Grounding

To receive a shock two things must occur simultaneously: you must touch a hot wire (or a metal object in contact with a hot wire) and you must be grounded (Fig. 29-1). An electrical circuit has three wires. The hot wire, which is covered by black insulation (or any color but white, green, or gray) runs side by side with the neutral and ground wires. The neutral wire has a white or gray insulation. The ground wire is either bare copper or has green insulation. Homes built before 1960 often don't have a ground wire.

The hot wire carries the electrical power generated by your local utility. It's always poised and waiting to deliver its charge from inside an outlet or behind a switch, but current won't flow and release its power until it has a way to get back to its source, and to close the loop of the circuit. The neutral wire closes the loop. When you throw a switch to turn on an electric light bulb, you are essentially connecting the hot and neutral wires together and creating a circuit for electricity to follow.

The hot wire immediately senses this path and releases its energy. If nothing impeded the current flow, most of that energy would go unused. A light bulb or other electrical device standing in the path between the hot and the neutral wire uses up virtually all the energy available in the hot wire, leaving little for the neutral wire to carry back to the source. This is why you get a shock from touching the hot wire but not the neutral one, even when current is flowing.

When you get a shock from a hot wire, your body acts like a neutral wire and completes the circuit to the damp ground you are standing on. This is because the earth itself is also an excellent path that leads back to the power source and closes the loop. In fact, the electrical system uses the earth as an alternate path for safety purposes. The neutral wire is connected to the ground at the main service panel. From the main service panel, a wire goes to a copper-coated steel rod driven deeply

Hot bus bars

Hot bus bars

Grounding electrode conductor

Hot wire comes loose from terminal

Metal fixture & pull chain electrically charged

Grounding electrode conductor

Contact with ground

Hot wire comes loose from terminal

Metal fixture & pull chain electrically charged

Circuit breaker opens circuit

Contact with ground

Hot wire comes loose from terminal

Figure 29-1 How grounding prevents shocks.

into the earth beside the building, or to a metal water pipe that enters underground in older buildings. The power source is also grounded through a wire from the transformer on the utility pole to the earth. All building wiring is grounded. If you're not in contact with the damp ground, either by touching it directly or through wires, metal pipes or damp concrete in contact with the soil, you won't get a shock.

Your body is not as good an electrical path as a wire, even though it is about 90 percent water and water can be a good conductor. Your skin thickness, muscle, and other body traits make you a poor path for electrical current. Even so, your body is very vulnerable to electrical shocks. This is because shocks kill by stopping your heart. A steadily beating heart relies on tiny electrochemical nerve pulses that carry a current in the range of 0.001 A. Even a charge as small as 0.006 A can shatter the heart's microcircuitry and disrupt its beating rhythm. Often the nerves can't stabilize quickly enough to restore the circuitry and save your life.

An electric drill draws about 3 A and an electric mixer draws about 1 A, much more than the amount it takes for a fatal disruption of the heartbeat. Fortunately, it takes a fairly high voltage to push a significant amount of current through you. Generally, you won't get a shock from circuits under 24V. Electric toys fall into this range, as do doorbells, thermostats, telephones, security systems, cable TV, and low-voltage lighting. Even within this range, however, a shock can disrupt the heartbeat of a person with a pacemaker.

The best defense against a shock when you're handling electrical devices or appliances is to make sure your body is not grounded. Remember, current will go through you only when you are a path to the ground. Don't work with electricity while standing on damp ground or damp concrete, and don't work on a metal ladder that's resting on damp materials. Using electric tools and other electrical devices around the plumbing system can be dangerous, too. All these connect you with the ground.

The NEC has introduced three features that make electrical systems safer: the equipment ground, the ground fault circuit interrupter, and polarized plugs.

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