Prospective fault current calculator: PEFC, PSCC and Ipf
Prospective fault current is the current that would flow if a fault of negligible impedance happened at a point, normally the origin: 230 V divided by Ze for an earth fault, or by the line–neutral loop impedance for a short circuit. The larger of the two is recorded as Ipf, and every protective device must be rated to break it.
Ohm's law at the origin: 230 V through the loop you measured. It gives PEFC, PSCC and the Ipf to record, and says whether the board's devices can break it.
Ohm's law at the origin, and Regulation 434.5.1 of BS 7671 for the breaking capacity a device must have. Checked against BS 7671:2018+A4:2026, updated 3 September 2026. Free, and nothing to sign in to.
Measured with the earthing conductor disconnected, main switch off.
Leave blank if you have only the earth loop; PSCC is then not shown.
Prospective fault current at the origin, Ipf
0.77 kA
Devices can break it
0.77 kA against devices rated 6 kA.
- PEFC, line to earth 230 V ÷ Ze 0.35 Ω
- 657 A (0.66 kA)
- PSCC, line to neutral 230 V ÷ 0.30 Ω
- 767 A (0.77 kA)
How is prospective fault current worked out?
A fault of negligible impedance at the origin is driven by the supply voltage through whatever impedance the supply presents. To earth, that is 230 V ÷ Ze, the prospective earth fault current; between line and neutral it is 230 V through the line–neutral loop, the prospective short-circuit current. The schedule records the larger of the two as the fault current at the origin, and Regulation 434.5.1 requires every protective device to be rated to break at least that.
On a three-phase supply a fault between two lines is driven by 400 V rather than 230 V. Where the three-phase value has not been measured, Guidance Note 3 and the On-Site Guide accept twice the single-phase line–neutral figure as the estimate, which overstates the current rather than understating it. The tool marks the figure as an estimate when it uses it.
Breaking capacity
Domestic circuit-breakers are commonly rated 6 kA or 10 kA. A consumer unit to BS EN 61439-3 is normally also given a conditional rating, 16 kA with a 100 A BS 88-3 cut-out fuse upstream, because the fuse limits what reaches the breakers, an arrangement Regulation 434.5.1 allows. The check here is against the figure you give it; where the measured current is over the breakers' own rating, look at the assembly's conditional rating before condemning the board.
Worked example
A PME supply measures Ze = 0.35 Ω and a line–neutral loop of 0.30 Ω, feeding a consumer unit of 6 kA devices.
- PEFC = 230 ÷ 0.35 = 657 A (0.66 kA).
- PSCC = 230 ÷ 0.30 = 767 A (0.77 kA).
- Ipf recorded: 0.77 kA, the larger.
- 6 kA devices are rated well above 0.77 kA, so they are adequate.
Near a substation the same measurement can read 0.02 Ω, which is 11.5 kA: past a 6 kA breaker on its own, inside a board's 16 kA conditional rating behind its cut-out fuse.
Questions the trade asks
The current that would flow if a fault of negligible impedance happened at a point, normally measured at the origin, where it is highest. It is recorded on the certificate as Ipf, and Regulation 434.5.1 requires every protective device to have a rated short-circuit capacity at least equal to it.
PEFC is the prospective earth fault current, 230 V driven through the earth fault loop impedance Ze. PSCC is the prospective short-circuit current, 230 V through the line–neutral loop. The certificate records the larger of the two as the fault current at the origin.
A fault between two lines is driven by 400 V rather than 230 V. Where the three-phase value has not been measured, the guidance, Guidance Note 3 and the On-Site Guide, accepts twice the single-phase line–neutral figure as the estimate. It errs on the safe side, and this tool marks the figure as an estimate when it uses it.
On its own, only where the fault current at the board is 6 kA or less. A domestic consumer unit to BS EN 61439-3 is usually also given a conditional rating of 16 kA when fed through a 100 A BS 88-3 cut-out fuse, because the fuse limits what reaches the breakers. Regulation 434.5.1 allows this back-up arrangement. Check the assembly's own rating, not just the breakers.
Record what you measured at the origin, because that is what the certificate asks for. The distribution network operator's declared maximum, commonly 16 kA at the cut-out, is a ceiling the design has to be adequate for, because the network can change.
Next door
- Adiabatic equation calculator: minimum CPC sizeMinimum protective conductor size from fault current, disconnection time and k, with k worked out from the insulation rather than looked up. Free, no sign-up.
- Max Zs calculator: maximum Zs for MCBs, RCBOs and fusesMaximum earth fault loop impedance for any MCB, RCBO or fuse to BS 7671, with the 80% figure a site reading is judged against. Free, no sign-up.
The figures go straight onto the certificate
In Pascal the maximum Zs is filled in for every circuit as you pick the device, the schedule of test results carries all 32 columns, and the observation library suggests the wording, code and regulation as you type. EICs, EICRs and minor works, from £25 a month.
See the certificate softwareA calculator applies a method to the numbers you give it. It does not know the installation in front of you, and it is no substitute for BS 7671, the guidance or the judgement of the person signing the certificate. Check anything you rely on.