Maximum earth fault loop impedance (Zs) lookup
The maximum Zs for a circuit breaker or fuse under AS/NZS 3017:2022 Table 4.5, plus the maximum earth-conductor resistance from Table 4.6 — by device type, rating and disconnection time.
AS/NZS 3017:2022 Tables 4.5 & 4.6
Table 4.5 publishes only the 0.4 s column for MCBs.
Maximum Zs
AS/NZS 3017:2022 · Table 4.5 (C, 20 A, 0.4 s)
Maximum earth-conductor resistance (Rg): 0.5 Ω
AS/NZS 3017:2022 · Table 4.6 (C, 20 A, 0.4 s)
How this is worked out
- Maximum Zs is read straight from AS/NZS 3017:2022 Table 4.5 for the device type, rating and disconnection time. Nothing is interpolated: if the table has no cell, the tool says so.
- Table 4.5 publishes only the 0.4 s column for Type B, C and D MCBs; gG (HRC) fuses have both 0.4 s and 5 s columns.
- Maximum earth-conductor resistance (Rg) comes from Table 4.6, which is published up to 63 A.
- MCCBs and ACBs have no table. AS/NZS 3017 cl 4.8.4.1 derives Zs = 230 / Ia from the manufacturer's trip current for the required disconnection time — not covered by this lookup.
Common questions
What does a measured Zs above the maximum mean?+
The fault loop is too high for the protective device to disconnect within the required time on an earth fault. The circuit fails the earth fault-loop impedance test until the cause — a poor connection, an undersized earth, a long run — is fixed.
Should I compare my meter reading to Zs or to Rg?+
Zs is the live loop-impedance test result. Rg (Table 4.6) is the dead earth-conductor resistance limit shown on many meters as the alternative method. Use whichever test you performed.
Why is there no 5 s option for an MCB?+
AS/NZS 3017 Table 4.5 only publishes 0.4 s values for MCBs — final sub-circuits protected by MCBs must disconnect within 0.4 s. The 5 s column exists for gG fuses.
Related guide
AS/NZS 3000 verification: the tests every installation needsThe mandatory verification tests AS/NZS 3000 requires before a new installation is energised — in order, dead before live — and what goes on the report.
Recording this test?
Field Efficient picks the Table 4.5 limit from the breaker you record, judges the reading as you type it, and prints the clause on the report.
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Indicative only. Verify against the current edition of the standard before relying on a result for design, certification or a compliance decision. This tool is not a substitute for the standard or for a licensed electrician's judgement.