What is the difference between PSCC and PEFC?

PSCC (Prospective Short-Circuit Current) is the maximum current that would flow if the line and neutral conductors were shorted together at a given point — calculated as V/Ze at the origin. PEFC (Prospective Earth Fault Current) is the current that would flow in a line-to-earth fault — calculated as V/Zs. PSCC is always higher than PEFC because Ze is always lower than Zs. Both must be checked against the breaking capacity of protective devices.

Why must I check breaking capacity?

BS 7671 Regulation 434.5.1 requires that every protective device has a rated short-circuit breaking capacity not less than the prospective fault current at its point of installation. If the fault current exceeds the breaking capacity, the device may not be able to safely interrupt the fault — potentially causing an arc flash, fire, or explosion. This is a critical safety requirement verified during initial verification.

What is a typical PSCC for a UK domestic installation?

UK domestic installations typically see PSCC values between 1kA and 6kA at the origin. Supplies fed from older overhead lines may be as low as 0.8kA, while modern underground supplies in urban areas can reach 16kA or more. Most domestic MCBs and consumer units are rated at 6kA breaking capacity. If PSCC exceeds 6kA, you need devices rated to 10kA or higher, or an upstream device providing back-up protection.

How do I measure Ze for PSCC calculation?

Ze is measured at the origin of the installation using a loop impedance tester with the main earthing conductor disconnected from the MET. This gives the external earth fault loop impedance of the supply network alone. Typical Ze values: TN-S (cable sheath earth) = 0.35-0.8Ω, TN-C-S (PME) = 0.2-0.35Ω, TT = can be very high (1-200Ω or more, depending on electrode resistance).

Can I use PFC readings from a multifunction tester?

Yes — modern multifunction testers like the Megger MFT or Metrel measure PFC directly in kA. However, it's important to understand that the tester calculates PFC from the measured loop impedance using Ohm's law (I = V/Z). The tester assumes nominal voltage (230V), so if your supply voltage differs, the actual fault current will differ. This calculator lets you input your actual measured Ze and Zs to get accurate results.

PFC Calculator — Prospective Fault Current per BS 7671

Prospective Fault Current (PFC) Calculator

Calculate PSCC and PEFC from measured impedance values and verify against protective device breaking capacity per BS 7671 Regulation 434.5.1.

Ze (External Earth Loop Impedance)

Measured at origin — determines PSCC

Zs (Total Earth Fault Loop Impedance)

Ze + R1+R2 — determines PEFC

Device Breaking CapacityIcn rating of MCB/RCBO

Supply TypeDetermines voltage used in calculation

How We Calculate This

Prospective fault current is calculated using Ohm's law applied to the fault loop:

PSCC = Uo / Ze (short-circuit at origin)

PEFC = Uo / Zs (earth fault at furthest point)

Where Uo is the nominal line-to-neutral voltage (230V UK, used for both single-phase and symmetrical three-phase fault calculation per BS 7671 Appendix 3).

BS 7671 Requirements

Regulation 434.5.1: Breaking capacity must exceed prospective fault current
Regulation 411.4.6: Zs must be low enough for disconnection within required time
Typical domestic: 6kA breaking capacity for MCBs
Commercial: 10-25kA breaking capacity typically required
Back-up protection: Per Regulation 434.5.1, a device with lower breaking capacity is permitted if backed up by a device with adequate capacity