The severity of injury from exposure to electricity depends on two factors: the level of electrical current (amperage) and the duration the current passing through the body.
OSHA considers all voltages of 50 volts or above to be hazardous because, as we know, electric current, not voltage, passing through the human body causes injury, and the amount of current passing through an object depends on the resistance of the object.
The internal resistance of the human body is about 500 ohms, which is the minimum resistance of a worker with broken skin at the point of contact. The current through 500 ohms from a live part energized at 60 volts would be 120 milliamperes. This level of current, either ac or dc, is sufficient to cause serious injury.
Although OSHA's standards require guarding starting at 50 volts (AC or DC), it is not necessarily the case that voltages below that level are completely safe. Cases in which auto mechanics have sustained serious injuries working with 12-volt or 24-volt (DC) vehicle batteries. For instance, see these two examples of injuries while working around car batteries (NIH/Pubmed):
The table below summarizes what usually happens for a range of currents with a duration of one second at typical household voltages. Longer exposure times increase the danger to the shock victim. For example, a current of 100 mA applied for 3 seconds is as dangerous as a current of 900 mA applied for a fraction of a second (0.03 seconds).
|1 milliamp||Just a faint tingle.|
|5 milliamps||Slight shock felt. Disturbing, but not painful. Most people can "let go." However, strong involuntary movements can cause injuries.|
|6-25 milliamps (women)
9-30 milliamps (men)
|Painful shock. Muscular control is lost. This is the range where "freezing currents" start. It may not be possible to "let go."|
|50-150 milliamps||Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Heart fibrillation possible. Death is possible.|
|1-4.3 amps||Rhythmic pumping action of the heart ceases. Muscular contraction and nerve damage occur; death likely.|
|10 amps||Cardiac arrest and severe burns occur. Death is probable.|
|15 amps||Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit!|
The U.S. Department of Energy (DOE) Electrical Safety Guidelines classify high voltage as over 600 volts. Also, OSHA classifies any use of electrical service over 600 volts as high voltage.
Sometimes high voltages lead to additional injuries. High voltages can cause violent muscular contractions. You may lose your balance and fall, which can cause injury or even death if you fall into machinery that can crush you. High voltages can also cause severe burns due to arc flash.
At 600 volts, the current through the body may be as great as 4 amps, causing damage to internal organs such as the heart. High voltages also produce burns. In addition, internal blood vessels may clot. Nerves in the area of the contact point may be damaged. Muscle contractions may cause bone fractures from either the contractions themselves or from falls.
The amount of internal current a person can withstand and still be able to control the muscles of the arm and hand can be less than 10 milliamperes (milliamps or mA).
Currents above 10 mA can paralyze or "freeze" muscles. When this "freezing" happens, a person is no longer able to release a tool, wire, or other object. In fact, the electrified object may be held even more tightly, resulting in longer exposure to the shocking current. For this reason, hand-held tools that give a shock can be very dangerous.
If you can't let go of the tool, current continues through your body for a longer time, which can lead to respiratory paralysis (the muscles that control breathing cannot move). You stop breathing for a period of time.
People have stopped breathing when shocked with currents from voltages as low as 49 volts. Usually, it takes about 30 mA of current to cause respiratory paralysis.
Currents greater than 75 mA may cause ventricular fibrillation (very rapid, ineffective heartbeat). This condition will cause death within a few minutes unless a special device called a defibrillator is used to save the victim.
Heart paralysis occurs at 4 amps, which means the heart does not pump at all. Tissue is burned with currents greater than 5 amps.
As we know, the severity of an electrical shock in the body is determined by several factors that influence the amount of current and the duration of exposure. These factors include:
This Puget Sound Energy video does a great job in explaining the hazards of downed powerlines. Remember, if you come in contact with a downed power line, stay clear and call 911 immediately. Do not touch them or the wire.
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