Record the higher of L-N and L-E measurements; check device breaking capacity covers it.
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This calculator converts a measured loop impedance into prospective fault current — the current that would flow in a dead short at that point in the installation. UK electricians record PFC (the higher of the prospective short-circuit and earth fault currents) on every certificate, and this tool gives the figure in both amps and kA from Ohm's law.
Enter the supply voltage — 230V by default — and the measured loop impedance in ohms. The lower the impedance, the higher the fault current, which is why PFC is highest at the origin of the installation and why the breaking capacity of your protective devices matters most at the board.
Ipf = V / Z
Ipf is the prospective fault current in amps. V is the supply voltage, taken as 230V nominal for a single-phase supply unless you enter a different value. Z is the loop impedance in ohms of the fault path being considered — line to neutral for short-circuit current, line to earth for earth fault current. The tool also expresses the result in kA, which is how device breaking capacities are stated.
230V is preset for a single-phase UK supply. Change it if you are working at a different voltage.
Type in the measured impedance in ohms — the L-N loop for short-circuit current or the L-E loop for earth fault current. Measure both and use each in turn.
The result appears in amps and kA. Record the higher of the L-N and L-E derived values as the PFC, and confirm every protective device has a breaking capacity at or above it.
Prospective fault current must be determined at the origin of every installation, and protective devices must be able to interrupt it safely — that is what the breaking capacity (for example 6kA or 10kA marked on an MCB) refers to. A device asked to break more current than its rating can fail destructively.
PFC is highest where impedance is lowest, normally at the main incoming terminals, and falls as you move out along circuits. Testing at the origin therefore captures the worst case for the installation. On single-phase supplies, measure both line-to-neutral and line-to-earth and record the higher figure.
Remember this is Ohm's law on a measured impedance: the accuracy of the result depends on the loop test. Meter readings on very low impedances have real measurement uncertainty, so treat marginal results against a device's breaking capacity with caution and follow BS 7671 for the installation's specific requirements.
Record the higher of L-N and L-E measurements; check device breaking capacity covers it.
It is the current that would flow if a fault of negligible impedance occurred at the point considered — a dead short between live conductors (prospective short-circuit current) or line to earth (prospective earth fault current). The value recorded on certificates is the higher of the two, normally measured at the origin.
Divide the supply voltage by the measured loop impedance: 230V across a 0.28Ω loop gives about 821A, or 0.82kA. Do this for both the L-N and L-E measurements and take the higher result as the PFC for the installation.
At least the prospective fault current at the point where they are installed. Domestic consumer unit devices are commonly rated 6kA, which suits most domestic supplies, but the measured PFC decides it — if it exceeds the device rating you need higher-capacity devices or backup protection arrangements. Verify against BS 7671 and the manufacturer's data.
Because loop impedance is at its minimum there, so fault current is at its maximum. Devices further into the installation see lower fault levels, so a check at the origin covers the worst case, though devices at distribution boards should also be adequate for the fault level at their position.
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