Step & Touch Potential
When electricity enters the ground, it doesn’t stay in one spot. It spreads out in rings, and the voltage is different at each ring. That difference can shock you even if you never touch the wire. Two ideas explain how: step potential and touch potential.
- Electrical
- Utilities
- Construction
How electricity spreads in the ground
When a power line falls, a boom touches a line, or a fault sends current into the earth, the current flows outward through the ground. The voltage is highest at the point of contact and drops with distance, like ripples in a pond. The ground around the contact point is energized, even if nothing looks wrong. The effect is worse on wet ground and with higher voltage.
Step potential
- Step potential is the voltage difference between your two feet when they stand at different distances from the contact point.
- That difference pushes current up one leg and down the other. It can cause muscle contractions, make you fall, and put more of your body on the ground, where the current can pass near the heart.
- Taking a long stride makes it worse. Feet closer together mean a smaller difference.
Touch potential
- Touch potential is the voltage between something you touch (a metal fence, a vehicle, a boom, a pole or a guy wire) and the ground where you stand.
- If the object is energized and you stand at a lower voltage, current passes through your hands, chest and legs.
- It’s what makes touching a truck that has hit a power line deadly, even from the ground. See When Equipment Contacts a Power Line.
Where it happens
- Downed power lines, broken poles and lines fallen on fences or cars. See Downed Power Lines.
- Cranes, booms, dump trucks and lifts that touch overhead lines. See Overhead Power Lines.
- Substations, transformer pads, grounding grids and pole bases during a fault.
- Lightning strikes, which also create ground voltage rings.
- Pad-mounted equipment, pumps and metal structures with a fault to ground.
What to do
- Stay at least 35 feet away from a downed line or from equipment touching a line. Farther is better. Don’t go closer to help.
- Warn others, call 911 and the utility, and wait until the utility says the line is de-energized and grounded.
- If you feel a tingle in your feet or legs, the ground is energized. Keep your feet together and move away with small shuffling steps or hops with both feet together, never one foot lifted. Walk away from the contact point.
- Operators stay in the cab unless there is fire or another immediate danger. If you must get out, jump clear with both feet together and land together, then shuffle or hop away. See When Equipment Contacts a Power Line.
- Never touch a person, vehicle, fence or tool that is touching the line.
- Don’t run or take long strides away from an energized area.
- Don’t stand in puddles or on wet ground near the contact.
- Don’t try to move a wire with a stick, a rope or equipment.
- Don’t approach a vehicle in contact with a line, even to rescue someone, unless the utility has confirmed power is off. The people inside should stay put unless the vehicle is on fire.
Protecting crews
- Plan the job. Find the lines, ask the utility for voltage, and use a spotter for equipment near lines. See Overhead Power Lines.
- Use grounding correctly where the work calls for it, with qualified workers and approved procedures. Utility workers use equipotential zones, grounding mats and insulated gloves and boots. Don’t improvise.
- Limit access around substations and energized equipment with fences and signs, and follow the site rules.
- Train everyone on the shuffle and the 35-foot rule. Include it in toolbox talks. See Toolbox Talks.
After exposure
Anyone who felt a shock, even a mild one, should get medical care. A shock can cause heart rhythm problems that show up later. See Electrical Injuries and Electric Shock Response.
Quick reminder
- Treat every downed line and any equipment on a line as live
- Stay at least 35 feet back
- Feet together; shuffle or hop, don’t stride or run
- Never touch the machine, load, fence or person in contact
- Call 911 and the utility; wait for confirmation of power off
Standards & references
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