Electrical Safety
By Stefanie R. Fishman, Project Assistant
Work around electrical power is one of the greatest dangers facing highway crews. For their safety, they must know about electricity and how to work around it. This article will describe the effects of electrical shock, the electrical power system, safety around electrical distribution lines, and preventive measures.
Current is the killing factor in electrical shock. Voltage is important only in that it determines how much current will flow through the body’s resistance. Current is measured in amperes or milliamperes (1/1000th of an ampere). Electric utilities classify effects of current if it breaks through the skin, since that risk can occur in emergencies. The following table summarizes effects of 60-hertz current on an average human body for that situation.
Current Values through Body Trunk in Milliamperes (ma) |
Effect on Average Human Body |
| 1 ma or less | No sensation |
| 1-8 milliamperes | Sensation of shock. Person can let go. |
| 8-15 milliamperes | Painful shock. Person can let go at will, if muscle control not lost. |
| 15 to 20 milliamperes | Painful shock. Muscle control to adjacent muscles lost. |
| 20-50 milliamperes | Painful. Severe muscular contractions. Breathing is difficult. |
| 50-100 milliamperes | Probable Death |
| Greater than 100ma | Death |
To put these values in perspective, the current necessary to operate a 120-watt light bulb is eight to ten times more current than would kill a person. A 12-watt bulb carries 90 milliamperes (ma), which can be fatal if it breaks through skin and the body’s resistance. The current in other everyday objects such as Christmas tree lights (60 to 90ma), electric razors (100ma), and hair dryers (8000ma) could also be fatal. These common objects are fatal if the current actually breaks through the skin and body resistance.
Above ground wires, however, carry enough force to break through the body’s resistance immediately upon contact. The force in these wires can be up to 35,000 volts. The current flow at the bodies resistance level and 120 volts is 240ma, over twice that which causes death. Thus, any contact with down lines means death instantaneously.
As shown in the following diagram, a typical above ground electrical system includes generating stations, step-substations, transmission lines, step-down substations, distribution lines, and residential, commercial and industrial customer service installations.
The distribution lines along highways are 2,400 volts and higher, and very dangerous.
In emergencies, public works personnel might encounter fallen wires and cables. In such situations, workers can be more concerned about correcting the public works emergency than their own safety. The workers’ safety should always be put first, especially around down wires. In all instances of down lines, workers should call the electric utility.
In routine, that is, non-emergency operations, each crewmember should be fully aware of the hazards of operating equipment near power lines. They should know the proper safety practices to follow.
Before beginning operations in non-emergency situations always notify DigSafe so that underground utilities can be marked. Request the cooperation of the line owner to de-energize and ground the lines or to help provide insulated barriers.
The National Institute for Occupational Safety and Health (NIOSH) encourages employers to consider de-energization as the primary mean of preventing injury from contact between equipment and power lines.
The first step when starting an emergency or non-emergency project near power lines is to determine the best placement for machinery during operations and the size and type of machinery to be used. Find the locations and voltage of all overhead power lines at the job site.
Everyone around the site should be alert. Don’t let the quiet, "harmless appearance lull you into a false sense of security." Although many overhead wires are covered, the covering is often designed to protect the wire from the weather or tree contact, not to protect you from the wire. Never consider a covered wire any safer than a bare wire.
Stay away from hanging or downed power lines and anything they are touching, including puddles of water, fences, trees, or people. Be aware that lightning or fallen wires could electrify wire or metal structures. Also remember that where there is one broken wire, there could be others.
Thus, in putting safety first one should remember:
Several preventive maintenance actions can decrease workers chances of having to deal with down lines on a site.
When trees or limbs fall, they often fell electrical lines as well. Supervisors and workers should be on the look out for trees growing close to wires. When a tree branches are close to wires, they should notify the local power company to trim the tree limbs. Municipal crews should never work on trees that are touching power lines.
Workers can prevent accidents by watching out for power lines around machinery, such as dump bodies, backhoes, and crane booms.
Special thanks to:
Ernest A. Guimond, Safety & Health Administrator, Public Service of New Hampshire (603) 634-2621
Sources:
Coming to Terms with Power Outages, Training Manual, Northeast Utilities
Preventing Electrocutions of Crane Operators and Crew Members Working Near Overhead Power Lines, National Institute for Occupational Safety and Health.
Return to UNH T2 Center, 33 College Road, Durham NH 03824 603-862-2826 Fax 603-862-2364