A retail fitout runs lighting down a long tenancy. The circuit is 1.5 mm² on a 10 A device and the route reaches 60 m. Small conductors and long runs are the combination that fails an earth fault loop impedance check. This page works that circuit and shows what the device curve does to the limit.
The inputs
- External loop impedance Ze: 0.35 Ω
- Phase to earth voltage: 230 V
- Device: B curve, 10 A
- Route length: 60 m
- Active conductor: 1.5 mm² copper
- Earth conductor: taken from the table
Ze is a site value. It comes from a measurement at the origin or from the network operator. The tool does not calculate it.
The clause and the tables
AS/NZS 3000:2018 Cl 5.7.4 sets the earth fault loop impedance requirement. AS/NZS 3000:2018 Table 8.1 gives the maximum Zs for the device curve and rating. AS/NZS 3000:2018 Table 5.1 sets the earth conductor size for the active size. AS/NZS 3008.1.2:2017 Tables 34 and 35 give the conductor resistance at the operating temperature.
The steps
- Earth conductor size: 1.5 mm²
- R1 + R2 per km at the conductor operating temperature: 33 Ω/km
- R1 + R2 over the run: 1.98 Ω
- Earth fault loop impedance Zs: 2.33 Ω
The active and the earth are both 1.5 mm². The loop therefore carries two conductors of the same resistance over the whole route. A small conductor moves Zs fast for that reason. 33 Ω/km over 60 m adds 1.98 Ω to a Ze of 0.35 Ω.
The result
- Earth fault loop impedance: 2.33 Ω
- Maximum permitted Zs: 5.75 Ω
- Prospective fault current: 98.71 A
- Maximum length at the limit: 163.64 m
The circuit passes. 2.33 Ω sits well under the 5.75 Ω limit. The 60 m route is far short of the 163.64 m that the B curve allows.
The curve moves the limit, not the circuit
Change the device to a C curve at the same 10 A rating and hold everything else.
- Earth fault loop impedance: 2.33 Ω, unchanged
- Maximum permitted Zs: 3.07 Ω
- Maximum length at the limit: 82.32 m
Zs did not move, because the cable did not move. The limit fell from 5.75 Ω to 3.07 Ω, and the allowed run fell from 163.64 m to 82.32 m. A C curve device needs more fault current to trip in its magnetic region, so it tolerates less loop impedance.
The same circuit at 60 m passes on either curve. A 100 m run passes on the B curve and fails on the C curve. On a fitout with long tenancy runs, the curve choice sets the reach of the circuit. Past that reach the conductor has to grow.
What else to check
The prospective fault current of 98.71 A flows on an earth fault at the far end of this circuit. The device has to see that current to operate in the intended time. A measured Zs on commissioning should land near 2.33 Ω. Allow for the temperature difference between a cold test and a loaded conductor.
Ze of 0.35 Ω is an assumption until it is measured. On this circuit Ze is a small part of the 2.33 Ω total. A higher site value therefore moves the result less than the run length does. On a short circuit near the board, Ze dominates instead.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Max cable length for a 20 A C curve subcircuit, AS/NZS 3000 Table 8.1: max Zs by curve and rating and Loop impedance and disconnection time.