Earthing System Calculator

Parameters

Every field on the form, what it feeds, the range accepted, and which fields only apply to certain electrode types. Some fields behave in ways that are not obvious from the label, so read the notes before relying on the output.

Soil resistivityohm metres, 10 to 10000

The single most influential input. It appears directly in every electrode resistance formula and in the step voltage expression, so the result scales linearly with it. Measure it on site with a Wenner four-pin test if you can. Typical Australian ranges are 20 to 100 for clay, 50 to 500 for loam, 100 to 2000 for sand, 100 to 3000 for gravel, and 1000 to 10000 for rock. Measure in the driest part of the year, because resistivity can double or worse in a dry summer.

Soil typeclay, loam, sand, rock or gravel

Reference only. It appears in the results for context but it does not enter any formula and it does not change the resistivity value used. Selecting rock while leaving resistivity at 200 ohm metres will calculate at 200 ohm metres. Set the resistivity number yourself.

Electrode typerod, strip, plate or ring

Chooses the resistance formula. Rod uses resistivity divided by 2 pi times length, times the natural log of 4 times length divided by diameter. Strip uses resistivity divided by pi times length, times the natural log of 2 times length divided by width. Plate uses resistivity divided by 4 times the square root of the plate area. Ring uses resistivity divided by 4 pi times the ring radius.

Electrode lengthmetres, 0.5 to 100

What this means depends on the electrode type. For a rod it is the driven depth. For a strip it is the buried run length. For a plate it is one side of the plate, paired with the width field to give the area. For the ring option it is fed into the ring formula as a diameter in millimetres, so a ring entry only makes sense if you treat this field as the ring diameter in millimetres rather than metres. That is a known quirk of the current implementation, so cross-check any ring result by hand.

Electrode diametermillimetres, 8 to 50, rod only

The diameter of the driven rod. It sits inside a natural logarithm, so its influence is weak: doubling the rod diameter reduces resistance by well under 20 percent, whereas doubling the rod length nearly halves it. Length beats girth every time. A typical Australian copper-clad steel rod is 16 millimetres.

Electrode widthmillimetres, 50 to 2000, strip and plate only

For a strip it is the tape width, which also sits inside a logarithm and therefore has modest effect. For a plate it multiplies the length to give the buried area, which sits under a square root, so it matters far more there.

Burial depthmetres, 0.5 to 10

The depth to the centre of the electrode. It is validated and it triggers a warning below 0.5 metres about unreliable earthing in dry conditions, but it does not appear in any resistance formula. Changing it will not change the calculated resistance. In reality, depth matters a great deal because deeper soil holds moisture through summer.

Number of electrodescount, 1 to 100

Parallel electrodes. The calculator divides the single electrode resistance by this number and then applies a flat 0.95 practical factor. Real parallel electrodes suffer mutual interference and never achieve a straight division, especially when spaced closer than about twice their length, so treat multi-electrode results as optimistic.

Electrode spacingmetres, not exposed on this form

The underlying engine accepts a spacing value and has a branch that penalises closely spaced electrodes, but the web form does not send one, so that branch never runs. Parallel electrodes are modelled as a plain division by the count.

Fault currentamps, 10 to 100000

The prospective earth fault current that flows through the electrode. It drives three outputs: the prospective touch voltage, which is this current times the practical earth resistance, the step voltage, and the minimum earth conductor size. Be careful what you enter here. On an Australian MEN installation most fault current returns through the neutral, not through the electrode, so putting a switchboard fault level into this field will produce enormous and unrealistic touch voltages.

Fault clearance timeseconds, 0.05 to 10

The time the protective device takes to clear. It sets the touch voltage limit band and it appears under the square root in the conductor sizing equation. The bands used are 50 volts at 0.4 seconds and longer, 65 volts from 0.2 up to 0.4 seconds, and 80 volts from 0.1 up to 0.2 seconds. Shorter clearance times permit higher touch voltages because the body tolerates a brief shock better than a sustained one.

System typeTN-S, TN-C-S, TT or IT

Selects the earth resistance limit the result is graded against: 10 ohms for TN-S, TN-C-S and TT, and 1000 ohms for IT. That is the only thing this field changes. It does not alter the touch voltage limit, the step voltage limit or the conductor size. Australian MEN installations correspond most closely to TN-C-S.

Assumptions and limits

This is a first-pass electrode sizing tool built on classical soil resistance formulas. It is deliberately pessimistic about touch and step voltage, and understanding why is essential to reading the output correctly.

What the calculator assumes

• Uniform, single-layer soil. One resistivity value for the whole volume around the electrode. Real ground is layered, and a rod that passes from dry sand into damp clay behaves nothing like the single-layer model predicts. Seasonal variation is not modelled either.
• Parallel electrodes divide cleanly. Resistance is divided by the electrode count and then multiplied by a flat 0.95 factor. There is no mutual resistance term and no spacing penalty, because the form does not collect spacing. Real arrays of closely spaced rods deliver considerably less than the ideal division.
• Touch voltage is the full electrode voltage rise. It is calculated as fault current times practical earth resistance, which is the whole ground potential rise rather than the fraction a person would actually bridge. There is no surface potential profile, no body impedance model, no allowance for footwear, and no allowance for a crushed rock surface layer. Critically, there is no credit for the MEN connection or the supply neutral return path, so for a normal Australian MEN installation this figure is far more onerous than the real hazard. Expect the touch voltage check to fail on ordinary inputs.
• Step voltage is a single simplified gradient. Fault current times resistivity, divided by twice the electrode length. It is a rough indicator, not an IEEE 80 surface potential calculation, and the limit it is compared against is simply 1.5 times the touch voltage limit.
• Touch voltage limits are banded by clearance time only. 50, 65 or 80 volts. There is no distinction between ordinary locations and special locations such as bathrooms, pools or medical areas, where lower limits apply.
• Conductor sizing is copper only. The adiabatic equation runs with a fixed material constant of 115 and no aluminium option. Use the earth conductor calculator if you need aluminium or a different constant.
• The formulas are classical, and the limits are indicative. The resistance expressions are standard textbook forms. The touch voltage bands, the resistance limits by system type and the material constant have not been transcribed from a licensed copy of AS/NZS 3000.

What this should not be used for

• Substation or high voltage earth grid design. That is the domain of IEEE Standard 80, AS 2067 and the ENA EG-0 framework, all of which model surface potential, grid geometry, split factors and decrement factors that this tool does not.
• Certifying that touch and step voltages are safe. The touch voltage output here is a worst-case ground potential rise and will read alarmingly high on realistic fault currents.
• Replacing a fall-of-potential test. Calculated resistance is a design estimate. The installed value must be measured and recorded.
• Earth fault loop impedance and automatic disconnection verification, which involve the whole loop rather than just the electrode. Use the cable sizing calculator for the loop length check.
• Lightning protection earthing to AS 1768, which has its own electrode and bonding requirements.
• Ring electrode design until the length field behaviour noted above is verified by hand.

None of this has been validated by a chartered professional engineer. Verify every result against the current edition of AS/NZS 3000 Section 5, measure the installed earth resistance on completion, and have the responsible person for the installation sign off the final design.

Worked examples

Three configurations run end to end: a single rod in average soil, an array of rods in good clay, and a horizontal strip. The arithmetic is written out so you can reproduce it.

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 Clause 5.7.2, typically 0.4 seconds for final subcircuits supplying socket outlets rated up to 63 A, hand-held Class I equipment or portable equipment, and 5 seconds for other circuits including submains and circuits supplying fixed or stationary equipment. Table 8.1 gives the matching maximum Zs values. 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: where the requirements actually come from

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.