Application Note

Electrical noise is the result ofmore or less random electricalsignals getting coupled into cir-cuits where they are unwanted,i.e., where they disrupt informa-tion-carrying signals. Noiseoccurs on both power and signalcircuits, but generally speaking, itbecomes a problem when it getson signal circuits. Signal and datacircuits are particularly vulnerableto noise because they operate atfast speeds and with low voltagelevels. The lower the signal volt-age, the less the amplitude of thenoise voltage that can be toler-ated. The signal-to-noise ratiodescribes how much noise a cir-cuit can tolerate before the validinformation, the signal, becomescorrupted.

Noise is one of the more mysterious subjects in powerquality, especially since it mustbe considered with its equallymysterious twin, grounding. Tolessen the mystery, there are twokey concepts to understand:

• The first is that electricaleffects do not require directconnection (such as throughcopper conductors) to occur.For an electrician who’s beentrained to size, install and testwiring, this may not be intu-itive. Yet think of lightning, orof the primary and secondaryof an isolation transformer, orof the antenna to your radio:there’s no direct, hard-wiredconnection, but somehowcomplete electrical circuits arestill happening. The sameelectrical rules-of-behavior arein operation for noise coupling,as will be explained below.

• The second concept is that wecan no longer stay in therealm of 60 Hz. One of thebenefits of 60 Hz is that it’s alow enough frequency thatpower circuits can be treated(almost) like dc circuits; inother words, basic Ohm’s Lawwill get you most places youneed to go. But when it comes

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Electrical noiseand transients

to noise, we need to keep inmind that signal circuits occurat high frequencies, that noiseis typically a broad spectrum offrequencies, and that we needto consider the frequency-dependent behavior ofpotential sources of noise.

Coupling mechanismsThere are four basic mechanismsof noise coupling. It pays tounderstand them and how theydiffer one from the other becausea lot of the troubleshooter’s jobwill be to identify which couplingeffect is dominant in a particularsituation.

1. Capacitive couplingThis is often referred to as electrostatic noise and is a volt-age-based effect. Lightningdischarge is just an extremeexample. Any conductors sepa-rated by an insulating material(including air) constitute a capac-itor—in other words, capacitanceis an inseparable part of any cir-cuit. The potential for capacitivecoupling increases as frequencyincreases (capacitive reactance,which can be thought of as theresistance to capacitive coupling,decreases with frequency, as can be seen in the formula: XC = 1/2πfC).

2. Inductive couplingThis is magnetic-coupled noiseand is a current-based effect.Every conductor with currentflowing through it has an associ-ated magnetic field. A changingcurrent can induce current inanother circuit, even if that circuitis a single loop; in other words,

20 - 30 Vlogic signal

3 - 5 Vlogic signal

Noise

Noise

Figure 1. Lower voltage, faster signals increase sensitivity to noise.

2 Fluke Corporation Electrical noise and transients

the source circuit acts as a trans-former primary with the victimcircuit being the secondary. Theinductive coupling effectincreases with the following fac-tors: (1) larger current flow, (2)faster rate of change of current,(3) proximity of the two conduc-tors (primary and secondary) and(4) the more the adjacent con-ductor resembles a coil (rounddiameter as opposed to flat, orcoiled as opposed to straight).

Here are some examples ofhow inductive coupling can causenoise in power circuits:• A transient surge, especially if

it occurs on a high-energy cir-cuit, causes a very fast changein current which can coupleinto an adjacent conductor.Lightning surges are a worstcase, but common switchingtransients or arcing can do thesame thing.

• If feeder cables are positionedsuch that there is a net mag-netic field, then currents canbe induced into ground cablesthat share the raceway.

• It is well known that signalwires and power conductorsshould not be laid parallel toeach other in the same race-way, which would maximizetheir inductive coupling, butinstead be separated andcrossed at right angles when

necessary. Input and outputcables should also be isolatedfrom each other in the samemanner.Magnetic fields are isolated by

effective shielding. The materialused must be capable of conduct-ing magnetic fields (ferrousmaterial as opposed to copper).The reason that a dedicated cir-cuit (hot, neutral, ground) shouldbe run in its own metal conduitwhen possible is that is in effectmagnetically shielded to mini-mize inductive coupling effects.

Both inductive and capacitivecoupling are referred to as nearfield effects, since they dominateat short distances and distancedecreases their coupling effects.This helps explain one of themysteries of noise—how slightphysical repositioning of wiringcan have such major effects oncoupled noise.

3. Conducted noiseWhile all coupled noise ends upas conducted noise, this term isgenerally used to refer to noisecoupled by a direct, galvanic(metallic) connection. Included inthis category are circuits thathave shared conductors (such asshared neutrals or grounds).Conducted noise could be highfrequency, but may also be 60 Hz.

Common examples of connec-tions that put objectionable noisecurrents 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 thathave shorted from line or neu-tral to ground, or that have notfailed but have normal leakagecurrent. This leakage current islimited by UL to 3.5 mA forplug-connected equipment, butthere is no limit for perma-nently wired equipment withpotentially much higher leak-age currents. (Leakage currentsare easy to identify becausethey will disappear when thedevice is turned off).

• Another common example isthe so-called isolated groundrod. When it is at a differentearth potential than the sourcegrounding electrode, a groundloop current occurs. This is stillconducted noise, even thoughthe direct connection isthrough the earth.

• Datacom connections that provide a metallic path fromone terminal to another canalso conduct noise. In the caseof single-ended, unbalancedconnections (RS-232), the con-nection to terminal ground ismade at each end of the cable.This offers a path for groundcurrents if the equipment ateach end is referenced to adifferent power source with adifferent ground.

4. RFI (Radio FrequencyInterference)

RFI ranges from 10 kHz to the 10 s of MHz (and higher). Atthese frequencies, lengths of wirestart acting like transmitting andreceiving antennas. The culpritcircuit acts as a transmitter andthe victim circuit is acting as areceiving antenna. RFI, like theother coupling mechanisms, is afact of life, but it can be con-trolled (not without some thoughtand effort, however).

Ø - Ø

Ø - G N - GNoise Coupling

Ground

ØA

ØB

N

ØC

Figure 2. Noise coupling. Ground noise measured as ø-G or N-G noise.

3 Fluke Corporation Electrical noise and transients

RFI noise reduction employs anumber of strategies:• Fiber optic cable, of course, is

immune to electrical noise.• Shielded cabling (such as coax

cables) attempts to break thecoupling between the noiseand signal.

• Balanced circuits (such astwisted pair) don’t break thecoupling, but instead takeadvantage of the fact that theRFI will be coupled into bothconductors (signal and return).This noise (called CommonMode noise) is then subtracted,while the signal is retained. Ineffect, the balanced circuit cre-ates a high impedance for thecoupled noise.

• Another example of the high-impedance-to-noise approachis the use of RF chokes.Whether used with data orpower cables, RF chokes canoffer effective high-frequencyimpedance (XL increases withfrequency).

• A low-impedance path can beused to shunt away the noise.This is the principle behind filtering and the use of decou-pling caps (low impedance tohigh frequency, but open atpower line frequencies). But asometimes-overlooked, yetcritical, aspect is that theground path and plane mustbe capable of handling high-frequency currents. High-frequency groundingtechniques are used to accom-plish this. The SRG (SignalReference Grid), first devel-oped for raised floor computerroom installations, is an effec-tive solution. It is essentiallyan equipotential ground planeat high- frequency. (For furtherinformation on high-frequencygrounding, see the referenceslisted on the back page.)

Signal groundingTo understand the importance of “clean” signal grounds, let’sdiscuss the distinction betweenDifferential Mode (DM) vs.Common Mode (CM) signals.Imagine a basic two-wire circuit:supply and return. Any currentthat circulates or any voltageread across a load between thetwo wires is called DM (the termsnormal mode, transverse modeand signal mode are also used).The DM signal is typically thedesired signal (just like 120V at areceptacle). Imagine a third conductor, typically a groundingconductor. Any current that flowsnow through the two originalconductors and returns on thisthird conductor is common toboth of the original conductors.The CM current is the noise thatthe genuine signal has to over-come. CM is all that extra trafficon the highway. It could havegotten there through any of thecoupling mechanisms, such asmagnetic field coupling at powerline frequency or RFI at higherfrequencies. The point is to con-trol or minimize these ground orCM currents, to make life easierfor the DM currents.

MeasurementCM currents can be measuredwith current clamps using thezero-sequence technique. Theclamp circles the signal pair (or,in a three-phase circuit, allthree-phase conductors and theneutral, if any). If signal andreturn current are equal, theirequal and opposite magneticfields cancel. Any current readmust be common mode; in otherwords, any current read is cur-rent that is not returning on thesignal wires, but via a groundpath. This technique applies tosignal as well as power conduc-tors. For fundamental currents, aClampMeter or DMM + clampwould suffice, but for higher frequencies, a high bandwidthinstrument like the Fluke 43Power Quality Analyzer orScopeMeter should be used witha clamp accessory.

A Matter of Life and DeathSometimes PQ troubleshooting isa matter of life and death.

Dave was the on-site fieldengineer at the hospital. Oneday he got a call from a veryconcerned nurse in the ER. Oneof their patients had died. But asupsetting as that was, it wasn’tthe main source of concern.What was really unusual wasthat this particular corpse had aheartbeat.

Dave soon arrived at thescene. A quick glance told himthat the dead had not come backto life. The problem lay else-where. The nurses pointed outwhat they had seen, a signal onthe EKG indicating a heartbeat.But there was somethingunusual about this signal (aboveand beyond the fact that itseemed to be coming from adead body). He noticed that thesignal was a 60 Hz sine wave

(slightly flat-topped). A furtherlook at the signal wires told himthat they had been laid parallelto the power cord. The couplingbetween signal and power wirescaused the 60 Hz “Heartbeat” onthe EKG machine. The moral ofthe story is to always isolate thesignal and power conductors—before it becomes a matter of lifeand death.

4 Fluke Corporation Electrical noise and transients

Transients

Transients should be distin-guished from surges. Surges are aspecial case of high-energy tran-sient which result from lightningstrikes. Voltage transients arelower energy events, typicallycaused by equipment switching.

They are harmful in a numberof ways:• They deteriorate solid state

components. Sometimes a sin-gle high energy transient willpuncture a solid state junction,sometimes repetitive lowenergy transients will accom-plish the same thing. Forexample, transients whichexceed the PIV (peak inversevoltage) rating of diodes are acommon cause of diode failure.

• Their high-frequency compo-nent (fast rise times) causethem to be capacitively cou-pled into adjoining conductors.If those conductors are carry-ing digital logic, that logic willget trashed. Transients alsocouple across transformerwindings unless specialshielding is provided.Fortunately this same high frequency component causestransients to be relativelylocalized, since they aredamped (attenuated) by theimpedance of the conductors(inductive reactance increaseswith frequency).

• Utility capacitor switchingtransients are an example of a commonly-occurring high-energy transient (still by nomeans in the class of light-ning) that can affect loads atall levels of the distributionsystem. They are a wellknown cause of nuisance tripping of ASDs: they haveenough energy to drive a transient current into the dclink of the drive and cause anovervoltage trip.Transients can be categorized

by waveform. The first categoryis “impulsive” transients, com-monly called “spikes,” because ahigh-frequency spike protrudesfrom the waveform. The capswitching transient, on the otherhand, is an “oscillatory” transientbecause a ringing waveform rideson and distorts the normal wave-form. It is lower frequency, buthigher energy.

CausesTransients are unavoidable. Theyare created by the fast switchingof relatively high currents. Forexample, an inductive load like amotor will create a kickbackspike when it is turned off. Infact, removing a Wiggy (a sole-noid voltage tester) from ahigh-energy circuit can create aspike of thousands of volts! A

capacitor, on the other hand, cre-ates a momentary short circuitwhen it’s turned on. After thissudden collapse of the appliedvoltage, the voltage rebounds andan oscillating wave occurs. Notall transients are the same, but asa general statement, load switch-ing causes transients.

In offices, the laser copier/printer is a well-recognized “badguy” on the office branch circuit.It requires an internal heater tokick in whenever it is used andevery 30 seconds or so when it isnot used. This constant switchinghas two effects: the current surgeor inrush can cause repetitivevoltage sags; the rapid changesin current also generate tran-sients that can affect other loadson the same branch.

Measurement and recordingTransients can be captured byDSOs (Digital Storage Oscillo-scopes). The Fluke 43 PQAnalyzer, which includes DSOfunctions, has the ability to cap-ture, store and subsequentlydisplay up to 40 transient wave-forms. Events are tagged withtime and date stamps (real timestamps). The VR101S VoltageEvent Recorder will also capturetransients at the receptacle. Peakvoltage and real time stamps areprovided.

Figure 3. Fluke 43B can capture and save upto 40 transients.

Cursor moves todisplay peakMin/Max values.

Real-time stamp.Date:hr:min:sec

5 Fluke Corporation Electrical noise and transients

Voltage susceptibility profileThe new ITIC profile (InformationTechnology Industry Council) isbased on extensive research andupdates the CBEMA curve. TheCBEMA curve (Computer BusinessEquipment ManufacturersAssociation, now ITIC) was theoriginal voltage susceptibilityprofile for manufacturers of com-puters and other sensitiveequipment. Similar curves are

Transient voltage surge suppressors (TVSS)Fortunately, transient protectionis not expensive. Virtually allelectronic equipment has (orshould have) some level of pro-tection built in. One commonly-used protective component is theMOV (metal oxide varistor) whichclips the excess voltage.

TVSS are applied to provideadditional transient protection.TVSS are low voltage (600 V)

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For more information call:In the U.S.A. (800) 443-5853 or Fax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 or Fax (31 40) 2 675 222In Canada (800) 36-FLUKE or Fax (905) 890-6866From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116Web access: http://www.fluke.com

©2004 Fluke Corporation. All rights reserved.Printed in U.S.A. 10/2004 2403183 A-US-N Rev A

Fluke.Keeping your worldup and running.

being developed for 230 V/50 Hzequipment and for adjustablespeed drives. Sensitive equip-ment should be able to surviveevents inside the curve. Eventsoutside of the curve could requireadditional power conditioningequipment or other remedialaction. A major change in ITIC isthat the ride-through times foroutages as well as the tolerancefor sags have both been

Figure 4. ITIC Curve.

500

400

300

200

140

120110100908070

40

0

Applicable to 120, 120/208, and120/240 nominal voltages

110 V

90 V

1 µs 1 ms 3 ms 20 ms 0.5 s 10 s Steadystate

0.001 c 0.01 c 0.1 c 0.5 c 1 c 10 c 100 c 1000 c

Duration of disturbance in cycles (c) and seconds (s)

Perc

ent

of n

omin

al v

olta

ge (R

MS

or p

eak

equi

vale

nt)

Voltage-Toleranceenvelope

devices and are tested and certi-fied to UL 1449. UL 1449 ratesTVSS devices by Grade, Classand Mode. As an example, thehighest rating for a TVSS wouldbe Grade A (6000 V, 3000 A),Class 1 (let-through voltage of330 V max) and Mode 1 (L-Nsuppression). The proper ratingshould be chosen based on theload’s protection needs:• A lower Grade might result in

a TVSS that lasts one year

instead of ten years. The solidstate components in a TVSSwill themselves deteriorate asthey keep on taking hits fromtransients.

• A lower Class might permit toomuch let-through voltage thatcould damage the load. Class 1is recommended for switchmode power supplies.

• A Mode 2 device would passtransients to ground, wherethey could disrupt electroniccircuit operation.

increased. The field trou-bleshooter must keep in mindthat the profiles are recommen-dations and that a particularpiece of equipment may or maynot match the profile. Having saidthat, the profiles are still usefulbecause, when recorded eventsare plotted against them, theygive a general idea of the voltagequality at a particular site.