Application Note

Power Quality Troubleshooting Fluke Corporation 17
•
A transient surge, especially
if it occurs on a high-energy
circuit, causes a very fast
change in current which can
couple into an adjacent con-
ductor. Lightning surges are a
worst case, but common
switching transients or arc-
ing can do the same thing.
•
If feeder cables are posi-
tioned such that there is a
net magnetic field, then cur-
rents can be induced into
ground cables that share the
raceway.
•
It is well known that signal
wires and power conductors
should not be laid parallel to
each other in the same race-
way, which would maximize
their inductive coupling, but
instead be separated and
crossed at right angles when
necessary. Input and output
cables should also be isolated
from each other in the same
manner.
Magnetic fields are isolated by
effective shielding. The material
used must be capable of con-
ducting magnetic fields (ferrous
material as opposed to copper).
The reason that a dedicated cir-
cuit (hot, neutral, ground)
should be run in its own metal
conduit when possible is that is
in effect magnetically shielded
to minimize inductive coupling
effects.
Both inductive and capacitive
coupling are referred to as near
field effects, since they domi-
nate at short distances and dis-
tance decreases their coupling
effects. This helps explain one
of the mysteries of noise—how
slight physical repositioning of
wiring can have such major
effects on coupled noise.
3. Conducted noise
While all coupled noise ends up
as conducted noise, this term is
generally used to refer to noise
that is coupled by a direct, gal-
vanic (metallic) connection. In-
cluded in this category are
circuits that have shared con-
ductors (such as shared neutrals
or grounds). Conducted noise
could be high frequency, but
may also be 60 Hz.
These are some common
examples of connections that
put objectionable noise currents
directly onto the ground:
•
Sub-panels with extra N-G
bonds
•
Receptacles miswired with
N and G switched
•
Equipment with internal solid
state protective devices that
have shorted from line
or neutral to ground, or that
have not failed but have nor-
mal leakage current. This
leakage current is limited by
UL to 3.5 mA for plug-con-
nected equipment, but there
is no limit for permanently
wired equipment with poten-
tially much higher leakage
currents. (Leakage currents
are easy to identify because
they will disappear when the
device is turned off).
•
Another common example is
the so-called isolated ground
rod. When it is at a different
earth potential than the
source grounding electrode,
a ground loop current occurs.
This is still conducted noise,
even though the direct con-
nection is through the earth.
•
Datacom connections that
provide a metallic path from
one terminal to another can
also conduct noise. In the
case of single-ended, unbal-
anced connections (RS-232),
the connection to terminal
ground is made at each end
of the cable. This offers a
path for ground currents if
the equipment at each end
is referenced to a different
power source with a different
ground.
4. RFI (Radio Frequency
Interference)
RFI ranges from 10 kHz to the
10s of MHz (and higher). At
these frequencies, lengths of
wire start acting like transmit-
ting and receiving antennas.
The culprit circuit acts as a
transmitter and the victim circuit
is acting as a receiving antenna.
RFI, like the other coupling
mechanisms, is a fact of life, but
it can be controlled (not without
some thought and effort, how-
ever).
Figure 4.2 Noise coupling. Ground noise measured as ø-G or N-G noise.
Ø - Ø
Ø - G N - G
Noise Coupling
Ground
ØA
ØB
N
ØC