To understand step and touch potential, we first need to understand how energy dissipates across conductive objects. During broken pole or downed wire conditions, some really good conductors exist that provide a path to ground including metal fences, wet soil and puddles. Other conductors exist that may not be so good, yet still allow current to travel to ground, such as trees, wood fences and utility poles. Wood is typically thought of as an insulator, but wet wood will conduct electric current.
When an energized conductor falls across a chain-link fence or directly to the ground, the object and immediate area become energized, creating a zone of high voltage in relation to the ground. The actual voltage depends on the source, resistance of the object and soil conditions, which include material and moisture. The dissipation of voltage from a grounded conductor – or from the grounded end of an energized grounded object – is called the ground potential gradient. Voltage drops associated with this dissipation of voltage are called ground potentials. The voltage decreases rapidly with increasing distance from the grounded end.
Another way of describing this is the example of a stone dropped in a pond. The stone creates ripples eventually fading as they move from the center. Voltage is highest at the source and fades as the energy moves across the ground.
Touch potential is the voltage between any two points on a person’s body – hand to hand, shoulder to back, elbow to hip, hand to foot and so on. For example, if an overhead conductor falls on a car, and person touches the car, current could pass from the energized car through the person to the ground.
Above all else, always consider all equipment, lines and conductors to be energized. Be cautious and if you notice downed wires or damaged electrical equipment, contact appropriate utility personnel. Remember circuits do not always turn off when a power line falls into a tree or onto the ground. Even if they are not sparking or humming, fallen power lines can kill you if you touch them or even the ground nearby.
What to do in vehicle accidents that involve power lines
Instincts can help us to avoid danger but in some situations, our natural inclinations can lead to tragic results. If your car hits a utility pole or otherwise brings a power line down, getting out of vehicle, with few exceptions, is the wrong thing to do until the line has been de-energized. Know the right steps to take to save your life:
- You are almost always better off to stay in the car, especially if the line is in contact with the vehicle.
- Call or signal for help. It is safe to use a cell phone.
- Warn others who may be nearby to stay away, and wait until the electric utility arrives to makes sure power to the line is cut off.
- If the power line is still energized and you step outside, your body becomes the path to ground for electricity, and electrocution is the tragic result. Wait until the electric utility arrives and shuts off the power.
- The only exception would be if fire or other danger, like the smell of gasoline, is present. In that case, the proper action is to jump – not step – with both feet hitting the ground at the same time. Jump clear. Do not allow any part of your body to touch the vehicle and ground at the same time. Hop to safety, keeping both feet together as you leave the area. Like ripples in a pond or lake, the voltage diminishes the farther out it is from the source. Stepping from one voltage level to another allows the body to become a path for electricity.
- Even if a power line has landed on the ground, there is still the potential for the area near your car to be energized. Stay inside the vehicle unless there is fire or imminent risk of fire.
- The same rules apply with situations involving farm equipment and construction equipment coming in contact with overhead lines. Those working with large equipment should stay inside the vehicle if equipment extensions come in contact with power lines.