NZ Earth Fault Loop Impedance Calculator — AS/NZS 3000 Table 8.1 & 8.2
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 NZ calculator works the equation backwards
from the disconnection time required by AS/NZS 3000 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
earth conductor or an exposed conductive part. The loop runs from the
fault point, back through the protective earth conductor, through the
MEN link at the switchboard, back to the transformer star point via
the supply neutral, and out again on the active conductor. The lower
the Zs, the higher the fault current, the faster the
protective device trips.
The formula every electrician should know
The maximum permitted earth-fault loop impedance is:
Zs(max) = Uo / Ia
Where Uo is the nominal voltage to earth (230 V on
single-phase NZ supplies) and Ia is the current that causes
automatic disconnection of the protective device within the time
required by AS/NZS 3000 — typically 0.4 s for final
sub-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
NZ 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, with appropriate test margin. AS/NZS 3000
Clause 8.3.6 covers the in-service test.
Calculate Zs for
your specific circuit →
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