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.
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 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.
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 →
NZ earth fault loop impedance (Zs) calculator with AS/NZS 3000 Table 8.1 & 8.2 maximum Zs values for MCB Types B, C and D. Free, per-breaker-type Ia values.