Earthing System Calculator

Q: How do I calculate touch and step voltage per IEEE 80?

IEEE Std 80-2013 provides the methodology for calculating permissible touch and step voltages in substations. The tolerable touch voltage is Etouch = (1000 + 1.5 x Cs x rho_s) x 0.116 / sqrt(ts) for a 50kg person, where rho_s is the surface material resistivity, Cs is a correction factor for surface layer, and ts is the fault duration in seconds. Step voltage uses the coefficient 6.0 instead of 1.5. The actual mesh and step voltages are then calculated using Equations 86 and 92 based on grid geometry, conductor spacing, and soil resistivity. Both must be below tolerable limits.

Q: How does soil resistivity affect earthing design?

Soil resistivity (measured in ohm-metres) is the dominant factor in earth electrode resistance. Values range from 1 ohm-m (sea water/marshy ground) to over 10,000 ohm-m (dry rock). IEEE 80 Clause 12 describes the Wenner four-pin method for measurement, using progressively wider pin spacings to characterise resistivity at different depths. IEC 60364-5-54 Clause 542 requires that the earthing arrangement account for seasonal variations. In high-resistivity soils, techniques such as deep-driven rods, ground enhancement materials (bentonite/GEM), or extended horizontal electrodes may be necessary.

Q: How do I size an earthing conductor per BS 7671?

BS 7671 Regulation 543.1 requires earthing conductors to be sized by the adiabatic equation S = sqrt(I2t) / k, where I is the fault current in amperes, t is the disconnection time in seconds, and k is a material factor from Table 54.2 to Table 54.6 (k = 143 for copper with PVC insulation, k = 76 for aluminium). Alternatively, Table 54.7 provides minimum sizes based on the phase conductor size: for phase conductors up to 16mm2, the earthing conductor must be the same size; for 16-35mm2, minimum 16mm2; above 35mm2, half the phase conductor size. The main earthing terminal must satisfy Regulation 542.4.

Q: What is the difference between TN, TT, and IT earthing systems?

IEC 60364-1 Clause 312 defines three earthing system types. TN systems (TN-S, TN-C, TN-C-S) have the supply source directly earthed and exposed conductive parts connected to that earth point via a protective conductor. TT systems have the supply source earthed but exposed conductive parts connected to a separate local earth electrode, requiring RCDs for fault protection. IT systems have the source isolated or impedance-earthed, with exposed conductive parts locally earthed; a first fault only causes a warning, and a second fault must be disconnected. AS/NZS 3000 primarily uses MEN (a form of TN-C-S), while UK uses TN-S, TN-C-S, and TT.

Q: What grounding requirements does NEC Article 250 specify?

NEC Article 250 is the comprehensive grounding and bonding article. Section 250.24 requires grounding of the supply at the service. Table 250.66 sizes the grounding electrode conductor (GEC) based on the largest service entrance conductor: for example, 2/0 AWG copper service requires a minimum 4 AWG copper GEC. Table 250.122 sizes equipment grounding conductors based on the overcurrent device: 20A requires 12 AWG, 100A requires 8 AWG, 400A requires 3 AWG. The grounding electrode system per 250.50 must include all available electrodes (water pipe, building steel, concrete-encased electrode, ground ring).

IEEE 80-2013 Guide for Safety in AC Substation Grounding. Touch & step voltage, mesh/step analysis, conductor sizing.

Calculation Mode

Simple resistance

Electrode resistance only

IEEE 80 full analysis

Touch/step voltage + conductor sizing

NEC compliance

Article 250 compliance check

Soil Properties

Soil TypeSoil Resistivity (ohm.m)

Electrode Configuration

Electrode TypeBurial Depth (m)

Rod Length (m)Rod Diameter (m)

Target Resistance

⚡

Configure electrode parameters and click Calculate to see earthing system analysis.

Earth fault loop impedance is the total impedance of the fault current path from the source through the phase conductor, fault, and protective conductor back to the source. BS 7671 Regulation 411.4 requires that this impedance be low enough to ensure the protective device disconnects the supply within the specified time to prevent electric shock.

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How to Size an Earth Protective Conductor

1
Determine the fault current — Calculate or obtain the maximum earth fault current at the point of installation. This requires knowledge of the earth fault loop impedance and the supply voltage.[IEC 60364-5-54 Clause 543.1]
2
Determine device disconnection time — Find the disconnection time of the protective device at the calculated earth fault current from its time-current characteristic curve. This is the duration the conductor must withstand the fault.[BS 7671 Regulation 411.3.2]
3
Select the k factor — Look up the k value for the protective conductor material and insulation from IEC 60364-5-54 Table A.54.1 through A.54.6. Copper with PVC insulation has k = 115.[IEC 60364-5-54 Tables A.54.1-6]
4
Apply the adiabatic equation — Calculate the minimum conductor size using S = sqrt(I2t) / k, where I is the fault current, t is the disconnection time, and k is the material factor from the previous step.[IEC 60364-5-54 Clause 543.1]
5
Apply minimum size rules — Verify the calculated size meets the minimum requirements from BS 7671 Table 54.7 or use the simplified method: CPC size equals phase conductor size up to 16mm2, half the phase size above that.[BS 7671 Table 54.7]

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How Earthing System Works

The earthing calculator designs ground electrode systems and verifies that touch and step voltages remain within safe limits during earth fault conditions.

Using IEEE Std 80-2013 methodology, the calculator determines the ground potential rise (GPR), mesh voltage, and step voltage for a given grid geometry and soil resistivity. The tolerable touch voltage is calculated as Etouch = (1000 + 1.5 x Cs x rho_s) x 0.116 / sqrt(ts), where Cs is the surface layer derating factor, rho_s is the surface material resistivity, and ts is the fault duration.

IEC 60364-5-54 defines earthing arrangements (TN, TT, IT systems) and minimum conductor sizes. NEC Article 250 establishes grounding electrode requirements. AS/NZS 3000 Section 5 covers earthing and bonding. Results include the required earth grid geometry, electrode sizing, touch and step voltage compliance, earth resistance, and conductor cross-section.

Maximum Earth Fault Loop Impedance Zs (BS 7671)

Device Rating	Type B (Ω)	Type C (Ω)	Type D (Ω)
6A	7.67	3.83	1.92
10A	4.60	2.30	1.15
16A	2.87	1.44	0.72
20A	2.30	1.15	0.57
32A	1.44	0.72	0.36
40A	1.15	0.57	0.29
63A	0.73	0.36	0.18

Source: BS 7671:2018 Table 41.3

Frequently Asked Questions

What earth resistance value is required per AS/NZS 3000?

AS/NZS 3000:2018 Clause 5.6.2 and Table 5.1 require the earth resistance of the main earthing system to be low enough to ensure protective device operation within the required disconnection time. For TN systems, this is automatically satisfied by the MEN (Multiple Earthed Neutral) connection. For TT systems, the resistance must satisfy Ra x Ia <= 50V, where Ra is the total earth resistance and Ia is the operating current of the RCD. With a 30mA RCD, this allows a maximum earth resistance of 1667 ohms, but practical values of 10-50 ohms are targeted for reliability.

How do I calculate touch and step voltage per IEEE 80?

IEEE Std 80-2013 provides the methodology for calculating permissible touch and step voltages in substations. The tolerable touch voltage is Etouch = (1000 + 1.5 x Cs x rho_s) x 0.116 / sqrt(ts) for a 50kg person, where rho_s is the surface material resistivity, Cs is a correction factor for surface layer, and ts is the fault duration in seconds. Step voltage uses the coefficient 6.0 instead of 1.5. The actual mesh and step voltages are then calculated using Equations 86 and 92 based on grid geometry, conductor spacing, and soil resistivity. Both must be below tolerable limits.

How does soil resistivity affect earthing design?

Soil resistivity (measured in ohm-metres) is the dominant factor in earth electrode resistance. Values range from 1 ohm-m (sea water/marshy ground) to over 10,000 ohm-m (dry rock). IEEE 80 Clause 12 describes the Wenner four-pin method for measurement, using progressively wider pin spacings to characterise resistivity at different depths. IEC 60364-5-54 Clause 542 requires that the earthing arrangement account for seasonal variations. In high-resistivity soils, techniques such as deep-driven rods, ground enhancement materials (bentonite/GEM), or extended horizontal electrodes may be necessary.

How do I size an earthing conductor per BS 7671?

BS 7671 Regulation 543.1 requires earthing conductors to be sized by the adiabatic equation S = sqrt(I2t) / k, where I is the fault current in amperes, t is the disconnection time in seconds, and k is a material factor from Table 54.2 to Table 54.6 (k = 143 for copper with PVC insulation, k = 76 for aluminium). Alternatively, Table 54.7 provides minimum sizes based on the phase conductor size: for phase conductors up to 16mm2, the earthing conductor must be the same size; for 16-35mm2, minimum 16mm2; above 35mm2, half the phase conductor size. The main earthing terminal must satisfy Regulation 542.4.

What is the difference between TN, TT, and IT earthing systems?

IEC 60364-1 Clause 312 defines three earthing system types. TN systems (TN-S, TN-C, TN-C-S) have the supply source directly earthed and exposed conductive parts connected to that earth point via a protective conductor. TT systems have the supply source earthed but exposed conductive parts connected to a separate local earth electrode, requiring RCDs for fault protection. IT systems have the source isolated or impedance-earthed, with exposed conductive parts locally earthed; a first fault only causes a warning, and a second fault must be disconnected. AS/NZS 3000 primarily uses MEN (a form of TN-C-S), while UK uses TN-S, TN-C-S, and TT.

What grounding requirements does NEC Article 250 specify?

NEC Article 250 is the comprehensive grounding and bonding article. Section 250.24 requires grounding of the supply at the service. Table 250.66 sizes the grounding electrode conductor (GEC) based on the largest service entrance conductor: for example, 2/0 AWG copper service requires a minimum 4 AWG copper GEC. Table 250.122 sizes equipment grounding conductors based on the overcurrent device: 20A requires 12 AWG, 100A requires 8 AWG, 400A requires 3 AWG. The grounding electrode system per 250.50 must include all available electrodes (water pipe, building steel, concrete-encased electrode, ground ring).

Standards Reference

IEEE Std 80-2013 — Substation grounding
IEC 60364-5-54 — Earthing arrangements
NEC Article 250 — Grounding
AS/NZS 3000:2018 — Section 5

Earthing System Calculator

Calculation Mode

Soil Properties

Electrode Configuration

Target Resistance

How to Size an Earth Protective Conductor

How Earthing System Works

Maximum Earth Fault Loop Impedance Zs (BS 7671)

Frequently Asked Questions

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