Earthing System Calculator

Earthing System Design for AS/NZS 3000 Section 5

The earthing system bonds non-current-carrying metalwork to earth so that fault current returns through a low-impedance path and protective devices clear quickly. This calculator estimates ground resistance for common electrode types in the soil resistivity you specify, and flags touch and step voltage risks during a fault. All calculations follow AS/NZS 3000 Section 5 and the supporting equations from AS/NZS 3000 Appendix B.

How to calculate maximum earth resistance per AS/NZS 3000

AS/NZS 3000 does not specify a single maximum earth resistance value. Instead it requires the earth fault loop impedance (Zs) be low enough that the protective device disconnects within the time set in Table 5.1, typically 0.4 seconds for socket outlets up to 32 A, 5 seconds for fixed appliances and distribution circuits. The calculator works backwards from these limits to determine the maximum acceptable earth resistance for your circuit.

For most Multiple Earthed Neutral (MEN) installations the effective ceiling on the consumer earth electrode resistance works out to roughly 10 to 30 Ω, but the exact figure depends on the upstream supply impedance, the protective device type and rating, and whether the circuit feeds a socket outlet or a fixed load.

Earth fault loop impedance vs earth resistance

Earth resistance (Re) is the resistance of just the electrode-to-soil path. Earth fault loop impedance (Zs) is the resistance of the entire loop: phase conductor → load → fault → earth conductor → MEN link → transformer secondary → phase conductor again. Zs is what determines whether the protective device sees enough fault current to trip in time. The calculator computes both, plus the prospective fault current Ifault.

Soil resistivity testing: Wenner method walkthrough

The Wenner four-pin method is the industry-standard soil resistivity test. Drive four equally-spaced auxiliary electrodes into the ground in a straight line, inject a known AC current between the outer pair, and measure the voltage drop between the inner pair. The apparent soil resistivity (ρ) at depth ≈ a is:

ρ = 2π × a × R

where a is the electrode spacing in metres and R is the resistance reading in ohms. Repeat at spacings of 1, 2, 4, and 8 m to characterise resistivity at progressively deeper layers. Use the harmonic mean of these readings as your design value.

Earth electrode types and when to use each

• Driven rod (1.2 to 2.4 m copper-clad steel): the default for residential and light commercial. Cheap, fast to install, effective in soils up to ≈ 200 Ω·m. Multiple rods in parallel for high-resistivity sites.
• Plate (600 × 600 mm copper or galv. steel): when ground is too rocky to drive rods. Buried at 600 mm depth minimum. Lower contact area than a rod for the same depth.
• Mesh / grid: at substations and large industrial sites. Provides equipotential bonding across a large area, critical for limiting touch and step voltages during a fault.
• Ring earth (buried bare conductor around the building): common for lightning protection and high- resistivity sites. Often combined with rods at corners.

Touch and step voltage: what AS/NZS 3000 Section 5 actually requires

When fault current flows through the earth electrode, the soil around it sits at an elevated potential. Touch voltage is what a person feels between an exposed metal part (e.g. a switchboard frame) and the ground 1 m away, limited to 50 V AC under fault for installations accessible to ordinary persons. Step voltage is what a person feels between their two feet (1 m apart) standing on the ground near the electrode. The calculator flags either if it exceeds the safe limit for the disconnection time of the upstream protective device.