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What
is the difference between series and parallel circuits, and how do they
differ in A.C. and D.C. applications onboard a boat? H.L., via
e-mail
A
basic circuit consists of a power source—a battery, shore power,
or genset—a switch, and a device such as a motor or lightbulb. There
are two kinds of currents that can run along a circuit: A.C., or alternating
current, and D.C., or direct current. Unlike D.C., which always flows
in one direction, A.C. reverses its direction at regular intervals called
cycles. The number of cycles per second is known as the frequency of the
current. In the United States, we operate on a 60-cycle, or 60-Hertz (Hz),
frequency. As long as the electricity can pass from one side of the source
to the other, through the circuit out from the positive and back to the
negative terminal, the circuit will be complete and the device will operate.
In
the simple circuit in the first diagram, closing the switch allows the
electricity to complete its path and light up the bulbs. Opening up
the switch prevents the flow of electricity from returning to the source,
so the bulbs go off. In its most basic form, what we have is a series
circuit, as there is only one path for the current to follow.
If
you add another device or two to this series circuit and flip that switch
off, everything in the circuit will go off. Moreover, if one device
or wire in the circuit goes bad, the entire circuit will fail because
there will be no complete path from one pole of the battery to the other.
In some ways, this is not a good idea onboard, especially for multiple
A.C. applications like a lamp and entertainment center circuit. If the
lamp shorts, the entire circuit will go out.
The
parallel circuit, shown in the middle diagram, provides three paths
through which electricity can flow. Should one be faulty—a break
in a wire, for instance—the other two will not be affected, and
there will still be a clear path for the electricity to flow from and
to the power source.
In
the circuit shown here, one switch controls three devices. Open the
switch, and all three shut off. But if you install three switches, as
illustrated by the third diagram, you can shut one or two off without
affecting the others.
The
question of whether to use A.C. or D.C. applications aboard comes down
to a practical solution. For example, a boat that needs 1,800 watts
of electricity to run its A.C. circuits would require 14-gauge wire
conductors. (Gauge is a measure of thickness where the larger the number,
the thinner the wire.) For the same application in 24-volt D.C., a two-gauge
wire conductor would be needed—larger than a shore-power cord.
Some A.C. circuits aboard your boat would include lamps, galley appliances,
and entertainment items, while D.C. would cover bilge pumps and electronics,
for example.
As
with all things electric aboard your vessel, unless you are qualified
to do so, let a marine electrician have a look at your system.
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