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Clearing Up Neutral-to-Ground Voltage Confusion
Because effects from N-G voltage can range from nonexistent to significant, youmust learn to identify true common-mode events
Feb 1, 2007Tom Shaughnessy, o!erC"T| Electrical Construction and Maintenance
Power quality questions continue to revolve around one underlying issue related to
electronic equipment: its ability to withstand the effects of electrical interference. If
equipment sensitivity was always well known and defined, then we would have few,
if any, doubts. In this perfect world, we would also know with a high degree of
certainty that a voltage sag of a known amplitude and duration would have either
no effect or a significant impact on equipment. Unfortunately, we seldom are privy
to such information. Therefore, the possible effects of neutraltoground !"#$
voltage are often left up in the air.
%hen you measure "# voltage, the measurement yields a simple voltage
differential, which a voltage potential on either the neutral conductor or grounding
conductor may create. &urthermore, this differential may be a simple byproduct of
neutral return current ' or may even be part of a comple( commonmode voltage
signal. The effects of these conditions vary greatly.
The simple question ' )%hat is the effect of "# voltage*+ ' isnt so simple
because it depends upon the magnitude, mode of propagation, timing,
energy-frequency content, and sensitivity of the equipment involved. ets try to
resolve this important and confusing question.
Voltage drop and the NEC
#$%&ig' ('#)$% Basic single-phase circuit and load' *s the load dra!s current, a voltage drop developsacross the supply and return conductors' N-G voltage measurements at the load !ill reflect the voltage
drop across the return +neutral conductor'
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Figure 1shows a simple diagram of a singlephase load connected to a voltage
source. /s the load draws current, a voltage drop develops across the supply and
return conductors. "# voltage measurements at the load will reflect the voltage
drop across the return !neutral$ conductor.
The "ational 0lectrical 1ode, in 2ec. 345.46!/$, &P" "o.7, states: )1onductors for
branch circuits as defined in /rt. 455, si8ed to prevent a voltage drop e(ceeding 9
at the farthest outlet of power, heating, and lighting loads, or combination of such
loads, and where the ma(imum total voltage drop on both feeders and branch
circuits to the farthest outlet does not e(ceed ;, provide reasonable efficiency of
operation.+ This amounts to a 5s, some equipment manufacturers installed power supply
and motherboard grounding in configurations that made them e(tremely
susceptible to earth-groundreferenced offset. In response, a few surge suppressor
manufacturers introduced T=22 products with "# components and e(tremely low
transient voltage clamping levels ' with disastrous consequences in certain cases.
?owever, over time those design deficiencies were corrected ' presentday test
requirements usually prevent the widescale introduction of such products.
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Figure 2 shows the basic diagram of a power supply system. %hat possible effect
could neutral return losses have upon a system with this configuration* /fter all,
there are no groundreferenced components on the input to the power supply that a
voltage potential on the neutral conductor could upset. In fact, U power supply
tests reverse the polarity of voltages applied to a power supply. 1onsequently, the
power supply must withstand 435= with respect to earth-ground for both normal
and reverse polarities.
The voltage sensing and feedback circuitry also must meet electrical isolation
requirements for safety purposes. The bond between the grounding of the system
and the electronics occurs on the secondary of the highfrequency transformer
inside the power supply or system. If the system is well designed, the effects of low
frequency voltage potentials appearing on the neutral conductor should have no
adverse effects. In fact, if a power supply has a switched input capability !e.g.,
>;=/1 to 3
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%hile the industry has focused much attention on power frequency voltages
measured with a voltmeter and involving "# voltages, a much larger problem
arises when you consider higher frequency "# voltages that require measurement
with better instrumentation. This poses realistic performance challenges for
electronic equipment.
#$%&ig' '#)$% "xample of true common-mode interference recorded !ith a po!er monitor'
Figure !shows an e(ample of true commonmode interference, which was
recorded with a power monitor. The red trace is linetoground !#$, and the blue
trace is "#. The earth-groundreferenced potential is common to both current
carrying conductors, and the only path for this interference is through
earth-groundreferenced circuitry within equipment powered from that circuit. The
commonmode potential is only about ;5= to B5=, but the frequency content of this
potential is fairly high !appro(imately 35k?8$. The negative effects of these
interference signals can range from power supply reset to damaged I-C ports !A2
393$.
Figure "shows another e(ample of a form of true commonmode interference
signal. ?ere, Power10T recorded " and "# voltages with a line decoupler and
recorded the resulting current signals in 0thernet cabling with 1hannels 9 and 7
with highfrequency current probes. The commonmode interference signal drives
interference currents through the 0thernet and associated intersystem cabling.
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#$%&ig' .'#)$% True common-mode interference affecting system net!or/s' This signal drives
interference currents through the "thernet and associated intersystem ca$ling systems'
If the staff was to measure only from neutral to ground, then the true common
mode nature of these signals would not be apparent. &or instance, referring back to
&ig. 4, the impedance of the neutral conductor will support impulse propagation as
loads cycle on and off. The resulting transient voltages, however, are developed
from a relatively high impedance. Therefore, their potential to wreak havoc is
limited. In comparison, commonmode interference signals, as shown in &igs. 9 and
7, not only have more available paths through the system, but their energy and
frequency content may be higher.
#$%&ig' 0'#)$% Ground current induced voltage' Current surging through the grounding system of afacility caused this event'
%hat about current flowing through earth-ground that is measured as an "#
signal*Figure #shows an event caused by current surging through the groundingsystem of a facility. The "# voltage waveform truncates at 75= peak because the
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waveform e(ceeded the input range of the digital storage oscilloscope. ?owever,
you can follow the slope of the lines and e(trapolate that the peak voltage easily
reached and probably e(ceeded 455=D This event caused hard drive failures and
data loss.
$ppling what we%ve learned
/s many power monitor setups do not use # connections along with " and "#
connections, you must ferret out true commonmode events. %ith sole usage of "
# connections to detect commonmode events, you may develop a tendency to
ignore low amplitude "# transients. /fter all, if recorded transients are always
present at a given level and you can determine no adverse effects, why waste
monitor memory recording those events* Increasing the monitor threshold to avoid
capturing lower level and commonly occurring "# events may leave the truecommonmode events undetected.
%e can sum up our e(periences as follows:
"# voltages less than 9= and developed at power frequencies seldomcause
adverse effects.
ow level "# transients less than 3;= peak and caused by load cycling
usually donot
cause adverse effects. ?owever, the potential for adverse effectwill increaseas frequency content and amplitude increases.
?igher frequency, true commonmode events cancause adverse effects, but
you may not be able to detect or correctly identify their presence.
Eeasuring "# voltages with a multimeter is a valid procedure, and the
measurements you make may help identify wiring problems that cause e(cessive
voltage drop. Aemember, high levels of "# voltage invariably arise from
grounding-bonding problems.