Part 4 of 5: In the first three weeks of this series about power line safety we have explored the standard clearances required by OSHA, as well as the requirements that must be met when working near power lines. These measures were put in place to safeguard workers from harm. However, it is important for personnel to know what they should do if a crane, load line, or load contacts an energized wire. These next two weeks we are going to look at how personnel may be affected and what steps to take if this were to happen.
It is important to know that when any part of the crane, load line or load contacts energized power, the crane will become electrified and the ground around the crane will also become electrified. This is called ground gradient which is where the voltage in the ground around the crane dissipates over distance. The ground around the crane will carry different voltages and these areas are called ground potentials. Unfortunately there is no way to tell how far these areas extend.
The illustration below depicts how a ground gradient occurs and how ground potential areas are created (represented by the jagged lines surrounding the crane, with the red numbers are used to identify different potentials). NOTE: there may be many more potentials and this is only to provide a representation of how voltage dissipates as it moves farther away from the crane.
Injuries occur when individuals find themselves in more than one potential. Step potential is the voltage difference between a person's two feet when they are standing within the ground gradient. A person could be at risk of serious injury or death simply by standing in two potentials. Touch potential is the voltage difference between the potential where the person is standing and their hands in contact with the crane, load, rigging, etc. A person could be at risk of serious injury or death simply by touching the crane or load while standing on the ground.
In the illustration above, the operator is shown in green and is usually in only one potential, as long as they do not move from the cab (unless they are in imminent danger from fire, explosion, or other emergency that necessitates leaving the cab). The electricity is at a constant voltage when in the same potential and it is able to flow thru the individual without impacting them.
Persons in red are exposed to step or touch potential, due to either their feet being apart, by touching the crane or touching the load. In these cases, the person is exposed to an unknown amount of voltage and is subject to severe injury or death. This is why OSHA requires that not-conductive tag lines always be used.
The persons represented in black are not impacted in their current position. The individual with their feet together is in one potential and the individual standing with feet apart is outside of the area of electrified ground. Both individuals are safe as long as they do not move.
Next week we will take a closer look at how one is to react if equipment becomes energized. Many times a worker may not be impacted at the time of contact, but how they react after the fact may be the difference between life and death.
So, dare we ask, when was the last time you checked your tag lines or looked at your policy for idle contact on a working crane?
Remember, if you ever need clarity about a possible encroachment concern or how to work around power lines, feel free to call us to talk through the situation. After all, Safety through Education is more than our tagline, it is our guiding principle!
