Step potential is the voltage difference between your two feet when you stand on ground that has become electrically energized. When a source of electricity, like a downed power line or a lightning strike, contacts the earth, current spreads outward through the ground. The voltage is highest at the point of contact and drops off as it radiates away in irregular concentric rings. If your feet land on two different rings, current can flow up one leg and down the other, passing through your body.
How Voltage Spreads Through the Ground
Electricity always seeks a path to dissipate, and soil conducts it. When a live conductor touches the earth, current fans out in all directions. The ground nearest the contact point carries the highest voltage, and each successive zone farther away carries less. This creates what engineers call a “ground gradient,” a series of voltage zones radiating outward like ripples in a pond.
The danger comes from straddling two of those zones at once. Your feet are typically about two to three feet apart when walking. If one foot sits in a higher-voltage zone and the other in a lower-voltage zone, that difference pushes current through your lower body. The closer you are to the energized source, the steeper the voltage drop between zones, and the greater the current that flows through you. Farther away, the difference between zones shrinks and the risk drops.
Why Step Potential Is Dangerous to the Body
The human body’s internal resistance is roughly 300 ohms, essentially the resistance of wet, salty tissue beneath the skin. Skin contact resistance varies widely, from about 1,000 to 100,000 ohms depending on moisture, skin condition, and surface area. Wet conditions dramatically lower resistance, which means more current flows through the body for the same voltage difference.
Even relatively low voltages can force dangerous amounts of current through a foot-to-foot path. Research has shown that as little as 12.6 milliamps can cause involuntary muscle contraction strong enough that a person cannot override it with voluntary effort. In a step potential scenario, leg muscles may lock up, causing you to fall. Falling is especially dangerous because it increases the distance between contact points on your body, potentially exposing your chest and heart to the current path.
Step Potential vs. Touch Potential
Step potential specifically refers to the voltage between your two feet on the ground. Touch potential is broader: it’s the voltage between any two points on your body. If a power line falls on a car and you touch that car while standing on the ground, current can travel from your hand through your torso and out through your feet. That hand-to-foot path is more dangerous than a foot-to-foot path because it crosses the heart.
Both hazards stem from the same underlying physics. The ground around a fault carries a voltage gradient, and any two points of contact at different voltages create a path for current. Step potential is the version you encounter simply by walking, which is what makes it so insidious: you don’t have to touch anything to be at risk.
Lightning and Animals
Step potential is one of the leading causes of mass livestock deaths during thunderstorms. When lightning strikes the ground, the same voltage gradient radiates outward. Four-legged animals are especially vulnerable because their legs are spread much wider apart than a human’s feet, which means a larger voltage difference between their front and hind legs. The current path also runs through the animal’s torso, crossing vital organs.
This is why you sometimes see news reports of dozens of cattle or horses killed by a single lightning strike. The bolt doesn’t hit each animal directly. Instead, the ground current from one strike radiates outward and passes through every animal standing within the high-gradient zone.
How Engineers Reduce the Risk
Electrical substations and industrial sites use several strategies to keep step potential within safe limits. The most visible one is the layer of crushed rock or gravel covering the ground surface. Crushed stone has significantly higher resistivity than ordinary soil, which means it resists the flow of current between the ground and a person’s feet. This effectively insulates workers from the energized soil underneath, reducing the current that can enter the body.
Underground grounding grids also play a role. These buried networks of conductors are designed to equalize voltage across the ground surface so there’s less difference between any two nearby points. The goal is to make the voltage as uniform as possible, minimizing the gradient that creates step potential in the first place.
Safety standards set limits on how much step voltage is acceptable. The IEC/IEEE 80005-1 standard caps touch and step voltage at 30 volts under normal conditions. However, the permissible level depends heavily on the duration of exposure and surface conditions. For a half-second shock on wet concrete, engineering calculations allow step voltages up to about 1,820 volts because the exposure is so brief. For a 10-second exposure on the same surface, the limit drops to roughly 407 volts. Shorter shock durations are less dangerous because the body can tolerate brief surges of current that would be lethal over several seconds.
What to Do Near a Downed Power Line
If you find yourself near a downed power line or any situation where the ground may be energized, the single most important rule is to keep your feet as close together as possible. The voltage difference between your feet is what drives current through your body, so minimizing the gap between them reduces your exposure.
The recommended technique is the shuffle walk: take very small steps, keeping your feet close together and flat on the ground at all times. Never let the heel of one foot pass the toes of the other. This ensures both feet stay in nearly the same voltage zone, keeping the difference between them as close to zero as possible. Some safety trainers also describe a “bunny hop” with feet together, though shuffling is generally considered more stable and less likely to result in a fall.
Move away from the source, not toward it. The voltage gradient is steepest close to the point of contact, so every foot of distance you gain makes you significantly safer. Do not run, and do not take large strides. A single long step near an energized source can bridge enough of a voltage difference to cause muscle lockup, which means you may collapse and widen your contact with the ground. If you’re in a vehicle that a power line has fallen on, stay inside. The tires provide insulation, and the metal body of the vehicle keeps you at a uniform voltage. Stepping out creates a path between the energized vehicle and the ground.