0195136047.pdf

216 THREE-PHASE CIRCUITS AND RESIDENTIAL WIRING

SPDT SPDT

Travelers

3-way switch #1 3-way switch #2

W

B

G

Figure 4.4.5Three-way

switch connections to operate a

device.

• Real, reactive, and apparent powers in three-phase circuits.


• Measurement of three-phase power by two single-phase wattmeters.


• Basic notions of residential circuit wiring, including grounding and safety considerations.

4.6 PRACTICAL APPLICATION: A CASE STUDY

Physiological Effects of Current and Electrical Safety

Various effects that electric current may have on the human body and the current levels at which

they occur are illustrated in Figure 4.6.1. Note that small currents can have serious effects. The

ranges in Figure 4.6.1, however, represent variations among individuals in body size, condition,

and tolerance to electric shock. The region of the body through which the current passes is crucial.

The current can easily be estimated by the application of Ohm’s law,

I=V/R

whereRis the resistance of the circuit of which a person’s body might become a part. Most serious

electric shocks occur where a person is in simultaneous contact with the so-called hot wire and

the ground. Ground may be the dry or moist earth, the plumbing of a house, or even a concrete

floor that is connected to the plumbing. The circuit resistance would include not only the body

Threshold of

perception

Let-go current

Respiratory paralysis, fatigue, pain

Ventricular fibrillation

Burns, injury

Sustained myocardial

contraction

1 mA 10 mA 100 mA 1 A 10 A 100 A

Figure 4.6.1Physiological effects of electricity. (Adapted from J. G. Webster,Medical Instrumentation,

Application and Design,Houghton Mifflin, 1978.)