0195136047.pdf

4.4 RESIDENTIAL WIRING AND SAFETY CONSIDERATIONS 213

The three-line cable coming out of the secondaries of the distribution transformer on the
utility pole passes through the electric meter that measures energy consumption in kilowatt-hours
and terminates at themain panel. Figure 4.4.2 shows a typical wiring arrangement for a residence.
At the main panel,circuit breakersserve the joint role of disconnecting switches and overcurrent
protection; the neutral is connected to abusbar(bus) and in turn to the local earth ground; the hot
lines are connected to individual circuits for lighting and appliances, as illustrated in Figure 4.4.2.
The circuit breaker labeled GFCI (ground-fault circuit interruption), used for safety primarily
with outdoor circuits and in bathrooms, has additional features that will be described later. Note
that every outgoing “hot” wire must be connected to a circuit breaker, whereas every neutral wire
and ground wire must be tied directly to earth ground at the neutral busbar.
Today most homes have three-wire connections to their outlets, one of which is shown in
Figure 4.4.3. The need for both ground and neutral connections needs to be explained, since the
ground conductor may appear to be redundant, playing no role in the actual operation of a load that
might be connected to the receptacle. From the viewpoint ofsafety, the ground connection is used
to connect the metallic chassis of the appliance to earth ground. Without the ground conductor
connected to the metal case of the appliance, as shown in Figure 4.4.4(a), the appliance chassis
could be at any potential with respect to ground, possibly even at the “hot” wire’s potential if a part
of the “hot” wire were to lose some insulation and come in contact with the inside of the chassis.
An unintended connection may occur because of the corrosion of insulation or a loose mechanical
connection. Poorly grounded appliances can thus be a significant hazard by providing a path to
ground through the body of a person touching the chassis with a hand. An undersized ground
loop current limited by the body resistance may flow directly through the body to ground and
could be quite harmful. Typically, the circuit breaker would not operate under such circumstances.

R

Main

breaker

20 A

15 A

G

G

W

W

W or G

GFCI

R Outdoor

lighting

(120-V circuit

with GFCI)

Electric stove

(240-V circuit)

Kitchen appliances

(120-V circuit)

Bedroom lighting

(120-V circuit)

Basement lighting

(120-V circuit)

Earth ground

Neutral busbar (bus)

R

B

B

20 A

20 A

20 A

Bus R Bus B

B W

20 A

G

W

B

G

W

B

Figure 4.4.2Typical wiring arrangement for a residenceNote:Currents
and voltages shown are rms magnitudes.