Earthing System Design Guide 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 the protective device clears quickly. AS/NZS 3000 Section 5 sets out how to design that path for an Australian installation. This guide walks through the whole process: soil resistivity measurement, electrode selection, earth resistance calculation, earth fault loop impedance verification, touch and step voltage analysis, and documentation. By the end you will know how to design a compliant earthing system from scratch, and where the Earthing System Calculator fits into the workflow.

Why earthing matters

Three things go wrong if the earthing system is undersized or incorrectly bonded. First, an earth fault current may not be high enough to trip the upstream protective device, leaving live metalwork energised at hazardous voltage. Second, even if the device does trip, it may take long enough that the touch voltage on exposed metal exceeds the threshold for ventricular fibrillation (around 50 V AC for 5 seconds, much less for shorter exposures). Third, lightning strikes and switching transients can develop enormous transient voltages between exposed metalwork and earth if the equipotential bonding network is not sized for it.

AS/NZS 3000 Section 5 mitigates all three by mandating a low impedance earth fault loop, equipotential bonding of all exposed conductive parts, and disconnection times that limit how long a worker or occupant can be exposed to fault voltage. The numbers in the standard are not arbitrary, they trace back to IEC 60479 body impedance research.

The legal framework

Earthing in Australia is governed by AS/NZS 3000:2018 (the Wiring Rules), specifically Section 5 (Earthing arrangements and earthing conductors) and the related Tables 5.1 (disconnection times), Table 5.2 (minimum earth conductor sizes), and Section 5.3 (MEN system requirements). Supplementary standards apply for specific contexts: AS/NZS 3017 for verification testing, AS 2067 for high voltage substation earthing, AS/NZS 4836 for safe working on or near electrical equipment, and AS/NZS 3008.1.1 for cable sizing (because cable impedance contributes to the fault loop).

Two practical points come up repeatedly. AS/NZS 3000 distinguishes between socket outlet circuits (0.4 second disconnection time, more stringent) and fixed appliance circuits (5 second disconnection time, less stringent). And the standard does not set a single maximum earth resistance value; it sets a maximum disconnection time, and you size the earthing system to meet it.

Soil resistivity testing using the Wenner method

Soil resistivity (rho, measured in ohm metres) is the single biggest input to earth electrode performance. A 1.2 metre rod in 200 ohm metre soil might give 100 ohms of earth resistance; the same rod in 2000 ohm metre rocky soil might give 1000 ohms. The Wenner four-pin method is how you find rho on site.

Drive four auxiliary electrodes into the ground in a straight line, equally spaced. Inject a known AC current (usually a few milliamps at 100 to 200 Hz to avoid mains pickup) between the outer pair. Measure the voltage between the inner pair. The apparent resistivity at a depth roughly equal to the electrode spacing is:

rho = 2 * pi * a * R

where a is the electrode spacing in metres and R is the measured resistance in ohms (voltage divided by current). Repeat at spacings of 1, 2, 4, and 8 metres to build a soil profile at progressively deeper layers. Australian soils typically come in around 100 to 1000 ohm metres for sandy and rocky country, 10 to 100 ohm metres for clay and damp soils, and well above 1000 for solid rock.

Picking an electrode type

Four electrode geometries cover almost every Australian site.

• Driven rod (1.2 to 2.4 metre copper-clad steel) is the default for residential and light commercial. Cheap, fast to install, effective in soils up to about 200 ohm metres. Use multiple rods in parallel for higher-resistivity sites.
• Plate (typically 600 by 600 mm copper or galvanised steel) buried at least 600 mm deep. Use when the ground is too rocky to drive rods. Lower contact area than a rod for the same depth, so usually higher resistance for the same soil.
• Mesh or grid (a network of buried bare conductors, often combined with rods at intersections). Used at substations and large industrial sites where the touch and step voltage requirements demand a large equipotential area.
• Ring earth (a continuous bare conductor buried around the perimeter of the building). Common for lightning protection and as a base for electrodes in high-resistivity sites. Often combined with rods at corners.

Calculating earth resistance for a single rod

The Dwight formula gives the resistance of a single vertical rod driven into uniform soil:

R = (rho / (2 * pi * L)) * (ln(8 * L / d) - 1)

where rho is soil resistivity in ohm metres, L is rod length in metres, and d is rod diameter in metres. For a 2.4 metre 14 mm copper-clad rod in 200 ohm metre soil, this gives roughly 75 ohms.

Two practical observations. First, doubling the rod length only halves the resistance approximately, so very long single rods give diminishing returns. Second, rod diameter has very little influence (it is inside a logarithm), so using a fatter rod is rarely worth the cost.

Calculating combined resistance for parallel rods

When a single rod cannot meet the required earth resistance, drive additional rods and bond them in parallel. The combined resistance is not simply the single rod resistance divided by the rod count, because each rod sits in the voltage gradient created by its neighbours. The diminishing-returns factor depends on the spacing.

A common rule of thumb: two rods spaced at one rod length apart give about 60 percent of the resistance of one rod (not 50 percent). Four rods at the same spacing give about 35 percent. For tighter spacings the factor gets worse. The Earthing System Calculator applies the appropriate combination factor for the rod count and spacing you specify, so you do not have to look it up.

Verifying earth fault loop impedance

Earth resistance (Re) on its own is not what AS/NZS 3000 cares about. The standard cares about earth fault loop impedance (Zs), which is the total impedance of the entire fault loop:

1. Phase conductor from the supply
2. Through the load (effectively a short during a fault)
3. Down the earth conductor to the MEN link or main earth
4. Through the earth electrode to the soil mass
5. Back through the supply earthing system to the transformer

Zs is what determines the prospective fault current, which in turn determines whether the upstream protective device clears the fault within the time AS/NZS 3000 Table 5.1 requires. The Earth Conductor Sizing Calculator handles step 3, and the Earthing System Calculator handles step 4 and combines the rest.

Touch and step voltage analysis

For substations, large LV switchboards, or any installation where a worker or member of the public might stand within a few metres of the electrode during a fault, AS/NZS 3000 Section 5 requires touch and step voltage compliance.

Touch voltage is the difference between an exposed conductive part (a metal switchboard frame, for example) and a person standing 1 metre away on the ground. Limited to 50 V AC for disconnection times around 0.4 to 5 seconds, lower for longer exposures.

Step voltage is the difference between two points on the ground 1 metre apart, where a person could be standing during a fault. Allowed limits are typically higher than touch voltage but still hazardous, and they depend strongly on the soil surface (gravel reduces hazard, asphalt or wet ground increases it).

Both are calculated from the voltage gradient around the electrode during a fault. Mesh and grid earths are designed specifically to flatten the gradient across the area where workers stand, which is why they are mandatory for substations.

MEN, TT, and TN-S systems explained

Australia uses three earthing system topologies. Each affects how the earth fault loop closes and what the consumer earth electrode needs to do.

MEN (Multiple Earthed Neutral). The standard Australian residential and commercial configuration. The supply neutral is earthed at the transformer and bonded to the consumer earth at the main switchboard. The fault current returns mostly through the neutral conductor, with the consumer electrode acting as a backup path. Earth resistance requirements are relatively relaxed because the neutral provides a low-impedance return.

TT. Separate consumer earth, no bond to supply neutral. Common in rural Australia where the local distribution network does not provide a continuous earthed neutral. Fault current must return entirely through the consumer earth electrode, so the electrode resistance has to be much lower (often less than 5 ohms) for an RCD to detect the fault and clear it within Table 5.1 times.

TN-S. Dedicated earth conductor back to the source, no neutral earth bond at the consumer end. Used in some industrial installations where neutral injection of harmonic currents must be avoided. The earth electrode plays a backup role similar to MEN.

Documentation and post-install testing

Once the earthing system is built, three measurements demonstrate compliance. A fall of potential test gives the as-built earth electrode resistance and confirms it matches the design figure. An earth fault loop impedance test (clamp-on or short-circuit) gives Zs, which is what AS/NZS 3000 Table 5.1 actually checks. A continuity test confirms every exposed conductive part is bonded to the main earthing terminal.

Document the design: soil resistivity readings (date, location, method, raw measurements), electrode geometry (count, depth, spacing, material), calculated earth resistance and Zs, protective device coordination, and the as-built test results. AS/NZS 3017 requires this documentation for the certificate of electrical safety. WorkSafe and the supply authority can audit it at any time, so keep it on file.

Common mistakes

• Designing to earth resistance rather than Zs. AS/NZS 3000 cares about disconnection time, not Re alone. Always calculate the full loop.
• Ignoring soil drying. Earth resistance measured in winter can double or triple in a dry summer. Design with margin.
• Using estimated soil resistivity for substation designs. Wenner test on every substation site, no exceptions.
• Forgetting to bond exposed metalwork. The earth electrode is useless if the switchboard frame is not bonded to it.
• Underrating the earth conductor. An electrode of 5 ohms paired with a 4 mm² earth conductor that vapourises in the first 50 milliseconds of a fault is no protection at all.

Where the calculator fits in

The ElecCalc Earthing System Calculator handles steps 3 to 5 of the design process: it takes soil resistivity, electrode count, spacing, and depth, and returns the earth resistance, prospective fault current, and a flag if the touch or step voltage may be problematic. Pair it with the Earth Conductor Sizing Calculator for step 3 of the loop, and the RCD Selection Calculator for protective device coordination.

For arc flash analysis on the same switchboard, the Arc Flash Calculator uses the fault current you derive from the earthing system design as one of its inputs.