Maximum Zs and route length from device and cable.
Table 8.1 maximum Zs and a Paragraph B5.2 route-length check for copper conductors up to 120 mm², using a measured Ze or the 80% Zs assumption. Cable inductance, aluminium conductors and RCD-only protection are not modelled.
Results
Select device rating, type, and disconnection time, then calculate.
Method and assumptions
Maximum Zs is from AS/NZS 3000:2018 Table 8.1. Circuit-breaker values are 230 V ÷ (k × In) with the mean tripping multiples of Paragraph B4.5 (Type B 4, Type C 7.5, Type D 12.5). Table 8.1 gives 5 s values for fuses only.
Clause 5.7.2: 0.4 s for final subcircuits supplying socket-outlets up to 63 A, hand-held Class I or portable equipment; 5 s for other circuits where people are not exposed to unsafe touch voltages.
The route check uses Paragraph B5.2 with copper conductors at 22.5 × 10⁻³ Ω·mm²/m. With a measured Ze, Zs = Ze + Zint is compared with Table 8.1. Without one, Zint is limited to 80% of Zs (Equation B7), which reproduces the Table B1 route lengths.
Verify by testing the installed circuit (Clause 8.3.9).
For guidance only. Verify against the current AS/NZS 3000 and your fault-loop design.
Common questions
Where do the maximum Zs values come from?
AS/NZS 3000:2018 Table 8.1, which gives the maximum earth fault-loop impedance for the whole circuit including the supply, at 230 V. For circuit-breakers the value is 230 V ÷ (k × In) using the mean instantaneous tripping multiples of Paragraph B4.5: 4 for Type B, 7.5 for Type C and 12.5 for Type D. Fuse values are taken from Table 8.1.
Which disconnection time applies, 0.4 s or 5 s?
Clause 5.7.2 requires 0.4 s for final subcircuits supplying socket-outlets rated up to 63 A, hand-held Class I equipment, or portable equipment moved by hand during use. Other circuits may use 5 s where people are not exposed to unsafe touch voltages. Table 8.1 gives 5 s values for fuses only.
How is the maximum route length worked out?
With Paragraph B5.2 of AS/NZS 3000. The circuit impedance uses copper resistivity at working temperature, 22.5 × 10⁻³ Ω·mm²/m. If you enter a measured Ze, Ze plus the circuit impedance is compared with Table 8.1. If not, the circuit impedance is limited to 80% of Zs (Equations B6 and B7), which reproduces the route lengths in Table B1.
Does an RCD change the result?
No. The calculator checks disconnection by the circuit-breaker or fuse against Table 8.1. RCDs required under Clause 2.6.3 are additional protection and are not used to relax the Table 8.1 values here.
