Loop Impedance
Check the earth fault loop impedance against the maximum Zs.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the loop impedance calculator do?
The tool adds the external impedance Ze to the resistance of the active and earth conductors over the run length. The sum is the earth fault loop impedance Zs. It compares Zs with the maximum for the protective device, and reports the prospective fault current and the longest run that still passes.
Which tables set the maximum Zs?
The maximum Zs comes from AS/NZS 3000:2018 Table 8.1 for the device curve and rating. The earth conductor size comes from Table 5.1. Conductor resistance comes from AS/NZS 3008.1.2:2017 Tables 34 to 35. A failed check cites AS/NZS 3000:2018 Clause 5.7.4.
Which inputs matter most for Zs?
Ze, run length and active conductor size change Zs the most. The device curve and rating set the limit that Zs is compared with. The earth conductor size is taken from Table 5.1 unless you enter one. Cables in parallel divide the active resistance only, not the earth resistance.
What are the common gotchas?
Ze is a site value and is not measured by the tool. A warning appears when Ze alone uses 90 percent or more of the maximum Zs. The prospective fault current is worked out at the phase to earth voltage, which defaults to 230 V. Inputs sit in the page address for sharing.