20.14

Safety and the Physiological Effects of Current

Electric circuits, although very useful, can also be hazardous. To reduce the danger inherent in using circuits, proper electrical grounding is necessary. The next two figures help to illustrate what electrical grounding means and how it is achieved.

The physics of safe electrical grounding.

Figure 20-42a shows a clothes dryer connected to a wall socket via a two-prong plug. The dryer is operating normally; that is, the wires inside are insulated from the metal casing of the dryer, so no charge flows through the casing itself. Notice that one terminal of the ac generator is customarily connected to ground by the electric power company. Part b of the drawing shows the hazardous result that occurs if a wire comes loose and contacts the metal casing. A person touching it receives a shock, since electric charge flows through the casing, the person’s body, and the ground on the way back to the generator.

Figure 20-42 (a) A normally operating clothes dryer that is connected to a wall socket via a two-prong plug. (b) An internal wire accidentally touches the metal casing, and a person who touches the casing receives an electrical shock.

Figure 20-43 shows the same appliance connected to a wall socket via a three-prong plug that provides safe electrical grounding. The third prong connects the metal casing directly to a copper rod driven into the ground or to a copper water pipe that is in the ground. This arrangement protects against electrical shock in the event that a broken wire touches the metal casing. In this event, charge would flow through the casing, through the third prong, and into the ground, returning eventually to the generator. No charge would flow through the person’s body, because the copper rod provides much less electrical resistance than does the body.

Figure 20-43 A safely connected dryer. If the dryer malfunctions, a person touching it receives no shock, since electric charge flows through the third prong and into the ground via a copper rod, rather than through the person’s body.

Serious and sometimes fatal injuries can result from electrical shock. The severity of the injury depends on the magnitude of the current and the parts of the body through which the moving charges pass. The amount of current that causes a mild tingling sensation is about 0.001 A. Currents on the order of 0.01−0.02 A can lead to muscle spasms, in which a person “can’t let go” of the object causing the shock. Currents of approximately 0.2 A are potentially fatal because they can make the heart fibrillate, or beat in an uncontrolled manner. Substantially larger currents stop the heart completely. However, since the heart often begins beating normally again after the current ceases, the larger currents can be less dangerous than the smaller currents that cause fibrillation.

The physics of the physiological effects of current.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.