Fault Loop Impedance Calculator

It is the impedance of the path a fault current takes from the supply, through the active conductor to the fault, back along the protective earth and the supply neutral. AS/NZS 3000:2018 Clause 5.7 requires it to be low enough that the fault current trips the protective device within the disconnection time of Clause 5.7.4 (0.4 s for final subcircuits up to 63 A supplying socket-outlets or hand-held equipment, 5 s for others).

Zs maximum = Uo / Ia, where Uo is the nominal phase-to-earth voltage (230 V) and Ia the current that operates the device instantaneously. For MCBs the calculator takes Ia as 4, 7.5 and 12.5 times the rating for B, C and D curves, the multiples behind the AS/NZS 3000:2018 Table 8.1 values. A C20 MCB at 230 V allows 230 / 150 = 1.53 Ω.

AS/NZS 3000:2018 Appendix B5.2.2 assumes that 80% of the nominal voltage is available at the circuit's origin during a fault; the other 20% is lost in the supply. Dividing the circuit's own active plus earth impedance by 0.8 gives the total loop impedance compared with Uo / Ia, without needing a measured external impedance.

The AS/NZS 3008.1.1:2025 a.c. resistance at the insulation's maximum operating temperature (75 °C for V-75 PVC, 90 °C for X-90 XLPE) for both the active and the earth, plus the tabulated reactance. That is the hot, worst-case loop the device must still clear.

No. AS/NZS 3000:2018 Clause 8.3.9 requires the fault loop to be verified when the installation is tested. The calculation is for design: it tells you before installation whether the cable and earth sizes and the route length work with the chosen device.

Shorten the route, increase the earth conductor (it is often smaller than the active), increase the active size, or use a device with a lower instantaneous trip current, such as a B curve or an RCD where permitted. The maximum length in the results shows how far the circuit can run as it is.