Application Note

16 Fluke Corporation Power Quality Troubleshooting
Electrical noise is the result of
more or less random electrical
signals getting coupled into cir-
cuits where they are unwanted,
i.e., where they disrupt informa-
tion-carrying signals. Noise oc-
curs on both power and signal
circuits, but generally speaking,
it becomes a problem when it
gets on signal circuits. Signal
and data circuits are particularly
vulnerable to noise because
they operate at fast speeds and
with low voltage levels. The
lower the signal voltage, the
less the amplitude of the noise
voltage that can be tolerated.
The signal-to-noise ratio de-
scribes how much noise a cir-
cuit can tolerate before the valid
information, the signal, becomes
corrupted.
Noise is one of the more mys-
terious subjects in PQ, especially
since it must be considered
with its equally mysterious
twin, grounding. To lessen the
mystery, there are two key
concepts to understand:
•
The first is that electrical
effects do not require direct
connection (such as through
copper conductors) to occur.
For an electrician who’s been
trained to size, install and
test wiring, this may not be
intuitive. Yet think of light-
ning, or of the primary and
secondary of an isolation
transformer, or of the an-
tenna to your radio: there’s
no direct, hard-wired con-
nection, but somehow com-
plete electrical circuits are
still happening. The same
electrical rules-of-behavior
are in operation for noise
coupling, as will be
explained below.
•
The second concept is that
we can no longer stay in the
realm of 60 Hz. One of the
benefits of 60 Hz is that it’s
a low enough frequency that
power circuits can be treated
(almost) like dc circuits; in
other words, basic Ohm’s
Law will get you most places
you need to go. But when it
comes to noise, we need to
keep in mind that signal cir-
cuits occur at high frequen-
cies, that noise is typically
a broad spectrum of frequen-
cies, and that we need to
consider the frequency-de-
pendent behavior of potential
sources of noise.
Coupling mechanisms
There are four basic mecha-
nisms of noise coupling. It pays
to understand them and how
they differ one from the other
because a lot of the trouble-
shooter’s job will be to identify
which coupling effect is domi-
nant in a particular situation.
1. Capacitive coupling
This is often referred to as
electrostatic noise and is a
voltage-based effect. Lightning
discharge is just an extreme
example. Any conductors sepa-
rated by an insulating material
(including air) constitute a ca-
pacitor—in other words, capaci-
tance is an inseparable part of
any circuit. The potential for
capacitive coupling increases as
frequency increases (capacitive
reactance, which can be
thought of as the resistance to
capacitive coupling, decreases
with frequency, as can be seen
in the formula: X
C
= 1/ 2πfC).
2. Inductive coupling
This is magnetic-coupled noise
and is a current-based effect.
Every conductor with current
flowing through it has an asso-
ciated magnetic field. A chang-
ing current can induce current
in another circuit, even if that
circuit is a single loop; in other
words, the source circuit acts as
a transformer primary with the
victim circuit being the second-
ary. The inductive coupling
effect increases with the follow-
ing factors: (1) larger current
flow, (2) faster rate of change of
current, (3) proximity of the two
conductors (primary and sec-
ondary) and (4) the more the
adjacent conductor resembles a
coil (round diameter as opposed
to flat, or coiled as opposed to
straight).
Here are some examples of
how inductive coupling can
cause noise in power circuits:
Section 4
Electrical Noise and Transients
Figure 4.1 Lower voltage, faster signals increase sensitivity to noise.
20 - 30V
logic signal
3 - 5V
logic signal
Noise
Noise