What is prospective fault current and why does it matter?

Prospective fault current is the maximum current that would flow during a short circuit or earth fault. BS 7671 Regulation 434.5.1 requires that the prospective fault current be determined at every point of the installation. The protective device must have a breaking capacity at least equal to the prospective fault current, otherwise it may not safely interrupt the fault and could cause fire or damage.

What is the difference between PSCC and PEFC?

PSCC (Prospective Short Circuit Current) is the fault current that flows in a line-to-neutral short circuit. PEFC (Prospective Earth Fault Current) is the fault current that flows in a line-to-earth fault via the circuit protective conductor. PSCC is typically higher because the neutral conductor usually has lower impedance than the CPC. Both must be checked against the protective device's breaking capacity.

How is prospective fault current calculated?

Prospective fault current is calculated using Ohm's law: I = Uo / Zs, where Uo is the nominal voltage (230V for single phase) and Zs is the total loop impedance. At the origin, PSCC = 230 / Ze. At any point in the circuit, the conductor impedances (R1, Rn or R2) are added to Ze.

What breaking capacity do I need for my MCBs?

Standard domestic MCBs typically have a 6kA breaking capacity, which is adequate for most domestic installations where PSCC is usually below 6kA. For installations close to substations or with very low Ze values, the PSCC may exceed 6kA and MCBs with higher breaking capacity (10kA or 16kA) or HRC fuses (BS 88 with 80kA) may be required.

What are typical Ze values for UK supply types?

TN-C-S (PME) systems typically have Ze values between 0.2 and 0.35 ohms. TN-S (lead sheath) systems typically range from 0.35 to 0.8 ohms. TT systems can have much higher values. The DNO (Distribution Network Operator) can provide the maximum expected Ze for your supply. Lower Ze means higher prospective fault current at the origin.

Prospective Fault Current Calculator — BS 7671

Calculate prospective short circuit current (PSCC) and prospective earth fault current (PEFC) from loop impedance values per BS 7671 18th Edition Regulation 434.5.1.

Supply Type

Single or three phase supply

Earthing System

Type of supply earthing arrangement

External Loop Impedance (Ze)

Measured at origin of installation

Circuit Length

One-way cable run length

r1 (Line conductor resistance)

Resistance per metre of line conductor

rn (Neutral conductor resistance)

Resistance per metre of neutral conductor

r2 (CPC resistance)

Resistance per metre of circuit protective conductor

How We Calculate This

This calculator determines the prospective fault current at the origin and at any point in a circuit using Ohm's law applied to the fault loop impedance.

Prospective fault current formula

At the origin: PSCC = Uo / Ze, where Uo is the nominal supply voltage (230V single phase) and Ze is the external earth fault loop impedance.

At any point: PEFC = Uo / Zs, where Zs = Ze + R1 + R2 (the total line-earth loop impedance including circuit conductor resistances).

Breaking capacity

BS 7671 Regulation 434.5.1 requires the breaking capacity of the protective device to be not less than the prospective fault current at the point of installation. The calculator compares the PSCC at the origin against the device breaking capacity.