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):
A 34-year-old male auto mechanic who was holding a wrench when his gold ring touched the positive terminal of a 12-volt car battery and the wrench touched both his ring and the negative terminal. He felt instant pain and had a deep partial-thickness circumferential burn at the base of his ring finger. No other soft tissues were injured. The cause of ring burns is most likely electrothermal burns.
A 21-year-old man sustained a band of deep burn around the wrist. A metal watchstrap that the patient was wearing, with evidence of the arching phenomenon on it, short-circuited the battery of the vehicle. Although the was an electrical accident, the current did not pass through any part of the patient's body, as what happens in an electrical injury.
1. The severity of injury from electrical shock depends on which two factors below?
a. Resistance and voltageThe 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).
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) 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! |
*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.
2. Cardiac arrest, severe burns, and death is probable if a worker receives an electrical shock of _____.
a. 1 microampThe 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.
3. OSHA classifies any use of electrical service _____ as high voltage.
a. 50 volts or greaterThe 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.
4. Electrical currents above _____ milliamperes may have a paralyzing or "freezing" effects on muscles.
a. 1As 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:
5. Which of the following will result in lower resistance and greater risk of injury when shocked?
a. Dirty skinA 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 his elbows on the 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!
6. What should you do to verify an electrical circuit is de-energized prior to working on it?
a. Ask someone if the electricity if offRead the material in each section to find the correct answer to each quiz question. After answering all the questions, click on the "Check Quiz Answers" button to grade your quiz and see your score. You will receive a message if you forgot to answer one of the questions. After clicking the button, the questions you missed will be listed below. You can correct any missed questions and check your answers again.
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.
This video explains the hazards of electricity in the workplace. It also shows some of the hazards that are present in nearly every workplace, and even in homes. Those hazards are electrical cords that are exposed to traffic.