Application Note

18 Fluke Corporation Power Quality Troubleshooting
RFI noise reduction employs
a number of strategies:
•
Fiber optic cable, of course,
is immune to electrical noise.
•
Shielded cabling (such as
coax cables) attempts to
break the coupling between
the noise and signal.
•
Balanced circuits (such as
twisted pair) don’t break the
coupling, but instead take
advantage of the fact that
the RFI will be coupled into
both conductors (signal and
return). This noise (called
Common Mode noise) is then
subtracted, while the signal
is retained. In effect, the bal-
anced circuit creates a high
impedance for the coupled
noise.
•
Another example of the
high-impedance-to-noise
approach is the use of RF
chokes. Whether used with
data or power cables, RF
chokes can offer effective
high-frequency impedance
(X
L
increases with frequency).
•
A low-impedance path can
be used to shunt away the
noise. This is the principle
behind filtering and the use
of decoupling caps (low im-
pedance to high frequency,
but open at power line fre-
quencies). But a sometimes-
overlooked, yet critical, as-
pect is that the ground path
and plane must be capable
of handling high-frequency
currents. High-frequency
grounding techniques are
used to accomplish this. The
SRG (Signal Reference Grid),
first developed for raised
floor computer room installa-
tions, is an effective solution.
It is essentially an equipo-
tential ground plane at high-
frequency. (For further infor-
mation on high-frequency
grounding, see the refer-
ences listed on the back page.)
Signal Grounding
To understand the importance of
“clean” signal grounds, let’s dis-
cuss the distinction between
Differential Mode (DM) vs. Com-
mon Mode (CM) signals. Imag-
ine a basic two-wire circuit:
supply and return. Any current
that circulates or any voltage
read across a load between
the two wires is called DM
(the terms normal mode, trans-
verse mode and signal mode
are also used). The DM signal is
typically the desired signal (just
like 120V at a receptacle).
Imagine a third conductor, typi-
cally a grounding conductor.
Any current that flows now
through the two original con-
ductors and returns on this third
conductor is common to both of
the original conductors. The CM
current is the noise that the
genuine signal has to overcome.
CM is all that extra traffic on the
highway. It could have gotten
there through any of the cou-
pling mechanisms, such as
magnetic field coupling at
power line frequency or RFI at
higher frequencies. The point is
to control or minimize these
ground or CM currents, to make
life easier for the DM currents.
Measurement
CM currents can be measured
with current clamps using the
zero-sequence technique. The
clamp circles the signal pair (or,
in a three-phase circuit, all
three-phase conductors and the
neutral, if any). If signal and re-
turn current are equal, their
equal and opposite magnetic
fields cancel. Any current read
must be common mode; in other
words, any current read is cur-
rent that is not returning on the
signal wires, but via a ground
path. This technique applies to
signal as well as power conduc-
tors. For fundamental currents, a
ClampMeter or DMM + clamp
would suffice, but for higher fre-
quencies, a high bandwidth in-
strument like the Fluke 43
Power Quality Analyzer or
ScopeMeter should be used
with a clamp accessory.
A Matter of Life and Death
Sometimes PQ troubleshooting
is a matter of life and death.
Dave was the on-site field
engineer at the hospital. One
day he got a call from a very
concerned nurse in the ER. One
of their patients had died. But
as upsetting as that was, it
wasn’t the main source of con-
cern. What was really unusual
was that this particular corpse
had a heartbeat.
Dave soon arrived at the
scene. A quick glance told him
that the dead had not come
back to life. The problem lay
elsewhere. The nurses pointed
out what they had seen, a sig-
nal on the EKG indicating a
heartbeat. But there was some-
thing unusual about this signal
(above and beyond the fact that
it seemed to be coming from a
dead body). He noticed that the
signal was a 60 Hz sine wave
(slightly flat-topped). A further
look at the signal wires told him
that they had been laid parallel
to the power cord. The coupling
between signal and power
wires caused the 60 Hz “Heart-
beat” on the EKG machine.
The moral of the story is to
always isolate the signal and
power conductors—before it
becomes a matter of life
and death.