Controlling Electrical Hazards ( Electricity: The Basics )

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What affects the flow of electricity?

Electricity flows more easily through some materials than others. Some substances such as metals generally offer very little resistance to the flow of electric current and are called “conductors.” A common but perhaps overlooked conductor is the surface or subsurface of  the earth. Glass, plastic, porcelain, clay, pottery, dry wood, and similar substances generally slow or stop the flow of electricity. They are called “insulators.” Even air, normally an insulator, can become a conductor, as occurs during an are or lightning stroke.

How does water affect the  flow of electricity?

Pure water is a poor conductor. But small amounts of impurities in water like salt, acid, solvents, or other materials can turn water itself and substances that generally act as insulators into conductors or better conductors. Dry wood, for example, generally slows or stops the flow of electricity. But when saturated with water, wood turns into a conductor. The same is true of human skin. Dry skin has a fairly high resistance to electric current. But when skin is moist or wet, it acts as a conductor. This means that anyone working with electricity in a damp or wet environment needs to exercise extra caution to prevent electrical hazards.

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What causes shocks?

Electricity travels in closed circuits, normally through a conductor. But sometimes a person’s body — an efficient conductor of electricity — mistakenly becomes part of the electric circuit. This can cause an electrical shock. Shocks occur when a person’s body completes the current path with:

• both wires of an electric circuit;

• one wire of an energized circuit and the ground;

• a metal part that accidentally becomes energized due, for example, to a break in its insulation; or

• another “conductor” that is carrying a current. When a person receives a shock, electricity flows between parts of the body or through the body to a ground or the earth.

What effect do shocks have on the body?

An electric shock can result in anything from a slight tingling sensation to immediate cardiac arrest. The severity depends on the following:

• the amount of current flowing through the body,

• the current’s path through the body,

• the length of time the body remains in the circuit, and

• the current’s frequency.

This table shows the general relationship between the amount of current received and the reaction when current flows from the hand to the foot for just 1 second.

Effects of  Electric Current in the Human Body

Current                                                               Reaction


Below 1 milliamp ere                                   Generally not perceptible

1 milliamp ere                                            Faint tingle

5 milliamp eres                                          Slight shock felt; not painful but

                                                               disturbing. Average individual can

                                                               let go. Strong involuntary reactions

                                                              can lead to other injuries.

6–25 milliamp eres (women)                      Painful shock, loss of muscular

                                                             control*

9–30 milliamp eres (men)                       The freezing current or “let-go”

                                                           range.* Individual cannot let go,

                                                          but can be thrown away from the

                                                         circuit if extensor muscles are

                                                        stimulated.

50–150 milliamp eres                         Extreme pain, respiratory arrest,

                                                       severe muscular contractions.

                                                       Death is possible.

1,000–4,300 milliamp eres                 Rhythmic pumping action of

                                                       the heart ceases. Muscular

                                                       contraction and nerve damage

                                                       occur; death likely.

10,000 milliamp eres                         Cardiac arrest, severe burns; death

                                                       Probable


 

* If the extensor muscles are excited by the shock, the person may be thrown away from the power source.

 

Source: W.B. Kouwenhoven, “Human Safety and Electric Shock,”

            Electrical Safety Practices, Monograph, 112, Instrument

           Society of America, p. 93. November 1968.

 

What kind of  burns can a shock cause?

Burns are the most common shock-related injury. An electrical accident can result in an electrical burn, arc burn, thermal contact burn, or a combination of  burns.

Electrical burns are among the most serious burns and require immediate medical attention. They occur when electric current flows through tissues or bone, generating heat that causes tissue damage.

Arc or flash burns result from high temperatures caused by an electric arc or explosion near the body. These burns should be treated promptly.

Thermal contact burns are caused when the skin touches hot surfaces of overheated electric conductors, conduits, or other energized equipment. Thermal burns also can be caused when clothing catches on fire, as may occur when an electric arc is produced.

In addition to shock and burn hazards, electricity poses other dangers. For example, arcs that result from short circuits can cause injury or start a fire. Extremely high-energy arcs can damage equipment, causing fragmented metal to fly in all directions. Even low-energy arcs can cause violent explosions in atmospheres that contain flammable gases, vapors, or combustible dusts.

Why do people sometimes “freeze” when they are shocked?

When a person receives an electrical shock, sometimes the electrical stimulation causes the muscles to contract. This “freezing” effect makes the person unable to pull free of the circuit. It is extremely dangerous because it increases the length of exposure to electricity and because the current causes blisters, which reduce the body’s resistance and increases the current.

The longer the exposure, the greater the risk of serious injury. Longer exposures at even relatively low voltages can be just as dangerous as short exposures at higher voltages. Low voltage does not imply low hazard.

In addition to muscle contractions that cause “freezing,” electrical shocks also can cause involuntary muscle reactions. These reactions can result in a wide range of other injuries from collisions or falls, including bruises, bone fractures, and even death.

What should you do if someone “freezes” to a live electrical contact?

If a person is “frozen” to a live electrical contact, shut off the current immediately. If this is not possible, use boards, poles, or sticks made of wood or any other no conducting materials and safely push or pull the person away from the contact. It’s important to act quickly, but remember to protect yourself as well from electrocution or shock.

How can you tell if a shock is serious?

A severe shock can cause considerably more damage than meets the eye. A victim may suffer internal hemorrhages and destruction of  tissues, nerves, and muscles that aren’t  readily visible. Renal damage also can occur. If you or a coworker receives a shock, seek emergency medical help immediately.

What is the danger of static electricity?

Static electricity also can cause a shock, though in a different way and generally not as potentially severe as the type of shock described previously. Static electricity can build up on the surface of an object and, under the right conditions, can discharge to a person, causing a shock. The most familiar example of  this is when a person reaches for a door knob or other metal object on a cold, relatively dry day and receives a shock.

However, static electricity also can cause shocks or can just discharge to an object with much more serious consequences, as when friction causes a high level of static electricity to build up at a specific spot on an object. This can happen simply through handling plastic pipes and materials or during normal operation of rubberized drive or machine belts found in many worksites. In these cases, for example, static electricity can potentially discharge when sufficient amounts of flammable or combustible substances are located nearby and cause an explosion. Grounding or other measures may be necessary to prevent this static electricity buildup and the results.

 

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