Prospective Fault Current Calculator
Calculate PSCC & PEFC to verify protective device breaking capacity
PEFC always uses 230V line-earth. The 400V option only affects PSCC.
DNO maximum Ze helpers
These helpers only populate the earth fault loop value for PEFC. Confirm the value by enquiry or measurement before relying on it.
Impedance Values
External or distribution-board earth fault loop impedance
Measured or calculated short-circuit loop impedance at the origin or DB
Select this only after confirming the relevant device or assembly rating.
Formulas Used
What is Prospective Fault Current (Ipf)?
Prospective fault current is the maximum current that would flow if a fault occurred at a given point in an installation. It determines the breaking capacity required for protective devices (Regulation 434.5.1).
PEFC vs PSCC
PEFC (Prospective Earth Fault Current) is the current during an earth fault (line to earth). PSCC (Prospective Short Circuit Current) is the current during a short circuit (line to neutral or line to line). The higher value is recorded as Ipf when both are known.
Why Does It Matter?
If the PFC exceeds the breaking capacity of a device, the device may fail to safely interrupt the fault current, potentially causing fire or explosion. BS 7671 verification and reporting model forms include prospective fault current and protective device breaking capacity details.
Use measured, calculated, or enquired values for the point being assessed. Current IET model forms include fields for Ipf, Ze and breaking capacity; conditional assembly ratings still need the relevant manufacturer and upstream protective-device context.
Domestic (PME)
Typically 2-5 kA at the origin. Lower external impedance means higher fault current.
Domestic (TN-S)
Typically 1-3 kA at the origin. Higher external impedance results in lower fault current.
Commercial
Can exceed 16 kA near transformer. May require higher rated protective devices.
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