Electric Shock & the Human Body

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2 Module Goals This section is intended to inform of the dangers of accidental contact and methods of protection that can be utilized to protect one s life. These concepts can be brought forward into streetlighting practices to help achieve the best system possible for safeguarding the public. Below are the items to be covered in this module To explain the effects of electricity on the body Resistance of the human body 2

3 Three primary factors affect the severity of the shock a person receives when he or she is a part of an electrical circuit: Amount of current flowing through the body (measured in amperes). Path of the current through the body. Length of time the body is in the circuit. Other factors that may affect the severity of the shock are: The voltage of the current. The presence of moisture in the environment. The phase of the heart cycle when the shock occurs. The general health of the person prior to the shock. 3

4 The human body, particularly the heart, is very vulnerable to electrical current. Muscle contraction, paralysis, or heart stoppage can result from the flow of current through the body. Burns of the skin may also occur. These effects depend upon the amount of current and the path the current takes through the body. The length of time during which the current flows through the body is also a significant factor. At currents ranging from to ampere, a person may start to sense that current flow is present. A current level of to ampere may cause the arm muscles to contract and pain to begin. A current level of 0.01 ampere induces both pain and muscle contraction. Uncontrolled contraction of muscles results from current between 0.01 and 0.02 ampere. Thus, a person cannot let go of an energized conductor. 4

5 If the current level is above 0.03 ampere, loss of voluntary controls over respiratory muscles may occur, causing breathing to stop. Normal respiration generally resumes if the current is interrupted. Current flow through the heart muscle will cause ventricular fibrillation. During this condition, the muscles of the heart operate in a disorganized manner and do not pump blood. Current levels as low as ampere can cause this condition, but the human body can normally tolerate this for a short time. The threshold for 60 Hz current is proportional to body weight and inversely proportional to the square root of shock duration time. A current of 1 ampere through the heart muscle for 1/60 of a second will cause fibrillation. 5

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7 Your body is 70 percent water, and water is an excellent conductor of electricity. So, when you touch an energized bare electrical wire or a faulty appliance, the electricity will use your body as the shortest path to the ground. If you are grounded, the electricity will instantly pass through you to the ground, causing a harmful and sometimes fatal shock. You can be killed by the tiny amount of electricity used by one 7.5-watt holiday light if it passes through your chest. If the shock doesn't kill you, it can still badly hurt you by causing serious falls, burns, cuts, or internal bleeding. A shock from a 100-watt bulb or a 1000-watt hair dryer will probably be fatal. 7

8 Current Level Probable Effect on Human Body 1 ma Level of perception, slight tingling sensation 9 ma Slight shock, not painful but disturbing 9-30 ma ma ma Let-go range: Painful shock, muscular control is lost Extreme pain, respiratory arrest, cannot let go, death is possible Ventricular fibrillation, nerve damage, death is likely 8

9 Resistance of the human body When a person touches an energized conductor, current flows through the body to the ground. Some of the current may flow through the heart, depending upon how contact occurred. The amount of current that flows depends upon the resistance of the skin, the voltage on the energized conductor, and the ground resistance. The skin typically provides a resistance on the order of 1,500 to 5,000 ohms. Skin puncture, which can result from burns caused by electrical current flow, reduces the resistance to as low as 500 ohms. 9

10 Now consider what happens when a person touches a 120-volt conductor. The typical skin resistances noted above permit a current flow of approximately to amperes, which can be enough to cause fibrillation, given sufficient time. When a person touches a 480- volt conductor, the current flow would be to amps. Wet conditions are common during low-voltage electrocutions. Under dry conditions, human skin is very resistant. Wet skin dramatically drops the body's resistance. 10

11 High-voltage electrical energy greatly reduces the body's resistance by quickly breaking down human skin. Once the skin is punctured, the lowered resistance results in massive current flow. Ohm's law is used to demonstrate the action. At 1,000 volts, Current = Volts/Ohms = 1,000/500 = 2 Amps which can cause cardiac standstill and serious damage to internal organs. 11

12 30 volts is generally considered to be a conservative threshold value for dangerous voltage. The cautious person should regard any voltage above 30 volts as threatening, not relying on normal body resistance for protection against shock. That being said, it is still an excellent idea to keep one's hands clean and dry, and remove all metal jewelry when working around electricity. Even around lower voltages, metal jewelry can present a hazard by conducting enough current to burn the skin if brought into contact between two points in a circuit. Metal rings, especially, have been the cause of more than a few burnt fingers by bridging between points in a low-voltage, high-current circuit. 12

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