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Unformatted text preview: ent 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. LOW VOLTAGE DOES NOT MEAN LOW HAZARD! Defibrillator in use. Page 6 Section 2 Effects of Electrical Current in the Human Body3,4 Current
Below 1 milliampere 1 milliampere 5 milliamperes 625 milliamperes (women) 930 milliamperes (men) 50150 milliamperes 1,0004,300 milliamperes 10,000 milliamperes 15,000 milliamperes reaction
Generally not perceptible. Faint tingle. Slight shock felt; not painful but disturbing. Average individual can let go. Strong involuntary reactions can lead to other injuries. Painful shock, loss of muscular control. The freezing current or "let-go" range. Individual cannot let go, but can be thrown away from the circuit if extensor muscles are stimulated.* Extreme pain, respiratory arrest (breathing stops), severe muscular contractions. Death is possible. Rhythmic pumping action of the heart ceases. Muscular contraction and nerve damage occur; death likely. Cardiac arrest and severe burns occur. Death is probable. Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit! *If the extensor muscles are excited by the shock, the person may be thrown away from the power source. The lowest overcurrent at which a typical fuse or circuit breaker will open is 15,000 milliamps (15 amps). 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 contractions 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 High voltages cause additional injuries! Higher voltages can cause larger currents and more severe shocks. Some injuries from electrical shock cannot be seen. Section 2 Page 7 DA N G E r S O F E L E C T r I C A L S H O C K
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 greater the current, the greater the shock! Severity of shock depends on voltage, amperage, and resistance. resistance--a material's ability to decrease or stop electrical current ohm--unit of measurement for electrical resistance Lower resistance causes greater currents. 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 flow 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 broken skin will drastically reduce resistance. The low resistance of wet skin Power drills use 30 times as allows current to pass into much current as what will kill. 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. 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 may be damaged. Contacting a live electrical part...
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This document was uploaded on 03/14/2014 for the course ECE 482 at University of Tennessee.
- Spring '09