UK Earth Fault Loop Impedance Calculator — BS 7671
The earth-fault loop impedance check is the test that decides
whether your protective device will actually trip in time during
an earth fault. WireWrite's UK calculator works the equation
backwards from the disconnection time required by BS 7671 to the
maximum permitted Zs for the breaker and circuit
you've selected.
What Zs actually measures
Zs (earth-fault loop impedance) is the total
impedance of the path a fault current takes when a live conductor
contacts the protective earth conductor or an exposed conductive
part. On a TN-S system the loop runs from the fault point, back
through the CPC, through the main earth terminal, back to the
transformer star point via the supply earth, and out again on the
live conductor. On TN-C-S (PME), the return path is the combined
neutral-and-earth (PEN) conductor as far as the supply head. The
lower the Zs, the higher the fault current, the faster
the protective device disconnects.
The formula every electrician should know
The maximum permitted earth-fault loop impedance is:
Zs(max) = (Cmin ×
Uo) / Ia
Where Uo is the nominal voltage to earth (230 V on
single-phase UK supplies), Cmin = 0.95 (the BS 7671
voltage-tolerance factor applied at calculation), and
Ia is the current that causes automatic disconnection
of the protective device within the time required by BS 7671
Regulation 411.3.2 — typically 0.4 s for final circuits up
to 32 A on TN systems, and 5 s for distribution circuits and
circuits above 32 A.
Ia is not the rated trip current of the breaker.
For a Type B MCB it's roughly 5 times the rated current; for
Type C it's roughly 10 times; for Type D, roughly 20 times. The
calculator uses manufacturer-published instantaneous trip
currents from common UK ranges.
Verify Zs against the measured value
The calculator gives you the maximum permitted Zs
for your circuit. The number you measure on site with a loop
tester must be less than this value. BS 7671 Regulation 643.7.3
covers the in-service test; the figures recorded should appear
on the Schedule of Test Results section of the EIC or EICR.
Calculate Zs for
your specific circuit →
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BS 7671 Certificates Supported
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BS 7671 Calculators
- Cable Sizing Calculator
- Voltage Drop Calculator
- Maximum Demand Calculator
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