Table 8.1 of AS/NZS 3000:2018 holds the maximum earth fault loop impedance for a circuit. It turns a disconnection time requirement into one number you can measure.
What the table governs
A protective device opens on an earth fault only if enough current flows. The current depends on the impedance of the fault loop. Table 8.1 gives the highest loop impedance that still lets the device operate in time.
Cl 5.7.2 sets the disconnection times behind the table. Cl 5.7.4 is the clause a failed check cites. Table 8.2 covers the longer permitted times for other circuit types. AmpSize compares against the Table 8.1 figure.
Two inputs index the table: the device curve and the device rating. The curve matters because the magnetic trip threshold differs. A B curve device trips at a lower multiple of its rating than a C curve. A C curve trips at a lower multiple than a D curve. A lower threshold allows a higher loop impedance.
Which tools read the table
- The loop impedance tool reads Table 8.1 for the limit and Table 5.1 for the earth conductor size.
- The cable size tool runs the same check on every candidate conductor when you enter a Ze.
Both take conductor resistance from Tables 34 and 35 of AS/NZS 3008.1.2:2017.
The inputs
A final subcircuit on a domestic board.
- External impedance Ze: 0.35 Ω
- Supply: 230 V
- Device: 16 A, B curve
- Route length: 30 m
- Active conductor: 2.5 mm²
The steps
The engine takes the earth conductor size from Table 5.1 and gets 2.5 mm². Both conductors are the same size, so each contributes the same resistance.
Combined, the active and earth conductors have a resistance of 18.02 Ω/km at the operating temperature. Over 30 m that gives an R1 + R2 of 0.54 Ω.
Ze plus R1 + R2 gives a Zs of 0.89 Ω.
The result
The circuit passes. Zs is 0.89 Ω against a Table 8.1 maximum of 3.59 Ω for a 16 A B curve device.
Two other figures come out of the same run. The prospective fault current is 258.25 A. The maximum length at the limit is 180.01 m. This cable has a long way to go before the clause bites.
Why the curve changes the answer
Hold every other input still and change the curve. On a 16 A C curve device the engine returns a maximum Zs of 1.92 Ω. On a 16 A D curve device it returns 1.15 Ω.
The same 0.89 Ω circuit passes all three. A circuit at 2 Ω would pass on B, fail on C and fail on D. The cable did not change. The device did.
A D curve device on a motor circuit therefore forces a larger active conductor or a shorter run. The limit it must meet is roughly a third of the B curve limit at the same rating.
Reading Ze correctly
Ze is a site value. The tool does not measure it and cannot check the figure you type. A high Ze eats the budget before the cable adds anything. The tool warns when Ze alone takes 90 % or more of the limit.
Test to Section 8 of AS/NZS 3000:2018 and compare the measured Zs with the calculated Zs. A large gap points to a wrong Ze, a wrong length or a poor connection.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Earth fault loop impedance and maximum Zs, Cable size for a 32 A single‑phase 25 m run and Loop impedance and disconnection time.