Application Note

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Application Note

New Fluke 117 Digital

Multimeter drives the

ghosts out of the attic

(and the electric circuits)

the wet, dirty plant environ-

ment, the potential problems

were many. “It should have

been either 110 volts, or

nothing.”

Hunting down the ghosts

In the past, tracking down such

a voltage indication could send

the technician off on a time-

wasting wild goose chase, look-

ing for a problem connection

that was never there. He could

ferret out the ghost voltage with

a low-impedance analog meter

or a solenoid tester, or ‘wiggy’ -

but that would require packing

or fetching an extra tool.

As an electrician, Eldon

Walstad’s job can be like that

of a radar operator when it

comes to detecting the invisible.

He has to be able to determine

what is real and what is not,

and separate what is dangerous

from what only appears to be.

Now Walstad has a new tool

that can help him make those

critical decisions—the Fluke 117

Digital Multimeter (DMM).

In an average week Wals-

tad, a journeyman electrician

with Rodgers Electric in Everett,

WA., handles all kinds of issues

related to electrical installa-

tion, maintenance and repair. A

frequent challenge is to deter-

mine why a device isn’t operat-

ing, and track down the faulty

contact, fuse, relay or broken

wire that’s preventing current

from flowing.

But occasionally a digital

multimeter will show voltage

present, even on a circuit that

should not be energized. Is it

real and potentially danger-

ous voltage, or merely “ghost

voltage” caused by capacitive

coupling between energized

wiring and adjacent unused

wiring?

“The other day I had a situ-

ation where I had 73 volts

showing up on a circuit,” says

Walstad, a second-generation

electrician with more than 30

years experience. The circuit in

question: a control circuit on a

concrete transfer cart used to

move material from the mixer to

the molds in a factory making

precast concrete light poles. In

Testing

Functions

Case

Study

Profile: Eldon Walstad,

Rodgers Electric

Tool: Fluke 117 Electrician’s

Multimeter

Measurements: High impedance for

troubleshooting sensitive electronic or

control circuits, and low impedance for

detecting ghost voltages

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

Summary of content (3 pages)

PAGE 1
New Fluke 117 Digital Multimeter drives the ghosts out of the attic (and the electric circuits) Testing Functions Case Study Tool: Fluke 117 Electrician’s Multimeter Profile: Eldon Walstad, Rodgers Electric Measurements: High impedance for troubleshooting sensitive electronic or control circuits, and low impedance for detecting ghost voltages As an electrician, Eldon Walstad’s job can be like that of a radar operator when it comes to detecting the invisible.
PAGE 2
But Walstad had extra help. The Fluke 117 Digital Multimeter he was using that day has dual impedance capability; incorporating both regular high impedance test capabilities and low impedance functions for detecting ghost voltages. By switching to the meter’s Auto-V/LoZ (low impedance) test setting, Walstad could see instantly that the 73 volts was only a ghost. “It saved me a trip back to the truck to get another meter,” Walstad says.
PAGE 3
Use the Auto-V/LoZ switch position on the DMM when readings are suspect (ghost voltages may be present) or when testing for the presence of voltage. When the leads are placed on an open circuit that contains a ghost voltage, the low input impedance will cause the ghost voltage to dissipate and the meter will display a reading near zero volts indicating no voltage present.