0195136047.pdf

214 THREE-PHASE CIRCUITS AND RESIDENTIAL WIRING

However, if the ground conductor is present and properly connected to the chassis of the appliance,

as shown in Figure 4.4.4(b), the metal case will remain at ground potential or, at worst, a few

volts from ground if a fault results in current through the ground wire. The “hot” conductor may

be shorted to ground under fault conditions, in which case the circuit breaker would operate.

The body resistance of a normal person ranges from 500 kdown to 1 k, depending upon

whether the skin is dry or wet. Thus, a person with wet skin risks electrocution from ac voltages

as low as 100 V. The amount of current is the key factor in electric shock, and Table 4.4.1 lists

the effects of various levels of 60-Hz ac current on the human body. Note that the 100–300-mA

range turns out to be the most dangerous.

The best possible shock protection is afforded by theground-fault circuit interrupter(GFCI)

shown in Figure 4.4.4(c). A sensing coil located around the “hot” and neutral wires in the GFCI

detects the imbalance of currents between the neutral and the live conductor under fault conditions

and opens the circuit in response when|IB−IW|>5 mA. The GFCI may be located either at

an outlet or at the main panel. Ground-fault interrupters are now required in branch circuits that

serve outlets in areas such as bathrooms, basements, garages, and outdoor sites.

Various codes, such as theNational Electrical Code, have been established to provide

protection of personnel and property, while specifying requirements for the installation and

maintenance of electrical systems. Only qualified and properly certified persons should undertake

installation, alteration, or repair of electrical systems. Safety when working with electric power

must always be a primary consideration. In addition to numerous deaths caused each year due

to electrical accidents, fire damage that results from improper use of electric wiring and wiring

faults amounts to millions of dollars per year.

Finally, Figure 4.4.5 illustrates how a device, such as a light, can be controlled independently

from two different locations using single-pole double-throw (SPDT) switches, commonly known

as three-way switches or staircase switches. When the “hot” wire is switched between two

“travelers” at the first switch and from the travelers to the device at the second, a complete

circuit is formed only when both switches are either up or down; flipping either switch opens the

circuit. Note that neutral and ground wires are never switched.

120 V

Ground (green or uninsulated bare wire)

Hot

(black or red wire)

Neutral

(white wire)

0

I

I

Figure 4.4.3Three-wire outlet.Note:Currents

are shown with rms magnitudes for normal-load

operation.

TABLE 4.4.1Effects of 60-Hz ac Current on Human Body

Range of Current Physiological Effects

1–5 mA Threshold of sensation

10–20 mA Involuntary muscle contractions; pain; “can’t let go” condition

20–100 mA Breathing difficulties; severe pain

100–300 mA Ventricular fibrilation;possible deathwith no intervention

300–500 mA Respiratory paralysis (heartbeat may stop and may restart if shock is removed before death)

1–10 A Severe burns; temporary heart contraction (not fatal unless vital organs are burned)