8.3 Prospective Fault Current (PFC/PSCC)

What Is Prospective Fault Current?

Prospective fault current (PFC) is the maximum current that would flow if a low-impedance fault occurred at a given point in the installation. It is the worst-case fault the protective device would ever be asked to clear. If the device's rated breaking capacity is lower than the PFC, the device may not safely interrupt the fault — it could fail to clear, sustain an arc, rupture, or cause a fire. For that reason, BS 7671 requires PFC to be measured (or determined by enquiry from the DNO) and recorded on the Electrical Installation Certificate (EIC) or Electrical Installation Condition Report (EICR).

The Two Components — PEFC and PSCC

A fault can be either phase-to-earth or phase-to-neutral (or phase-to-phase on a 3-phase system). These give two separate prospective currents:

• PEFC — Prospective Earth Fault Current: measured phase-to-earth (L–E). This is the fault current that flows around the earth fault loop and is governed by Ze (external) plus the installation earth path. PEFC = Uo / Ze at the origin.
• PSCC — Prospective Short-Circuit Current: measured phase-to-neutral (L–N), and for 3-phase systems also phase-to-phase (L–L). This is the current that flows when line and neutral (or two lines) are shorted, governed by the phase/neutral loop impedance.

For a single-phase installation, PEFC and PSCC are often similar in magnitude because the supply earth and neutral paths are similar. For a 3-phase installation, the phase-to-phase PSCC is higher — roughly √3 (≈1.73) times the single-phase L–N value across the same loop — because the driving voltage is 400 V rather than 230 V. On 3-phase systems PSCC therefore dominates.

Which value is recorded?

PFC recorded = the GREATER of PEFC and PSCC.

You measure both and enter the larger number on the EIC/EICR as the PFC for the installation. This is the value the protective devices must be able to break. Recording the smaller value would understate the duty on the device and is a defect.

Why PFC Is Measured at the Origin

Fault current is highest where loop impedance is lowest, and loop impedance is lowest closest to the source — the origin of the installation, just inside the consumer unit / distribution board, upstream of any sub-main length. The origin is therefore the worst-case location for PFC. Any fault downstream of the origin has additional conductor impedance in series, which reduces the available fault current.

Rule of thumb: PFC is measured at the origin (incoming supply position). You do not need to measure PFC at every accessory — by the time the fault reaches a final circuit the available current is lower, not higher. The origin value sets the duty on the upstream protective device(s).

Measurement Procedure

PFC is measured with a PFC tester, which is almost always combined with the loop impedance tester in a single instrument — the same tester that gives you Ze at the origin will give you PFC. The procedure:

1. At the origin, with the main switch off (so the installation is isolated) and the tester connected on the supply side of the main switch, measure:
   • PEFC between phase and earth (L–E).
   • PSCC between phase and neutral (L–N).
   • On a 3-phase system, also measure PSCC phase-to-phase (L1–L2, L2–L3, L1–L3) — take the highest.
2. Compare PEFC and PSCC (and the L–L value on 3-phase). The highest reading is the PFC.
3. Record that value on the EIC/EICR.

Because PFC and Ze are both origin tests, they are usually performed together at the start of the live testing sequence — GN3 lists them as the first live tests at the origin. Some testers display Ze and PFC from the same set of leads, halving the connections.

PFC measurement checklist

Relationship to Breaking Capacity

Every overcurrent protective device has a rated breaking capacity — the maximum fault current it can safely interrupt. For BS EN 60898 MCBs the standardised Icn (rated breaking capacity) ratings are 4500 A, 6000 A and 10000 A (4.5 kA, 6 kA, 10 kA). For BS 88 fuses and BS EN 61439 assemblies, Icn is much higher.

The compliance rule is:

Protective device Icn (or Ics) ≥ PFC at that point.

Icn is the ultimate breaking capacity (the device clears once at this current but may be unfit for further service). Ics is the service breaking capacity (the device clears, and is fit for further use). For practical verification on a 2391-52 inspection, compare the PFC against the device's Icn and record the device's breaking capacity on the schedule of test results.

Worked example

A single-phase domestic installation. At the origin:

• Measured PEFC (L–E) = 4.2 kA.
• Measured PSCC (L–N) = 5.1 kA.

PFC recorded = 5.1 kA (the greater).

The incoming device is a BS EN 60898 MCB rated Icn = 6 kA (6000 A). 6 kA ≥ 5.1 kA → satisfactory. If the PFC had been 7.2 kA, a 6 kA MCB would be inadequate and the device would need upgrading (e.g. to a 10 kA MCB or a fuse with a higher Icn).

3-phase worked example

A 3-phase installation. At the origin:

• PEFC (L–E) = 8 kA.
• PSCC (L1–N) = 8.5 kA.
• PSCC (L1–L2) = 14.7 kA (≈ √3 × 8.5).

PFC recorded = 14.7 kA (the L–L value, the greatest). The upstream switchgear / fuse must have a breaking capacity of at least 14.7 kA. This is why 3-phase boards often need fuses or high-breaking-capacity MCBs rather than standard domestic 6 kA devices.

Common Pitfalls

• Recording only one component. Measuring PEFC but not PSCC (or vice versa) means the recorded PFC may understate the duty on the device. Always measure both and record the greater.
• Ignoring the L–L value on 3-phase. The phase-to-phase PSCC is roughly 1.73× the L–N value; omitting it undercounts PFC by a large margin on 3-phase boards.
• Measuring at the wrong point. PFC is an origin test. Measuring at a final circuit gives a lower, non-worst-case value.
• Confusing Icn with In. In is the rated current (e.g. 32 A); Icn is the breaking capacity (e.g. 6000 A). The check is against Icn, not In.

Chapter 8.3 summary

• PFC = maximum prospective fault current at a point; the device must be able to break it.
• PFC = greater of PEFC (L–E) and PSCC (L–N / L–L). For 3-phase, L–L dominates.
• Measured at the origin, where loop impedance is lowest and fault current is highest.
• Usually measured together with Ze using a combined tester.
• Device Icn ≥ PFC; BS EN 60898 MCB Icn ratings are 4500 / 6000 / 10000 A.