-phase reading.
Fitting 6 kA devices where Ipf exceeds 6 kA without checking backup. The 16 kA conditional rating of a BS EN 61439-3 board applies only with the upstream fuse it was tested with. Through a different upstream device, 434.5.1 applies to the breakers on their own.
Confusing Ipf with Zs. Zs limits disconnection time and is a maximum; Ipf sets breaking capacity and is a minimum the device must exceed. Both come from loop measurements but they answer different questions. The Zs side is in What is Zs? Earth fault loop impedance explained and Maximum Zs Values BS 7671: Tables 41.2 to 41.5 and the 80% Rule.
Does Amendment 4 change anything about PFC?
No. Regulation 643.7.3.201 and Chapter 43 are unchanged in BS 7671:2018+A4:2026, and the Ipf box on the forms is as it was, with note (2) still allowing enquiry or measurement. The visible A4 changes are around it: the distribution board details block now reads Zdb rather than Ze for sub-boards, the circuit details columns are numbered 1 to 16 with breaking capacity at column 11, and the schedule of inspections for an EICR has item 4.13 (compatibility of protective devices, bases and other components; correct type and rating) and item 5.7 (adequacy of protective devices: type and rated current for fault protection, citing 411.3 and 530.3.201). Neither item names breaking capacity, so an undersized device is recorded as an observation in Section K with the regulation reference, not ticked off in the inspection schedule. For the full list of what moved in Amendment 4, see What Is BS 7671? The IET Wiring Regulations Explained for 2026.
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