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Circuit breaker panel

One circuit Circuit
breaker
switches

Load wire
Light

Wall
outlet
Switch

Ground

Radio
Toaster
Can
opener
FIGURE 6.39 Each circuit has its own breaker switch (or fuse), and this simplified sketch shows one circuit and the wiring from the
breaker panel to one light and four outlets. Household circuits are parallel circuits, and each appliance connected to such a circuit draws
current. Each circuit has its own designed voltage and ampere rating based on the intended use.

the same purpose, but it uses a different procedure. A circuit A household circuit requires two wires to each electrical

breaker uses the directly proportional relationship between device: (1) an energized “load-carrying” wire that carries elec-
the magnitude of a current and the magnetic field around the trical energy from the electric utility and (2) a grounded or

conductor. When the current reaches the preset level, the “neutral” wire that maintains a potential diff erence between
associated magnetic field opens the circuit by the magnetic the two wires. Suppose the load-carrying wire inside an appli-

field attracting a piece of iron, thus opening a spring-loaded ance becomes frayed or broken or in some way makes contact
switch. The circuit breaker is reset by flipping the switch back with the metal housing of the appliance. If you touch the hous-

to its original position. ing, you could become a part of the circuit as a current fl ows
In addition to overloads, fuses and circuit breakers are through you or parts of your body.
“blown” or “tripped” by a short circuit, a new path of lesser A three-pronged plug provides a third appliance-grounding
electrical resistance. A current always takes the path of least wire through the grounded plug. The grounding wire connects

resistance. A short circuit occurs when some low-resistance the metal housing of an appliance directly to the ground. If
connection or “shortcut” is accidentally established between there is a short circuit, the current will take the path of least
two points in an electric circuit. Such a “shortcut” could be resistance—through the grounding wire—rather than through
provided by a frayed or broken wire that completes the circuit. you. A polarized plug has one prong larger than the other. Th e
The electrical resistance of copper is very low, which is why cop- smaller prong is wired to the load-carrying wire, and the larger

per is used for a conducting wire. A short piece of copper wire one is wired to the neutral or ground wire. The ordinary, non-

involved in a short circuit might have a resistance of only polarized plug can fit into an outlet either way, which means

0.01 ohm. The current that could flow through this wire in a there is a 50-50 chance that one of the wires will be the one

120 volt circuit, according to Ohm’s law, could reach 12,000 amp that carries the load. The polarized plug does not take that gam-
if a fuse or circuit breaker did not interrupt the circuit at the ble since it always has the load-carrying wire on the same side
preset 15 amp load level. of the circuit. Thus, the switch can be wired in so it is always

A modern household electric circuit has many safety devices on the load-carrying wire. The switch will function on either
to protect people and property from electrical damage. Fuses or wire, but when it is on the ground wire, the appliance has an
circuit breakers disconnect circuits before they become over- energy-carrying wire, just waiting for a potential diff erence to
loaded and thus overheated. People are also protected from elec- be established through a ground, perhaps through you. When
trical shock by three-pronged plugs, polarized plugs ( Figure 6.40), the switch is on the load-carrying side, the appliance does not
and ground-fault interrupters, which are discussed next. have this potential safety hazard.
6-31 CHAPTER 6 Electricity 169