Fault current
Prospective fault current calculator
Estimate the prospective fault current (PFC) at a point in the installation from the nominal voltage and the loop impedance, and check it against your devices' breaking capacity.
PFC is about Uo divided by Z. Record the higher of the prospective short-circuit current and prospective earth-fault current.
Prospective fault current explained
Prospective fault current is the current that would flow if a dead short happened at a point in the installation. It tells you whether the protective devices there can interrupt that current safely. Every device has a rated breaking capacity, and BS 7671 requires it to be at least equal to the PFC where the device sits.
PSCC and PEFC: record the higher
There are two values. Prospective short-circuit current (PSCC) is a line-to-neutral fault. Prospective earth-fault current (PEFC) is a line-to-earth fault. Measure or calculate both and record the higher as the prospective fault current, because that is the worst case the device has to handle. The calculation is the nominal voltage divided by the impedance of the relevant loop.
Breaking capacity on site
Common domestic MCBs are rated 6 kA, which suits most homes. A stiffer supply, close to a substation or on larger services, calls for 10 kA devices, and you may rely on the back-up protection of the supplier cut-out (the Icn and Ics pair). If the result is above your devices' rating you need higher-rated devices or documented back-up protection. This shares the loop used by the max Zs check, used the other way round.
FAQs
How is prospective fault current calculated?
PFC is the nominal voltage divided by the loop impedance. At the origin, prospective earth-fault current uses Ze and short-circuit current uses the line-neutral loop impedance. Record the higher of the two.
What breaking capacity do I need?
Each device's rated breaking capacity (Icn) must be at least equal to the prospective fault current where it is installed. Common domestic MCBs are 6 kA. 10 kA types are used where the PFC is higher.
What is the difference between PSCC and PEFC?
PSCC is the prospective short-circuit current from a line-to-neutral fault. PEFC is the prospective earth-fault current from a line-to-earth fault. The figure you record is the greater of the two.