The complete guide to prospective fault current (PFC/Ipf). What it is, why it matters, how to measure it on site, typical values for domestic and commercial installations, breaking capacity of MCBs and fuses, and the BS 7671 regulatory requirements under Regulation 434.5.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
The short answer
Prospective fault current (PFC, or Ipf) is the maximum current that would flow if a fault of negligible impedance occurred — taken as the greater of the prospective short-circuit current (line-to-neutral) and the prospective earth fault current. It is measured at the origin and recorded on the certificate. Under BS 7671 Reg 434.5.1, every protective device must have a rated breaking capacity not less than the PFC at its point of installation.
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01 · BS 7671 Testing
Prospective fault current (PFC), also written as Ipf or PSCC (prospective short-circuit current), is the maximum current that would flow at a given point in an electrical installation if a fault of negligible impedance occurred at that point. It represents the worst-case scenario — the absolute maximum current the supply can deliver into a dead short circuit or a solid earth fault.
There are two types of prospective fault current to consider. The first is the prospective short-circuit current — the current that would flow if the line and neutral conductors were connected directly together (a dead short). This is typically the higher value. The second is the prospective earth fault current — the current that would flow if the line conductor made direct contact with the earth conductor or an earthed metallic part. This is related to the earth fault loop impedance (Ze).
PFC is determined by the supply impedance — the impedance of the path from the supply transformer to the point of measurement. The lower the impedance, the higher the prospective fault current. A property close to the transformer with a large supply cable has low impedance and therefore high PFC. A property at the end of a long supply run has high impedance and therefore lower PFC.
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02 · BS 7671 Testing
PFC matters for one critical reason: every protective device in the installation must be capable of safely interrupting the maximum fault current that could flow at the point where it is installed. This is a fundamental safety requirement set out in BS 7671 Regulation 434.5.1.
When a short circuit or earth fault occurs, an enormous current flows — potentially thousands of amperes. The protective device (MCB, RCBO, fuse) must interrupt this current safely, containing the arc within the device enclosure and disconnecting the circuit without fire, explosion, or danger to persons. If the fault current exceeds the device's rated breaking capacity, the device may fail catastrophically.
If the prospective fault current exceeds the breaking capacity of a protective device, the consequences can include: device contacts welding together (failing to disconnect), the device exploding and ejecting fragments of plastic and metal, sustained arcing that ignites the consumer unit or surrounding materials, and cascading failure of adjacent devices. This is not a theoretical risk — it is the reason PFC must be measured and verified on every installation.
PFC also matters for cable protection. The adiabatic equation (k²S² ≥ I²t) verifies that the cable conductor can withstand the heating effect of the fault current for the duration of the disconnection time. If the PFC is very high and the disconnection time is not short enough, the cable can be damaged by the fault current — the conductor can overheat, melting the insulation and creating a fire risk.
In the app
Elec-Mate's PFC calculator determines the fault current from supply impedance. The adiabatic calculator then verifies the cable can withstand it.
03 · BS 7671 Testing
PFC is measured using a multifunction tester (MFT) or a dedicated loop impedance tester. Most modern MFTs have a PFC function that measures the supply impedance and calculates the maximum fault current automatically. Where site measurement is not yet possible — for example on a new installation before energisation — PFC may be determined by enquiry to the DNO/distributor. On-Site Guide Section 1.3 requires the installer to establish with the local distributor, before starting work, the maximum prospective fault current (Ipf) at the supply terminals and the typical maximum external earth fault loop impedance (Ze), along with the type and rating of the distributor’s cut-out. The OSG states that measurement shall be made ‘if not determined by enquiry of the distributor’, so the two routes are equally valid under BS 7671.
Reg 643.7.3.201 requires PFC to be determined at the origin and at other relevant points in the installation. GN3 Reg 2.29 defines this precisely: “every relevant point shall mean every point where a protective device is required to operate under fault conditions, and includes the origin of the installation.” For a large commercial switchboard feeding multiple MCCBs, each MCCB location is a relevant point — not only the incoming incomer. Appendix 14 of BS 7671 gives further information on determining Ipf at such points, describing relevant points as the switchgear and protective devices that may have to operate and possibly disconnect a fault current.
PFC is measured at the origin of the installation — typically at the consumer unit or main distribution board. Connect the test instrument to the line, neutral, and earth terminals at the main switch (incoming side). The instrument measures both the line-to-neutral impedance and the line-to-earth impedance (Ze), then calculates the PFC from each. The higher of the two values is the prospective fault current that must be recorded on the certificate. Most instruments display the PFC value directly in kA (kiloamperes). Appendix 14 warns that the measurement should always be made on the output terminals of a suitably rated protective device, and never where there is no overcurrent protection between the point of connection and the supply transformer — fused test leads alone do not meet that requirement. Appendix 14 also notes one exception: in domestic (household) premises using a consumer unit to BS EN 61439-3 where the distributor declares a maximum prospective fault current of 16 kA, it is not necessary to measure or calculate the PFC at the origin of the supply.
PFC should also be measured at each sub-distribution board in the installation. The PFC at a sub-board is always lower than at the origin because the impedance of the submain cable between the main board and the sub-board adds to the total fault loop impedance, reducing the fault current. Connect the instrument to the incoming terminals of the sub-board and measure in the same way. The PFC at each board must not exceed the breaking capacity of the protective devices installed at that board.
The measured PFC is recorded in the "Prospective fault current Ipf" field on the EIC or EICR. Record the highest value (typically the line-to-neutral PFC). On the EICR, the PFC at the origin is recorded in the Supply Characteristics section. If the PFC exceeds the breaking capacity of any installed device, this must be recorded as an observation with the appropriate classification code.
04 · BS 7671 Testing
PFC values vary widely depending on the supply characteristics, the distance from the transformer, and the type of installation. Here are typical ranges for UK installations.
Domestic (end of long run)
0.5 — 2 kA
Domestic (typical urban)
2 — 4 kA
Domestic (near transformer)
4 — 8 kA
Commercial (small)
6 — 16 kA
Commercial/industrial (large)
16 — 50+ kA
Properties immediately adjacent to a ground-mounted transformer (pad-mount or mini-pillar) on a TN-C-S supply can have PFC values that exceed the 6kA breaking capacity of standard domestic MCBs. This is not unusual — it is simply a consequence of the very low supply impedance at close range. In these cases, higher-rated devices must be installed.
05 · BS 7671 Testing
Breaking capacity (also called rated short-circuit capacity) is the maximum fault current that a protective device has been tested and certified to safely interrupt. The device must be able to contain the arc, extinguish it, and open the circuit without fire, explosion, or danger. Different device types and standards specify different breaking capacities.
MCB (BS EN 60898)
6kA minimum (standard domestic), 10kA, 15kA available
MCB (BS EN 60947-2)
10kA, 16kA, 25kA, 36kA, 50kA (industrial)
RCBO (BS EN 61009)
6kA minimum (domestic), 10kA available
HRC fuse (BS 88)
80kA typical — very high breaking capacity
BS 3036 rewirable fuse
1kA to 4kA — very low breaking capacity
MCCB (moulded case)
16kA to 150kA depending on frame size
A critical point: BS 3036 rewirable fuses have very low breaking capacities, typically 1kA to 4kA. On many modern supplies with PFC values of 3kA or more, the BS 3036 fuse may not have adequate breaking capacity. This is one of the reasons why BS 3036 fuse boards are often identified as requiring upgrade during periodic inspection.
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AI board scanner, voice test entry, and automatic BS 7671 validation — finish the certificate before you leave the property. From £6.99/mo.
06 · BS 7671 Testing
Prospective fault current and Ze (external earth fault loop impedance) are directly related through Ohm's law. The prospective earth fault current is:
Ipf(earth) = Uo ÷ Ze
Uo = nominal phase voltage (230V) | Ze = external earth fault loop impedance
The lower the Ze, the higher the earth fault current. For a TN-C-S supply with a typical Ze of 0.20 ohms: Ipf = 230 / 0.20 = 1,150A (1.15kA) for the earth fault path. The prospective short-circuit current (line-to-neutral) is typically higher still because the neutral impedance is lower than the earth fault loop impedance.
Ze = 0.10Ω
2.3 kA
Ze = 0.20Ω
1.15 kA
Ze = 0.35Ω
657 A
Ze = 0.80Ω
288 A
Remember that the earth fault PFC is usually lower than the short-circuit PFC (line-to-neutral). The overall PFC recorded on the certificate should be the higher of the two values — typically the line-to-neutral short circuit current. The Ze is recorded separately in the supply characteristics section of the earthing arrangements data.
In the app
Prospective fault current calculator. Work Ipf out from Ze or supply impedance and check it against device breaking capacities to BS 7671:2018+A4:2026.
07 · BS 7671 Testing
PFC being "too high" means that the prospective fault current exceeds the rated breaking capacity of the installed protective devices. This is a serious safety defect that requires immediate action.
The most straightforward solution is to replace the protective devices with ones that have a breaking capacity exceeding the measured PFC. For example, replacing standard 6kA MCBs with 10kA MCBs. This may require a consumer unit upgrade if the existing consumer unit cannot accept higher-rated devices.
An HRC fuse (BS 88) installed upstream of the consumer unit can limit the fault current that reaches the downstream MCBs. BS 88 fuses have very high breaking capacities and can current-limit, reducing the peak fault current. This "back-up protection" arrangement can allow standard 6kA MCBs to be used downstream of a suitable HRC fuse — but only where the manufacturer of the downstream device declares the combination suitable. Reg 536.4.2.1 is explicit that where no manufacturer information is available, combined short-circuit protection shall not be used and every device must have the required short-circuit capability at its own point of installation.
If PFC exceeds device breaking capacity, this should typically be classified as C2 (potentially dangerous) on the EICR. In extreme cases where the margin is significant, C1 (danger present) may be appropriate. The observation should specify the measured PFC, the device breaking capacity, and the recommended remedial action.
08 · BS 7671 Testing
BS 7671 has several regulations that address prospective fault current. The key regulations are:
The rated short-circuit capacity of each protective device shall be not less than the prospective fault current at the point at which the device is installed. This is the fundamental requirement — every device must be able to safely interrupt the maximum fault current it could face.
Where the short-circuit breaking capacity of a device is lower than the maximum prospective short-circuit or earth fault current expected at its point of installation, Reg 533.3 requires compliance with the last paragraph of Reg 536.1 and with Reg 536.5. Reg 536.4.2.1 (combined short-circuit protection of overcurrent protective devices) sets the acceptance route: reference shall be made to the instructions of the manufacturer of the downstream device, and where no manufacturer information is available, combined short-circuit protection shall not be used and each device must have the required short-circuit capability at its point of installation. Reg 536.5 then requires that coordination documentation be added to the design documentation at initial verification. This is the regulatory basis for back-up protection arrangements — for example an HRC BS 88 fuse upstream of standard 6kA MCBs.
Regulation 643.7.3.201 (Part 6, Chapter 64) requires that the prospective short-circuit current and prospective earth fault current are measured, calculated, or determined by another method, at the origin and at other relevant points in the installation. This is the testing-stage duty. The design-stage obligation sits in Regulation 313.1, which requires the prospective short-circuit current at the origin of the installation and the external earth fault loop impedance Ze to be determined by calculation, measurement, enquiry or inspection, and in Regulation 434.1, which requires the prospective fault current to be determined at every relevant point of the installation by calculation, measurement or enquiry. Those figures inform protective device selection, consumer unit specification, and cable sizing.
Chapter 43 sets out the general principles of protection against overcurrent, including short-circuit current. The protective device must disconnect the circuit before the short-circuit current causes damage to the cable insulation or connections. The adiabatic equation (k²S² ≥ I²t) is the tool used to verify this requirement.
In the app
Elec-Mate's digital EICR records the PFC at the origin and checks it against the breaking capacity of every installed protective device.
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