Step and Touch Voltage: Difference and IEEE 80

What happens in the ground during a ground fault

When a ground fault occurs in a substation or a plant, part of the fault current returns to the source through the soil. As it enters the ground through the grounding grid, it raises the grid’s potential relative to a distant earth. This is the ground potential rise (GPR), equal to the current injected into the soil multiplied by the grounding impedance.

The potential is not the same over the whole surface. It is highest over the grid conductors and decreases between them and outward. Those differences are what put a person at risk:

• Touch voltage: between a grounded metal structure the person touches with a hand and the point where the feet stand, normally one meter away. The current passes through the chest.
• Step voltage: between the two feet one meter apart, touching nothing.
• Mesh voltage: the maximum touch voltage within a mesh of the grid, which usually occurs at the center of the corner meshes.
• Transferred voltage: the potential of the grid carried to another place by a conductor (a cable screen, a pipe, a rail, a fence), where the person stands on a remote earth.

What the tolerable values depend on

The IEEE 80 guide (Guide for Safety in AC Substation Grounding) starts from the current the body tolerates without ventricular fibrillation, which decreases with exposure time, and from the resistance of the body and of the contact of the feet with the soil. The tolerable step and touch voltages follow from there. For a 50 kg person, the equations are:

Where ρs is the resistivity of the surface layer (Ω·m), Cs is the reduction factor for the thickness of that layer and ts is the fault time in seconds. For 70 kg, the factor 0.116 is replaced by 0.157. The design adopts the weight according to the type of access to the area; 50 kg is the most conservative assumption.

Three practical conclusions:

1. Clearing time rules. A slow protection reduces the allowable voltage. If settings are changed, the grid must be reviewed.
2. The surface layer helps a lot. Crushed rock or asphalt increases the resistance between the feet and the soil and raises the tolerable voltages.
3. Step voltage is rarely the limit. The body tolerates more from foot to foot than from hand to feet, and touch voltage usually sets the design.

Local code and utility requirements

Electrical codes and utility standards in most jurisdictions require the grounding system to control step, touch and transferred voltages, and many give reference grounding resistance values for substations. A resistance value alone does not show that the voltages are safe: meeting a reference resistance does not remove the need to verify that step, touch and transferred voltages do not exceed the tolerable values. That is why the design of a substation grid does not end at the value in ohms. We adapt the design and the verification to the local code and to the requirements of the utility that apply to your project.

How they are calculated and verified

In the field, the IEEE 81 guide describes the measurement of surface, step and touch potentials by injecting a test current into the grid. It is the way to confirm the design in an existing installation or after an expansion. See earth resistance test.

How they are reduced

• More conductor and better distribution in the grid, especially at the perimeter and in the corners.
• Surface layer of high resistivity.
• Deep rods that reach layers of lower resistivity.
• Gradient control at fences, gates and accesses, with perimeter conductors.
• Insulation or sectioning of pipes, rails and fences that leave the area, to avoid transferred voltages.
• Faster protection, which reduces the exposure time.

In industrial plants, the critical points are often outside the substation: the fence, the metal pipes reaching the electrical room and the structures near the service entrance. See industrial grounding and substation grounding grid.

Where problems appear in a plant

Dangerous voltages do not appear only inside the substation yard. In industrial plants the points most worth checking are:

• The perimeter fence of the substation: if it is connected to the grid, someone touching it from outside can be subjected to a high touch voltage; if it is not, it can receive a transferred voltage. It is solved with an exterior perimeter conductor or with isolated sections, depending on the design.
• Doors and gates, which when opened sweep an area outside the grid.
• Pipes, rails and cable trays that enter or leave the substation area and carry its potential to other places in the plant.
• Equipment at the edge of the grid, such as outdoor switchboards or service entrance structures.
• Expansions: a new transformer or a neighboring substation changes the fault current and the potential distribution.

A periodic review with field measurements detects cut or corroded grid conductors, which worsen the voltages without the total resistance changing much.

We do your grounding study

Our team designs and verifies grounding grids of medium-, high- and extra-high-voltage substations and industrial plants with IEEE 80. The work includes the soil model, the fault current from the short-circuit study, and the step, touch and transferred voltages. If they do not comply, we propose improvements. We do the design remotely from your data, worldwide; field measurements are done in Colombia, or elsewhere on request. See our engineering design and studies and primary engineering services, and write to us through contact.

Related guides

• Substation grounding grid
• Earth resistance test
• Industrial grounding
• Short-circuit study
• Isolated or grounded neutral
• Lightning shielding

References

• IEEE 80, IEEE Guide for Safety in AC Substation Grounding.
• IEEE 81, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.

Frequently asked questions