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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.