Earth Fault Loop Impedance Calculator

Earth Fault Loop

Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.

Methodology

Total earth fault loop impedance is the sum of the external supply loop impedance and the circuit's own phase and protective earth conductor resistances:

where Z_e is the external supply loop impedance, R_1 is the phase conductor resistance, and R_2 is the protective earth (PE) conductor resistance for the circuit length. Prospective earth fault current follows using nominal phase-to-earth voltage (230V in Australia):

This is compared against an indicative instantaneous magnetic trip multiplier for common MCB curve types (Type B ≈5×, Type C ≈10×, Type D ≈20× rated current) to check whether automatic disconnection is likely within the required time.

Assumptions & Limitations

• The displayed Zs is calculated from the entered Ze plus computed phase and PE conductor resistance — it is not a measured value. If you have a Zs figure from an on-site loop impedance tester, that measured value already includes real-world factors (contact resistance, actual conductor temperature) this calculation cannot capture, and should be treated as more authoritative
• MCB trip multipliers are indicative typical values for the curve type — actual trip current varies by manufacturer and specific device. Always verify against the manufacturer's published trip curve.
• Does not account for cable temperature during fault (resistance increases with conductor temperature)
• This is a calculation and checking aid only — does not itself constitute a compliance certification against AS/NZS 3000
• RCD protection, where fitted, provides an additional disconnection path independent of this loop impedance calculation