Almost 3 million workers in the United States are at risk every day from uncontrolled energy when servicing equipment. There are roughly 3,600 disabling injuries and 4,000 nondisabling electrical contact injuries every year. Electrical hazards include electrocution, electric shock, burns, and falls.
Electric shock occurs in one of three ways. Individuals, while in contact with the ground, must come in contact with:
1. Both wires of the electric circuit
2. One wire on an energized circuit and the ground
3. A metallic part that has become “hot” by contact with an energized conductor
Electrocution results when a human is exposed to a lethal amount of electrical energy. In this case, the body becomes part of an active electrical circuit having a current capable of overstimulating the nervous system or causing damage to internal organs. The extent of injuries depends on the current’s magnitude, the pathway of the current, and the duration of current flow through the body.
How It Works
Because the skin offers most of the body’s electrical resistance, the point of electrical contact with the skin will determine the amount of shock received. The condition of the skin also affects resistance to electricity. Resistance is increased if electricity contacts in an area that has thick or callused skin or if the skin is dry. Resistance is decreased if the electricity contacts thin, wet or sweaty, or broken skin.
The presence of moisture from standing water, wet clothing, high humidity, or perspiration decreases resistance. Under dry conditions, the resistance offered by the human body may be as high as 100,000 ohms. Wet or broken skin may drop the body’s resistance to 1,000 ohms.
The level of current passing through the body is directly related to the resistance of its path through the body. Wet conditions, therefore, increase the possibility of a low-voltage electrocution.
People have different levels of resistance to electric shock because every human body is built differently. This is why a similar voltage shock can feel minor to one person and be deadly to another. Of course, there are other considerations than just resistance. For example, one person might have a heart that is more sensitive to ventricular fibrillation (twitching heart).
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Exposure Levels
“Current” is the flow of electrons from a source of voltage through a conductor and is measured in amperes (amps). The number of amps determines the degree of shock.
Remember that on a given circuit, voltage is consistent, but the resistance changes, which in turn changes the amps. As resistance goes up, amps decrease, and if the resistance goes down, amps will increase. When resistance is greater, fewer amps will be received, which lowers the chances of a harmful shock.
Here’s an example: Let’s say we’re dealing with a 120-volt circuit, and let’s assume a body has 2,000 ohms of resistance.
Using a formula for determining amps, we calculate that that 120 volts divided by a resistance of 2,000 ohms equals 0.06 amps. Because 1,000 milliamps (mA) equals 1 amp, 0.06 amps equals 60 milliamps. Therefore, an average person can be exposed to 60 milliamps with a typical home voltage of 120.
Here’s how different levels of exposure to electricity can affect people:
Exposure Level = 2-10 mA
Impact on Body: Minor shock is possible.
Other Potential Outcomes: If working from an elevation, could cause a person to fall
Exposure Level = 10-25 mA
Impact on Body: May lose muscle control and may not be able to release or let go of the circuit
Other Potential Outcomes: Rescuer may receive shock trying to assist a victim.
Exposure Level = 25-75 mA
Impact on the Body: Painful and may lead to collapse or even death
Other Potential Outcomes: The longer the person is exposed to this electrical current, the more likely death will occur.
Exposure Level = 75-300 mA
Impact on the Body: Even for a quarter of a second, this exposure is almost always immediately fatal.
Other Potential Outcomes: Causes ventricular fibrillation (the rhythmic pumping action of the heart ceases)
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Tomorrow, we’ll review rescue techniques for electrical emergencies.
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