Every once in a while, a newspaper reports that some crane operator got careless and let the crane boom touch a power line. Cranes are made of metal, and usually have metal outriggers used to stabilize the crane while it is lifting a load. This means the crane has an excellent connection to ground.
A typical 13,800 volt power distribution line in the United States is not insulated. When a crane boom touches one, the electric current can travel unimpeded down the boom, through the crane, through the operator's body, and into the ground. And 13,800 volt power lines can deliver a lot of electric current. The operator is usually killed immediately.
Unfortunately, the ground resistance under the crane is usually high enough that insufficient current is generated to trip the power distribution circuit breakers. This means that the power line remains live and electrically hot, and creates a very dangerous situation for everyone else. Not only is the crane itself dangerous to touch, but the ground immediately around the crane becomes dangerous to walk on.
Because the crane is grounding a power line, the ground immediately underneath the crane rises to several thousand volts. Because of the resistance of the ground, this voltage drops off sharply with distance in a radius around the crane. There can be a potential difference in the ground of several hundred volts from some point near the crane to a point a mere three feet closer to it.
It is human nature to want to help people in trouble, so the crane operator's coworkers will rush to his rescue. The fact that the ground is electrically hot is completely invisible and not intuitively obvious. When someone approaches the crane, one foot can be at a spot on the ground that is at a higher voltage potential than the other foot. This will cause an electric current to travel up one leg and down the other, injuring or even killing the person.
The best thing to do if a crane has touched a power line is to stay far away from it. Keep other people away from it as well, they probably won't know the ground is dangerous to walk on. Do not attempt to save the crane operator. Have someone call the electric company and get the power to the line shut off. The company will also send a team over to safely remove the crane from the power line.