Application Note

IntroductionEver measured an ac signal ona branch circuit that you knowto be dead or even discon-nected completely? It can be areal head scratcher. How can awire, with no connection ateither end, have a voltage onit? What you’re experiencing isa phenomenon aptly termed“ghost voltage.” While itsappearance may seem an inde-cipherable mystery, it usuallycan be solved without toomuch trouble.

The cluesDoes the wire, somewherealong its length, share a runwith many other wires? It’s not uncommon practice forelectrical contractors to thinkahead for their customers andpull extra circuits for possiblefuture expansion of the electri-cal system. Or you may havejust pulled a new wire(s) in arun with other wires. If so, weknow that our open wire isrunning alongside other circuitsthat are connected to the elec-trical system and are providingcurrent to real loads such aslights, motors, computers andheaters.

Two metal plates (the conductors) separated by theconductor’s insulation and airwould appear to fit the defini-tion of a capacitor. We knowfrom basic electrical theory thatac signals run across a capaci-tor are coupled while dc volt-ages are blocked. The amount

of ac signal passed through acapacitor is determined by itscapacitance and that capaci-tance is based on three factors:

Ghost voltages – phantomreadings can lead to thewrong diagnosis

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

1. The surface area of theplates

2. The distance between theplates

3. The make up of the insula-tion separating the plates.

ON

OFF

ON

OFF

?

Is the 60 Vreading real or ghost voltage

OffOn

Disconnect A Disconnect B

Motor A

Motor B

50 - 75 feetof conduit

2 Fluke Corporation Ghost voltages—phantom readings can lead to the wrong diagnosis

Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BD Eindhoven, The Netherlands

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

©2003 Fluke Corporation. All rights reserved.Printed in U.S.A. 8/2003 2105317 A-ENG-N Rev A

Fluke.Keeping your worldup and running.

In our ghost voltage case,the surface area of the plates isdetermined by the size of thewire and the length of the twoconductors that are laying side-by-side. The longer the run,the larger the surface area andtherefore, the higher the capac-itance. Increasing the numberof wires in a conduit will causethe wires to be packed closertogether, reducing the distancebetween conductors andincreasing capacitance. Theinsulation around the conduc-tors will stay pretty constantand won’t change the capaci-tance. However, external factors will change the effec-tive insulation between theconductors. For instance, whilepulling the bundle of wiresthrough a conduit, the insula-tion may be compressed,reducing distance betweenconductors. In addition, watermay get into a conduit andeffectively increase the capaci-tance as well. Another externalfactor may be carbon betweenconductors where an arc tookplace some time in the past. So in an electrical situation, theamount of ac signal coupledinto the disconnected wire isbased on a lot of factors.

But wait! This wire is openon both ends! Isn’t Ohms Lawstill working in this situation?To see voltage, don’t we needresistance and current flow? Of course you do! This is wherewe need to look at another part of the puzzle: The digitalmultimeter (DMM) making thevoltage measurement.

When you place your DMMleads between the open circuitand the neutral conductor, youeffectively complete the circuitthrough the input of the DMM.The capacitance between theconnected and live hot conduc-tor and the floating conductorforms a voltage divider in con-junction with the DMM inputimpedance. The DMM thenmeasures and displays theresulting voltage value. Withmost DMMs, like the Fluke 170and 180 Series, the inputimpedance is very high — onthe order of 10 megohms. Thisresults in a false reading on theDMM, which is actually meas-uring voltage coupled into thedisconnected conductor.

In most cases the amount ofac coupling between conduc-tors is very low, so the amountof current that can be pulledthrough this coupling is limited.However, these voltages, attimes, can be significantenough to cause confusionwhen we are troubleshootingcircuit problems.

The solutionTo reduce the confusion, andsave time troubleshooting anelectrical system, use a DMMwith a low input impedance.Fluke Models 7-600 and 12(with VCheck) are designed torespond differently to “ghostvoltages” than normal voltages.

The DMM’s input impedancevaries depending on what theinput circuitry senses. Initially,the input impedance is on theorder of two kilohms. So, whenthe leads are placed on theopen circuit that contains aghost voltage, the low inputimpedance will cause the ghostvoltage to dissipate and themeter will display “OL” with theΩ symbol indicating the meteris still in the resistance functionwith no voltage present. Whenthe leads are placed on a livecircuit however, the inputsenses the presence of a “hard”voltage and automaticallyadjusts the input impedance toa much higher value and thendisplays the actual voltagepresent. This variable imped-ance design effectively elimi-nates reading ghost voltages onopen circuits while still makingaccurate measurements on livepower circuits without havingto change meter settings.

So remember to put voltagereadings in perspective whentroubleshooting. If you feelthere shouldn’t be any voltagepresent, but there is, check forthe conditions listed in thisarticle. The voltage is not anapparition, just a result of thephysics of electricity.