in Chapter 9, one way to minimize its effects
is to create appropriate zones of separation
between RFI sources and equipment that may
be sensitive to its effects. In the discussion in
Chapter 9, however, we were focusing on radi-
ated noise. In this section, we will discuss how
to minimize the effects of noise transmitted
through the wiring on the boat via proper
grounding techniques.
Inductanceis a property of a conductor or
coil that determines how much voltage will be
induced in it by a change in current. Capaci-
tance, which is the ratio of the electric charge
transferred from one to the other of a pair of
conductors to the resulting potential differ-
ence between them, exists between any two
conductors insulated from each other. In
many cases, the two conductors may be carry-
ing equal but opposite electrical charges. The
difference between them can be described as a
difference in potential, which, as discussed
earlier, is synonymous with voltage.
Any wire or conductor that has electrical
current flowing through it will have a magnetic
field around it, and cabling that runs through
wire chases and conduits is a prime source of
RFI problems. Depending upon its frequency,
RFI traveling down a given conductor may,
quite easily, be able to couple with a parallel run
conductor both inductively and capacitively.
Both inductance and capacitance can cause
a number of electrical problems on board a
modern boat, including faulty readings on elec-
tronic engine instrumentation and even the
failure of an electronically controlled engine.
The best way to deal with this issue is to estab-
lish a separation distance between the conduc-
tors feeding these devices, but proper grounding
can also help.
Electrically shielded conductors are com-
mon on some systems, such as network wiring,
and the shielding will often take care of the
problem, but not always. Another approach is
the use of “twisted pair” wiring, in which two
or more conductors are twisted together for
their entire length. Both techniques are widely
used by electronics vendors and on systems
such as Mercury Marine’s SmartCraft network
and Teleflex’s i6000 control system. This is yet
another example of the blurring of the line
between the electrical and electronics sides of
the boat.
Noise generated by AC devices can be bro-
ken down into two categories: differential
mode and common mode. Differential mode
noise resides in hot (or ungrounded) and neu-
tral (grounded) conductors, both of which
carry current. Common mode noise, on the
other hand, describes noise that exists between
either the hot and the grounding conductors
or the neutral and grounding conductors. In
either case, the noise can easily manifest itself
as a hum emitted by audio equipment or as
flickering or diagonal lines on a television
screen. Equipment vendors use filtering sys-
tems to help control this, with some approaches
being more effective than others. One of the
ways to further minimize the noise is the use
of proper grounding techniques, such as spe-
cialized capacitor-based circuitry. The opera-
tive word here is “proper.” It’s possible, if done
incorrectly, to make matters worse.Ground Loops and the Antenna Effect
Aground loop is a situation where there is
more than one ground connection between
two (or more) pieces of equipment. These
duplicate ground paths create a loop antenna
that will pick up interference currents. The
inherent resistance in the cable leads trans-
forms the current into voltage fluctuations.
Subtle induced voltages migrate through the
ground loop system, making the electrical
potential within unstable and allowing desir-
able signals and noise to ride together. If the
desired signal is from an audio system, thegrounding systems 123