What if?
Pick one input and see how touch, step, GPR and grid resistance change across its range, with everything else held at the current design. The dashed line marks this design; shaded bands are where the voltage exceeds the allowed value.
What happens in the ground during a fault
When a phase conductor faults to grounded metal, the fault current has to get back to the neutral of the transformer that supplied it. Part of it goes home on metal: shield wires, neutrals, cable shields. The rest flows into the earth through the grounding system and spreads out through the soil.
Soil resists that current, so the grounding system rises in voltage relative to distant ("remote") earth. That rise is the ground potential rise (GPR): the grid current times the grid resistance. The ground surface above and around the grid rises too, but by less, and the further from the conductors, the less. A person standing on that surface and touching grounded metal bridges the difference. That difference, not the GPR itself, is what drives current through the body.
Use the Explore tab to change the soil, surface layer, fault, return paths and electrode and watch the touch and step voltages respond. Calculation shows the IEEE 80 working, What-if sweeps one input across its range.
The shock situations
IEEE 80 checks a person in each of these positions. In each case the voltage across the body is a difference between two surface or metal potentials, and the soil under the feet adds resistance in series with the body.
| Situation | Voltage across the person | What limits it |
|---|---|---|
| Touch | Grounded structure (at the GPR) minus the surface under the feet, standing within about 1 m of the structure. | Close grid spacing, conductors where people stand, surface layer, fast clearing. |
| Mesh | The worst touch voltage inside the grid, usually at the centre of a corner mesh where the surface sags most below the GPR. | Smaller corner meshes, perimeter rods, more conductors near the edge. |
| Step | Surface potential difference between the feet, 1 m apart, with no contact with metal. Largest just outside the grid edge and corners. | Burial depth, edge conductors and rods, surface layer. The step limit is much higher than the touch limit: IEEE 80 uses the same body current, but the two feet are in series (6Csρs) rather than in parallel (1.5Csρs). |
| Fence touch | A bonded fence is at the GPR; the person touching it from outside stands on ground that has already fallen away. | Run the grid (or a perimeter conductor) about 1 m outside the fence, extend the rock, or isolate the fence. |
| Transferred | A metallic path (communication shield, neutral, rail, pipe, fence) carries the GPR off site, or brings remote earth in. The difference can be nearly the full GPR. | Isolation (IEEE 487 for wire-line communications, fibre), bonding and insulating sections, keeping remote-earth conductors away from the station. |
| Metal-to-metal | Hand to hand between two metal objects at different potentials. No foot resistance helps. | Bond all metal that can be reached together. |
How much voltage a body can take
IEEE 80 uses Dalziel's fibrillation threshold: the current a person can survive is IB = k/√ts, with k = 0.116 for a 50 kg person (the usual choice where the public may be present) and 0.157 for 70 kg, valid from 0.03 to 3 s. The body is taken as 1,000 Ω and each foot as a 0.08 m radius plate on the surface, about 3ρs Ω. For touch the feet are in parallel, for step in series:
Faster clearing raises the allowed voltage as 1/√t; a surface layer raises the foot resistance. Two examples over 100 Ω·m soil, allowed touch / step in volts:
| Clearing time | Native soil, 50 kg | Native soil, 70 kg | 4 in (0.1 m) crushed rock 3,000 Ω·m, 50 kg | Rock, 70 kg |
|---|---|---|---|---|
| 0.1 s (6 cycles) | 422 / 587 | 571 / 794 | 1,528 / 5,010 | 2,068 / 6,781 |
| 0.25 s | 267 / 371 | 361 / 502 | 966 / 3,169 | 1,308 / 4,289 |
| 0.5 s | 189 / 262 | 255 / 355 | 683 / 2,241 | 925 / 3,033 |
| 1.0 s | 133 / 186 | 181 / 251 | 483 / 1,584 | 654 / 2,144 |
Cs is the surface layer derating factor: a thin, resistive layer over more conductive soil is less effective than an infinitely deep one. IEEE 80 gives Cs = 1 − 0.09(1 − ρ/ρs)/(2hs + 0.09). Use wet resistivities: clean crushed rock is a few thousand Ω·m wet; wet concrete only 20 to 100 Ω·m, about the same as soil. Asphalt is very resistive but cracks and patches weaken it.
Other standards set the limit differently. IEC 61936-1 and EN 50522 use the body impedance and heart-current curves of IEC/TS 60479-1 to give a permissible touch voltage that depends only on fault duration, then add foot and footwear resistance. The numbers differ from IEEE 80, so use the standard that applies to the project.
Where the fault current goes: split factor and return paths
Only the current that flows from the grid into the earth raises the GPR. IEEE 80 calls it the grid current IG = Cp · Df · Sf · 3I0, where Sf is the split (current division) factor, Df the decrement factor for DC offset (it depends on X/R and the fault duration) and Cp an allowance for growth.
- Local or remote source. For a fault inside a substation, current from that substation's own grounded-wye transformers returns to their neutrals through the grid conductors. It never enters the earth. Only the current from remote sources does. For a feeder fault outside a delta-wye distribution station the opposite happens: the station transformer supplies the fault, and current coming back through the earth must enter this grid to reach the transformer neutral. IEEE 80 asks for the worst case of both.
- Shield wires and neutrals take current two ways. The faulted phase current induces a current in a parallel return conductor (the coupling factor kc = Zm/Zs, about 0.1 to 0.3 for overhead shield wires, much higher for a concentric neutral wrapped around the conductor). The rest divides between the grid and the ladder of tower or pole grounds along the line in proportion to their impedances.
- Multigrounded neutrals help most. A 4-wire multigrounded system brings the neutral back to the source and grounds it at many points; the concentric neutral of an underground cable is coupled so tightly that only a small fraction of a padmount fault enters its rods. On a 3-wire (unigrounded) system there is no neutral to share the current, and the earth carries much more.
- The return conductor transfers voltage. The towers, poles or customer neutrals next to the site rise to a large share of the GPR during the fault. The Current split view shows that voltage profile along the line.
This tool models the faulted circuit's return conductor as a ladder of span impedances (Carson earth-return self and mutual impedances) with a ground at each tower, pole or padmount, solved exactly for that circuit. Other neutrals and shield wires at the site are semi-infinite ladders in parallel with the grid. IEEE 80 Annex C gives graphs and approximate methods for the same purpose; a detailed study (for example with a fault-current distribution program) is the basis for final design.
System grounding
| System | Ground-fault current | Where it returns | What it means for grounding |
|---|---|---|---|
| Solidly grounded, 4-wire multigrounded | High, often thousands of amperes | Largely on the neutral, which is grounded along the line and connected to every station and customer neutral | High fault current but a low split factor. Neutral-to-earth voltages and transferred voltage along the neutral matter. |
| Solidly grounded, 3-wire (unigrounded) | High | Through the earth, or on cable shields and shield wires where they exist | Large share of the fault current in the earth: higher GPR at equipment grounds. Each electrode matters more. |
| Low-resistance grounded | Limited by the neutral resistor, typically 200 to 1,000 A | Earth and equipment grounding conductors back to the resistor | Low GPR. Common in industrial medium-voltage systems; faults are tripped. |
| High-resistance grounded | A few amperes | Mostly capacitive and through the resistor | Negligible GPR; the system can run with one ground fault while it is located. |
| Ungrounded (delta) | Charging current only, a few amperes | Through the capacitance of the unfaulted phases | Small GPR for the first fault, but the unfaulted phases rise to line voltage to ground and a second fault becomes a phase-to-phase fault through two grounds. Arcing faults can cause overvoltage. |
What changes the result
| Change | Grid resistance | GPR | Touch | Step | Notes |
|---|---|---|---|---|---|
| More grid conductors (smaller meshes) | A little lower | A little lower | Much lower | Lower | The most direct way to lower mesh voltage. Concentrate conductors near the edge, where meshes run hottest. |
| Rods around the perimeter | Lower | Lower | Lower | Lower at the edge | Rods discharge current deep, flattening the surface gradient at the edge where step and corner-mesh voltages peak. |
| Longer (deep-driven) rods | Lower, a lot if they reach wetter or more conductive soil | Lower | Modest in uniform soil | Modest | Most effective in layered soil with a conductive lower layer (not modelled here). |
| Larger area | Lower, roughly with 1/√A | Lower | Depends on spacing | Depends | Extending the grid beyond the fence also fixes fence touch. |
| Deeper grid | Slightly lower | Slightly lower | Small, either way | Lower | The IEEE 80 equations show touch falling with depth; the numerical model shows a minimum near 0.5 m and a rise below that, as the surface sits further from the conductors. |
| Surface layer (crushed rock) | No change | No change | Allowed value much higher | Allowed value much higher | Keep it clean and thick; check outside the fence. |
| Faster clearing | No change | No change | Allowed value higher | Allowed value higher | Allowed voltage rises with 1/√t. |
| More return paths (shield wires, bonded neutrals) | No change | Lower (smaller split factor) | Lower | Lower | Watch the voltage transferred along those paths. |
| Higher soil resistivity | Proportionally higher | Higher | Higher | Higher | Touch and step are proportional to ρ for a given grid current; the split factor shifts toward the return conductors. |
Small installations: padmounts, vaults, poles and single rods
IEEE 80 is written for substations, and its mesh and step equations assume a grid of conductors in both directions with spacing above 2.5 m. A padmount transformer's ground ring and two rods, or a vault's ring, are too small for them, but the physics is the same. This tool's numerical model handles any of these electrodes. The touch check is within 1 m of the equipment footprint.
- On a 4-wire multigrounded system the cable concentric neutral and the transformer's secondary neutral carry most of the fault current, so the padmount rods see only a few per cent of it. Because a ring and two rods are 10 Ω or more, even that small share can raise the tank by hundreds to thousands of volts, and the result depends strongly on how many customer and secondary grounds are tied in. Switch the same padmount to a 3-wire system with a tape-shield cable and the share in the earth, and the GPR, rise sharply.
- A single 5/8 in by 8 ft rod in 100 Ω·m soil is about 40 Ω. For a building service, NEC 250.53(A)(2) requires a supplemental electrode for a single rod unless it is 25 Ω or less, and 250.53(A)(3) puts the two at least 6 ft (1.8 m) apart (check the edition that applies). Rods closer than about one rod length share the same soil and gain less; try it with two rods. Utility pole and equipment grounds fall under the NESC (IEEE C2), whose Rule 096 sets ground resistance expectations for supply systems.
- Utility construction standards set the actual padmount, vault and pole grounding details. Use the owner's standard; this tool shows why they work.
Measuring soil and grounds
- Soil resistivity (IEEE 81): the Wenner four-pin test gives apparent resistivity ρ = 2πaR for pin spacing a. Repeating it at several spacings reveals layering. This tool assumes one uniform value; where the soil is layered use a two-layer model and grounding software.
- Grid resistance: the fall-of-potential method with the current probe far away (several times the grid diagonal). In uniform soil the potential probe at 61.8 % of the distance gives the true value for a small electrode. Large grids need long leads or other methods.
- Surface material: test a wet sample of the actual rock. Fines and contamination can lower its resistivity by an order of magnitude.
What this tool calculates
- Tolerable voltages: IEEE 80-2013, Dalziel body current, 50 or 70 kg, Cs from the IEEE 80 empirical equation. Inside the surface layer area and outside it (native soil) separately.
- Grid resistance: Laurent-Niemann, Sverak and Schwarz equations (IEEE 80 clause 14), Dwight for a rod, and a numerical model of the actual electrode.
- Numerical model: the conductors and rods are divided into short segments that each leak a uniform current; the conductors are at one potential; the earth surface is handled by an image. The resulting surface potentials give touch voltage (GPR minus the surface) and step voltage (largest difference over 1 m) everywhere. Checked against the EPRI computer results printed in IEEE 80 Annex B: grid resistance 1 to 2 % lower, touch voltage 2 to 8 % lower, step voltage 3 to 7 % higher. Because it runs low on touch, the default verdict takes the larger of the numerical result and the IEEE 80 equations wherever those equations apply.
- Mesh and step voltage: the IEEE 80 Em and Es equations, reproducing Annex B.1 to B.3, with their range of validity flagged.
- Split factor: the ladder model described above; or enter Sf directly. Other paths at the site can be long (semi-infinite) or finite, such as a secondary run with a few customer grounds.
- Where the limits apply: touch is checked where a person can reach grounded metal (over the grid inside the fence, or within 1 m of an equipment footprint). Step is checked against the surface-layer limit inside the rock area and against the native-soil limit outside it.
- Conductor size: the IEEE 80 equation with Table 1 material constants and the connection temperature limit.
- Not modelled: layered soil, conductor longitudinal impedance (very large grids, high frequency), lightning, the effect of foundations and buildings, footwear resistance, and detailed telecommunication protection.
Scenarios
Distribution substation, 66/12 kV
Bus fault fed by two shielded lines; feeder neutrals share the current.
Same station, 3-wire feeders
No shield wire and no neutrals: watch the split factor go to 1.
Feeder fault outside the station
The station transformer feeds a remote fault; earth current returns through this grid.
Transmission substation, 230 kV
Large grid, 40 kA, local sources supply 30 %.
Padmount, 4-wire multigrounded
Ring and two rods; concentric neutrals carry most of the fault current.
Padmount, 3-wire
The same pad with a tape-shield cable and no neutral.
Underground vault
Ring and four rods around a 3 x 6 m vault under a concrete sidewalk.
Single ground rod
One 8 ft rod and the 25 Ω question.
Industrial, low-resistance grounded
A 400 A resistor keeps the GPR low.
Ungrounded 4.16 kV
Charging current only on the first fault.
IEEE 80 Annex B.1, B.2, B.3
The standard's worked examples, for checking.
Standards and references
Grounding design
- IEEE Std 80-2013, Guide for Safety in AC Substation Grounding (revision project P80 in progress)
- IEEE Std 81-2012, Measuring Earth Resistivity, Ground Impedance and Earth Surface Potentials
- IEEE Std 142-2007 (Green Book), Grounding of Industrial and Commercial Power Systems
- IEEE Std 837-2014, Qualifying Permanent Connections Used in Substation Grounding
- IEC 61936-1 and EN 50522, power installations above 1 kV AC and their earthing; IEC/TS 60479-1, effects of current on human beings
Codes, GPR and related practice
- IEEE C2, National Electrical Safety Code, Section 9 (grounding methods), including Rule 096
- NFPA 70, National Electrical Code, Article 250
- IEEE Std 367-2012, Determining the Electric Power Station GPR and Induced Voltage from a Power Fault
- IEEE Std 487-2015 and 487.1 to 487.5, protection of wire-line communication facilities serving electric supply locations
- IEEE Std 1048-2016, Protective Grounding of Power Lines (worker equipotential zones)
- CPUC General Orders 95 and 128 for California overhead and underground construction