Every apprentice hears it early: stay off the truck while the boom is up near the line, keep people back from the grounded structure, do not hand tools from the bucket to someone standing on the dirt. Those rules can feel like superstition until you understand step potential and touch potential, the two ways the ground itself can shock you during a fault.
This article explains the physics in plain terms, then connects it to the grounding and bonding habits your crew practices. It is not a substitute for your employer's training or procedures. It is the why behind them.
Why the ground is not always at zero volts
We talk about earth as "ground" as if it were a perfect zero-volt sink. It is not. Soil has resistance. When fault current flows into the earth, whether from an energized line touching a grounded truck, a pole, or a set of protective grounds, that current has to spread out through the soil, and pushing current through resistance creates voltage.
Hubbell Power Systems compares the effect to ripples spreading from a pebble dropped in a pond. Voltage is highest right at the point where current enters the earth and falls off as you move away. As a rough rule of thumb, Hubbell says voltage drops by about half for every 3 feet from that point, while noting that soil conditions change the real numbers.
That gradient is the whole story. If two points on the ground, or a point on the ground and a piece of equipment, are at different voltages, and your body bridges them, current flows through you.
What is step potential?
Step potential is the voltage difference between your two feet when you are standing in that gradient. One foot sits on a "ripple" closer to the fault, the other on a ripple farther out. The difference between them drives current up one leg and down the other.
A few practical consequences follow from that:
- Distance matters. The closer you stand to where current is entering the earth, the steeper the gradient and the bigger the difference between your feet.
- You do not have to touch anything. Step potential can hurt someone who is only walking past a grounded truck or structure during a fault.
- Bystanders are exposed. Groundmen, flaggers, inspectors and members of the public near the work area are all standing in the same soil.
What is touch potential?
Touch potential is the voltage difference between an object you are touching and the ground under your feet. Your hand is at the object's voltage; your feet are at whatever the soil is at where you stand. Current flows hand to feet.
Hubbell points out why this can be so dangerous: if the object is grounded far from where you are touching it, the touch potential can approach the full fault voltage. Its example is a digger derrick bonded to the system neutral that contacts an energized line. Anyone touching the truck or an uninsulated load line could be exposed to nearly the full fault voltage, because the truck is energized and the dirt under the worker's boots is not.
Why this matters on every job: Touch potential is the reason ground-level workers are told not to lean on, climb onto or hand material to a truck whose boom is working near energized conductors. If that boom makes contact, the truck becomes the object and your hand becomes the path.
Step potential and touch potential compared
| Step potential | Touch potential | |
|---|---|---|
| Current path | Foot to foot | Hand to feet |
| Who is exposed | Anyone standing in the voltage gradient, including bystanders | Anyone touching energized or grounded equipment while standing on earth |
| Typical trigger | Fault current entering earth at a pole, structure, truck or ground rod | Truck, boom, load line, guy or grounded conductor rising in voltage during a fault |
| Main defenses | Restricting access, keeping distance, equipotential mats | Equipotential zone, insulating gloves and gear, not touching equipment from the ground |
How an equipotential zone works
Here is the idea that confuses many new hands: grounding does not make everything zero volts during a fault. What it can do is make everything you can touch rise to nearly the same voltage. If your hands and feet are at the same potential, there is very little difference across your body, and very little current flows through it. That is an equipotential zone.
Hubbell describes two common ways of building one:
- Permanent grids, used in substations, where a buried bonded grid keeps the yard surface close to the same voltage as the equipment.
- Portable grounding mats for field work. The mat is bonded to the de-energized equipment the worker will touch, and a separate low-resistance connection runs to earth. Hubbell notes the mat itself is not meant to carry the fault current; the two connections work in parallel to keep the voltage across the worker low.
The key word is bonding. The worker, the mat and the equipment are tied together so they rise and fall together. Step off the mat while touching the equipment and you have broken the zone, and you are back to bridging two different voltages.
Other ways crews protect against step and touch
Equipotential zones are one layer. Hubbell also describes:
- Insulating equipment. Rubber gloves and similar gear protect workers who must handle grounded equipment. Hubbell stresses that the gear must be rated for the highest voltage that could appear on that grounded object during a fault, which is not necessarily the full system voltage.
- Restricting access. Keeping ground-level workers out of areas where step or touch voltages could be hazardous, especially near transmission structures. Workers should not handle grounded conductors or equipment unless they are inside an equipotential zone or protected by insulating gear.
These protections fit inside the broader framework of OSHA 29 CFR 1910.269, the standard covering electric power generation, transmission and distribution work, which includes requirements for training, job briefings and work on or near energized parts. Our 1910.269 explainer walks through how that standard is organized. Your employer's procedures and the training you receive under them are what govern how grounds, mats and barricades are actually set on your jobs.
Where induction fits in
De-energized does not always mean dead. Lines that run near energized circuits can pick up induced voltage, which is one more reason crews treat every conductor as energized until it has been tested and grounded under their procedures. The specifics of how your employer handles induced voltage, especially on transmission work and parallel circuits, belong in your job briefing and training, so ask about them directly.
Ground-level habits that come straight from this physics
Once you understand step and touch potential, many crew rules stop looking arbitrary. Use this as a self-check, and follow your employer's procedures where they are more specific:
- Do not touch a truck, boom or load line from the ground while the boom is working near energized conductors unless your procedures and protective setup allow it.
- Do not hand tools or material between someone in the bucket or on the structure and someone standing on earth.
- Stay on the mat whenever you are touching equipment it is bonded to; do not step off mid-task.
- Keep bystanders, flaggers and the public back from grounded structures and trucks during work near energized lines.
- Wear insulating gear rated for the voltage that could appear on what you are handling, and inspect it before use.
- Speak up in the job briefing if you do not know where the grounds are, where the zone is, or which equipment is bonded.
- If a contact or fault happens, do not rush toward the truck or structure. Wait for the line to be confirmed de-energized under your crew's emergency procedures.
New to the crew? Our guide to your first year as a groundman covers how to earn trust on the ground, and these habits are a big part of it.
The bottom line
Step potential and touch potential are why the dirt around a fault can be as dangerous as the line itself. Voltage spreads through the soil in rings, and your body gets hurt when it bridges two different voltages, foot to foot or hand to foot. Equipotential zones, insulating gear and restricted access all attack the same problem: keep you from becoming the path between two potentials. Learn the why, and the grounding and bonding rules on your crew will make sense every time you set them up.