A circuit is only safe if a fault disconnects fast enough. Work out Zs and the prospective fault current, and check it against the maximum Zs for your protective device.
The maximum Zs here is based on the device’s instantaneous trip multiple. Formal verification must use the manufacturer’s data and the disconnection times and rules in AS/NZS 3000.
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Start free trialThe earth fault loop impedance Zs is the external loop impedance Ze plus the circuit’s conductor resistances R1 (active) and R2 (protective earth).
Dividing the nominal voltage by Zs gives the prospective earth fault current — the current that would flow in an earth fault.
For safety, that current must trip the protective device fast enough. Comparing Zs against the maximum for the device tells you whether the circuit disconnects in time.
It’s the total impedance of the path a fault current follows in an earth fault — from the supply, through the active conductor to the fault, and back through the earth conductor. Written Zs, it equals the external impedance Ze plus the circuit’s R1 + R2. A lower Zs means a higher fault current and faster disconnection.
If a live conductor faults to earth, the protective device must disconnect the supply quickly enough to prevent electric shock. That only happens if the fault current is high enough — which means Zs must be low enough. Verifying Zs is a core part of proving a circuit is safe.
The maximum permitted Zs is the voltage divided by the current needed to operate the protective device within the required disconnection time. For MCBs this relates to the instantaneous trip current — roughly 5× rated current for Type B, 10× for Type C and 20× for Type D. This tool uses those multiples; the definitive figures come from the device data and AS/NZS 3000.
A Zs that’s too high means the device won’t disconnect fast enough. Options include using a lower-rated or more sensitive protective device, increasing conductor size to lower R1 + R2, shortening the run, or relying on an RCD for additional protection — all subject to design to the standards.
This calculator is an indicative aid using nominal MCB trip multiples, not a substitute for verification to AS/NZS 3000. Actual maximum Zs values depend on the specific device, disconnection times and measured impedances. All testing and verification must be performed by a licensed electrician.
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