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Course 715 - Electrical Safety for Technicians & Supervisors

Safety guides and audits to make your job as a safety professional easier

The Dangers of Electrical Shock

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Severity

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.

  1. The level of current is determined by both the voltage and resistance of an electrical pathway. The higher the voltage and lower the resistance, the greater the current.
  2. The next factor determining severity is the duration of exposure to electricity. The longer the employee is exposed, the greater the severity of injury.

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):

Real-life Examples

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 voltage
b. Current and duration
c. Duration and voltage
d. Resistance and current

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Low voltage - 600 Volts or Less

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).

Effects of Electrical Current Less Than 600 Volts on the Body

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!

2. Cardiac arrest, severe burns, and death is probable if a worker receives an electrical shock of _____.

a. 1 microamp
b. 5 milliamps
c. 2 amps
d. 10 amps

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High Voltage - Over 600 Volts

High Voltage - Greater than 600V.

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.

3. OSHA classifies any use of electrical service _____ as high voltage.

a. 50 volts or greater
b. above 120 volts
c. between 50 and 480 volts
d. over 600 volts

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Entrance Wound.

Current

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.

electrical burn on hand and arm
Exit Wound.

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. 1
b. 10
c. 100
d. 1000

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Factors that Determine Current Levels

Power drills use 30 times as much current as what will kill

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:

  • Voltage. Greater voltages produce greater currents.
  • Resistance. Resistance hinders current. The lower the resistance (or impedance in AC circuits), the greater the level of current.
  • Body type. Muscle structure also makes a difference. People with less muscle tissue are typically affected at lower current levels. Even
  • Duration. 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. Duration is important when you realize that a small power drill uses 30 times as much current required to cause death. However, if the shock is of short duration and the heart has not been damaged, a normal heartbeat may resume after contact with the electrical is eliminated. (This type of recovery is rare.)
  • Moisture. 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 broken 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.
  • Force. When more force is applied to the contact point or when the contact area is larger, the resistance is lower, causing stronger shocks.

5. Which of the following will result in lower resistance and greater risk of injury when shocked?

a. Dirty skin
b. Dry skin
c. Wet skin
d. Thick skin

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Case Study

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 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!

  • The victim should have tested the circuit to make sure it was de-energized.
  • Employers should provide workers with appropriate equipment and training. Using safety equipment should be a requirement of the job. In this case, a simple circuit tester may have saved the victim’s life.
  • Residential wiring should satisfy the National Electrical Code (NEC). Although the NEC is not retroactive, all homeowners should make sure their systems are safe.

6. What should you do to verify an electrical circuit is deenergized prior to working on it?

a. Ask someone if the electricity if off
b. Test the circuit to verify that it is deenergized
c. Flip the wall switch to make sure power is off
d. Work on the circuit live if it's under 50 volts

Check your Work

Read 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.

Video

Videos

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.

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