Grounding and Interfacing 1215cord-connected devices divert additional 60 Hz and
high-frequency noise currents into the safety ground
system, they often aggravate the very problem they
claim to solve. External, cord-connected filters, or those
built into outlet strips, can serve to band-aid badly
designed equipment. As shown in Fig. 32-24 (Section
32.4.2), some equipment is sensitive because
common-mode power line disturbances, especially at
high frequencies, have essentially been invited in to
invade the signal circuitry!Second, the advertised noise attenuation figures for
virtually all these power line devices are obtained in a
most unrealistic way. Measurements are made with all
equipment (generator, detector, and device under test)
mounted to a large metal ground plane. Although the
resulting specs are impressive, they simply don’t apply
to performance in real-world systems where ground
connections are made with mere wires or conduit.
However, these devices can be very effective when
installed at the power service entrance or a subpanel,
where all system safety grounds are bonded to a
common reference point.^55 For thorough, accurate infor-
mation about separately derived power distribution and
its application to equipment racks, the author highly
recommends reference 60.
Balanced power, more properly symmetrical power,
is another seductively appealing concept shown in Fig.
32-59. If we assumed that each system box had neatly
matched parasitic capacitances from each leg of the
power line to its chassis ground, the resulting noise
current flow into the safety ground system would be
zero, the interchassis voltage would be zero, and the
resulting system noise due to these currents would
simply disappear! For example, if C 1 and C 2 had equal
capacitance and the ac voltages across them were equal
magnitude but opposite polarity, the net leakage current
would indeed be zero. However, for the overwhelming
majority of equipment, these capacitances are not equal
or even close. In many cases, one is several times as
large as the other—it’s just a reality of power trans-
former construction. Even if the equipment involved has
two-prong ac power connections, actual noise reduction
will likely be less than 10 dB and rarely exceed 15 dB.
And it’s unlikely that equipment manufacturers will
ever pay the premium to match transformer parasitic
capacitances or use precision capacitors in power line
EMI filters. If the equipment involved has three-prong
(grounding) ac power connections, the leakage current
reduction, if any, provided by symmetrical power will
pale by comparison to the magnetically induced voltage
differences described in Section 32.3.4. In fact, many of
the benefits attributed to symmetrical power may result
from simply plugging all system equipment into the
same outlet strip or dedicated branch circuit—which is
always a good idea.
A GFCI (ground-fault circuit interrupter) works by
sensing the difference in current between the hot and
neutral connections at an outlet. This difference repre-
sents current from the hot conductor that is not
returning via neutral. The worst-case scenario assumes
that the missing current is flowing through a person.
When the difference current reaches 4–7 mA—
producing a very unpleasant but non-life-threatening
shock—an internal circuit breaker removes power in a
fraction of a second. Some power conditioners feature a
ground lift switch, touted to eliminate ground loop
problems, at their outputs. The National Electrical Code
requires that all balanced power units have
GFCI-protected outputs (among other restrictions on the
use of balanced power). Although safe, ground lifting
makes a GFCI-protected circuit prone to nuisance trips.
For example, consider the system hook-up shown in
Fig. 32-60.Figure 32-58. Power isolation transformer.Faraday shieldSafety groundLine
inputPri SecLine
outputFigure 32-59. Balanced power hopes to cancel ground currents.Line120 V0 V60 V60 VNeutral
Safety groundBox A Box BC 2C 1C 4C 3Inter-chassis
voltage