One of the most important advantages of the parallel configuration is that
if one branch of the configuration should fail (open circuit), the
remaining branches will still have full operating power.
In a home, the parallel connection is used throughout to ensure that if
one circuit has a problem and opens the circuit breaker, the remaining
circuits still have the full 120 V. The same is true in automobiles, computer
systems, industrial plants, and wherever it would be disastrous for
one circuit to control the total power distribution.
Another important advantage is that
branches can be added at any time without affecting the behavior of
those already in place.
In other words, unlike the series connection, where an additional component
reduces the current level and perhaps affects the response of some
of the existing components, an additional parallel branch will not affect
the current level in the other branches. Of course, the current demand
from the supply increases as determined by Kirchhoff's current law, so
you must be aware of the limitations of the supply.
The following are some of the most common applications of the parallel configuration.
Fig. no.1: Expanded view of an automobile's electrical system.
As you begin to examine the electrical system of an automobile, the
most important thing to understand is that the entire electrical system of
a car is run as a dc system. Although the generator produces a varying
ac signal, rectification converts it to one having an average dc level for
charging the battery. In particular, note the use of a filter capacitor in
the alternator branch in Fig. no.1 to smooth out the rectified ac waveform
and to provide an improved dc supply. The charged battery must
therefore provide the required direct current for the entire electrical system
of the car. Thus, the power demand on the battery at any instant is
the product of the terminal voltage and the current drain of the total
load of every operating system of the car. This certainly places an enormous
burden on the battery and its internal chemical reaction and warrants
all the battery care we can provide.
Since the electrical system of a car is essentially a parallel system, the total current drain on the battery is the sum of the currents to all the parallel branches of the car connected directly to the battery. In Fig.no.1, a
few branches of the wiring diagram for a car have been sketched to provide
some background information on basic wiring, current levels, and
fuse configurations. Every automobile has fuse links and fuses, and
some also have circuit breakers, to protect the various components of the
car and to ensure that a dangerous fire situation does not develop. Except
for a few branches that may have series elements, the operating voltage
for most components of a car is the terminal voltage of the battery,
which we will designate as 12 V even though it will typically vary
between 12 V and the charging level of 14.6 V. In other words, each
component is connected to the battery at one end and to the ground or
chassis of the car at the other end.
To examine the actual connection of elements in home, First, it is important to realize that except for some very special circumstances,
the basic wiring is done in a parallel configuration. Each
parallel branch, however, can have a combination of parallel and series
elements. Every full branch of the circuit receives the full 120 V or
240 V, with the current determined by the applied load. Fig. no.2 provides
the detailed wiring of a single circuit having a light bulb and two
outlets. Fig. no.2 shows the schematic representation. Note that
although each load is in parallel with the supply, switches are always
connected in series with the load. The power is transmitted to the lamp
only when the switch is closed and the full 120 V appears across the
bulb. The connection point for the two outlets is in the ceiling box holding
the light bulb. Since a switch is not present, both outlets are always
"hot" unless the circuit breaker in the main panel is opened. This is
important to understand in case you are tempted to change the light fixture
by simply turning off the wall switch. True, if you're very careful,
you can work with one line at a time (being sure that you don't touch the
other line at any time), but it is much safer to throw the circuit breaker
on the panel whenever working on a circuit.
Fig. no.2: Single phase of house wiring.
Note in Fig. no.2 that the feed wire (black) into the fixture from the panel is connected to the switch and both outlets at one point. It is not connected directly to the
light fixture because the lamp would be on all the time. Power to
the light fixture is made available through the switch. The continuous
connection to the outlets from the panel ensures that the outlets are "hot"
whenever the circuit breaker in the panel is on. Note also how the return
wire (white) is connected directly to the light switch and outlets to provide
a return for each component. There is no need for the white wire to
go through the switch since an applied voltage is a two-point connection
and the black wire is controlled by the switch.
Proper grounding of the system in total and of the individual loads is
one of the most important facets in the installation of any system. There
is a tendency at times to be satisfied that the system is working and to
pay less attention to proper grounding technique. Always keep in mind
that a properly grounded system has a direct path to ground if an undesirable
situation should develop. The absence of a direct ground causes
the system to determine its own path to ground, and you could be that
path if you happened to touch the wrong wire, metal box, metal pipe,
and so on.