Summary Statement
Student manual on electrical safety with information on recognizing, evaluating and avoiding hazards related to electricity.
January 2002
Currents greater than 75 mA 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.2
The table shows what usually happens for a range of currents (lasting 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). The muscle structure of the person also makes a difference. People with less muscle tissue are typically affected at lower current levels. Even low voltages can be extremely dangerous because the degree of injury depends not only on the amount of current but also on the length of time the body is in contact with the circuit.
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LOW
VOLTALGE DOES NOT MEAN LOW HAZARD!
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| Defibrillator in use |
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Effects
of Electrical Current* on the Body 3
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| Current | Reaction |
| 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)† | Painful shock. Muscular control is lost. This is the range where “freezing currents” start. It may not be possible to “let go.” |
| 9-30 milliamps (men) | |
| 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. Death is possible. |
| 1,000- 4,300 milliamps (1-4.3 amps) | Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely. |
| 10,000 milliamps (10 amps) | Cardiac arrest and severe burns occur. Death is probable. |
| 15,000 milliamps (15 amps) | Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit! |
| *Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns. †Differences in muscle and fat content affect the severity of shock. | |
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 (as seen on pages 9 and
10).
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 con-tractions themselves or
from falls.
A severe shock can cause much more damage to the body than is visible.
A person may suffer internal bleeding and destruction of tissues,nerves,
and muscles. Sometimes the hidden injuries caused by electrical shock
result in a delayed death. Shock is often only the beginning of a chain
of events. Even if the electrical current is too small to cause injury,
your reaction to the shock may cause you to fall, resulting in bruises,
broken bones, or even death.
The length of time of the shock greatly affects the amount of injury.
If the shock is short in duration, it may only be painful. A longer
shock (lasting a few seconds) could be fatal if the level of
current is high enough to cause the heart to go into ventricular fibrillation.
This is not much current when you realize that a small power drill uses
30 times as much current as what will kill. At relatively high currents,
death is certain if the shock is long enough. However, if the shock
is short and the heart has not been damaged, a normal heartbeat may
resume if contact with the electrical current is eliminated. (This type
of recovery is rare.)
The amount of current
passing through the body also affects the severity of an electrical
shock. Greater voltages produce greater currents. So, there is greater
danger from higher
voltages. Resistance hinders current. The lower the resistance (or impedance
in AC circuits), the greater the current will be. Dry skin may have
a resistance of 100,000 ohms or more. Wet
skin may have a resistance of only 1,000 ohms. Wet working conditions
or bro-ken skin will drastically reduce resistance. The low resistance
of wet skin allows current to pass into the body more easily and give
a greater shock. When more force is applied to the contact point or
when the contact area is larger, the resistance is lower, causing stronger
shocks.
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| Power drills use 30 times as much current as what will kill. |
The path of the
electrical current through the body affects the severity of the shock.
Currents through the heart or nervous system are most dangerous. If
you contact a live wire with your head, your nervous system will be
damaged. Contacting a live electrical part with one hand—while
you are grounded at the other side of your body—will cause electrical
current to pass across your chest, possibly injuring your heart and
lungs.
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| A male
service technician arrived at a customer’s house to perform
pre-winter maintenance on an oil furnace. The customer then left
the house and returned 90 minutes later. She noticed the service
truck was still in the driveway. After 2 more hours, the customer
entered the crawl space with a flashlight to look for the technician
but could not see him. She then called the owner of the company,
who came to the house. He searched the crawl space and found the
technician on his stomach, leaning on his elbows in front of the
furnace. The assistant county coroner was called and pronounced
the technician dead at the scene. The victim had electrical burns
on his scalp and right elbow. After the incident, an electrician inspected the site. A toggle switch that supposedly controlled electrical power to the furnace was in the “off” position. The electrician described the wiring as “haphazard and confusing.” Two weeks later, the county electrical inspector performed another inspection. He discovered that incorrect wiring of the toggle switch allowed power to flow to the furnace even when the switch was in the “off” position. The owner of the company stated that the victim was a very thorough worker. Perhaps the victim performed more maintenance on the furnace than previous technicians, exposing himself to the electrical hazard. This death could have been prevented!
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NEC—National
Electrical Code— a comprehensive listing of practices to protect workers and equipment from electrical hazards such as fire and electrocution |
| Electrical burn on hand and arm |
There have been
cases where an arm or leg is severely burned by high-voltage electrical
current to the point of coming off, and the victim is not electrocuted.
In these cases, the current passes through only a part of the limb before
it goes out of the body and into another conductor. Therefore, the current
does not go through the chest area and may not cause death, even though
the victim is severely disfigured. If the current does go through the
chest, the person will almost
surely be electrocuted. A large number of serious electrical injuries
involve current passing from the hands to the feet. Such a path involves
both the heart and lungs. This type of shock is often fatal.
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Arm
with third degree burn from high-voltage line.
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Summary
of Section 2
The danger from electrical shock depends on •••
the amount of the shocking current through the body,
the duration of the shocking current through the body,
and
the path of the shocking current through the body.
