Fluke 117 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

From the Fluke Digital Library @ www.fluke.com/library

2 Fluke Corporation New Fluke 117 Digital Multimeter drives the ghosts out of the attic

How impedance affects

testing

Most digital multimeters for

testing industrial, electrical and

electronic systems have high

impedance input circuits greater

than 1 megohm. This means

that when the DMM is placed

across a circuit for a measure-

ment, it will have little impact

on circuit performance. This is

the desired effect for most volt-

age measurement applications,

and is especially important for

sensitive electronics or control

circuits.

Older troubleshooting tools

such as analog multimeters

and solenoid testers gener-

ally have low impedance input

circuitry around 10 kilohms or

less. While these tools aren’t

fooled by ghost voltages, they

should only be used for testing

power circuits or other circuits

where the low impedance

will not negatively impact or

alter circuit performance. They

rarely comply with the current

IEC 61010 safety standards

and North American regulatory

requirements.

But Walstad had extra help.

The Fluke 117 Digital Multi-

meter he was using that day

has dual impedance capabil-

ity; 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.

Ghost voltage can

look real

Ghost voltages are caused when

energized circuits and non-

energized wiring are located in

close proximity to each other,

such as in the same conduit or

raceway. This condition forms a

capacitor and allows capacitive

coupling between the energized

wiring and the adjacent unused

wiring.

When you place your multi-

meter leads between the open

circuit and the neutral conduc-

tor, you effectively complete

the circuit through the input

of the multimeter. The capaci-

tance between the connected,

hot conductor and the floating

conductor forms a voltage

divider in conjunction with the

multimeter input impedance.

The multimeter then measures

and displays the resulting volt-

age value.

Most digital multimeters

today have an input impedance

that’s high enough to show

this ghost voltage, giving a false

impression of a live conductor.

The meter is actually measuring

voltage coupled into the discon-

nected conductor. But at times,

these voltages can be 80-85 %

of what the “hard” voltage

should be. If not recognized as

a ghost voltage, additional time,

effort and money will be lost

troubleshooting circuit problems.

The best of both worlds

With dual impedance meters,

technicians can safely trouble-

shoot sensitive electronic or

control circuits, as well as

circuits that may contain ghost

voltages, and can more reliably

determine whether voltage is

present on a circuit.

On the Fluke 114, 116 and

117 DMMs, the meter’s regu-

lar Vac and Vdc switch posi-

tions are high impedance. Use

these switch positions for most

troubleshooting scenarios and

especially on sensitive elec-

tronic loads.

The Fluke low impedance

function is called Auto-V/LoZ.

• Auto-V stands for automatic

volts. This feature automati-

cally determines whether the

measured signal is ac volt-

age or dc voltage, selects the

correct function and range,

and displays the correct infor-

mation.

• LoZ stands for Low Imped-

ance (Z). This feature presents

a low impedance input to

the circuit under test. This

reduces the possibility of

false readings due to ghost

voltages and improves

accuracy when testing to

determine absence or pres-

ence of voltage.

3 Fluke Corporation New Fluke 117 Digital Multimeter drives the ghosts out of the attic

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 indicat-

ing no voltage present. When

the leads are placed on a live

circuit, however, the input

senses the presence of “hard”

voltage and then displays the

actual voltage present.

Non-contact voltage

detection

The Fluke 117 DMM also

includes the built-in VoltAlert™

non-contact voltage func-

tion. Located on the top of the

meter above the LCD display,

it’s another feature handy for

detecting the invisible. The

VoltAlert function detects the

presence of ac voltage, sounds

an audible tone and activates

a red LED indicator light on the

top center of the LCD display.

Using this feature before

making a contact measurement

can add an extra layer of safety.

Users should, of course, still

test for the presence of voltage

using test leads with the meter

in the volts setting.

Four user groups,

four meters

The new Fluke DMMs include

four distinct models, each aimed

at a specific group of users. The

114 Electrical Multimeter is

ideal for electrical troubleshoot-

ing and straightforward ‘go/no

go’ testing in the residential

electrical setting, and also is

designed for use in electric util-

ity applications to test circuits

before turning power back on

at the meter. The 115 Multi-

meter is optimized for electrical

and electronic testing by field

service technicians. The 116

Multimeter features temperature

and microamps measurements

useful for heating, ventilation,

air conditioning and refrigera-

tion (HVAC/R) technicians. And

the 117 Electrician’s Multime-

ter tested by Eldon Walstad is

designed for commercial electri-

cians. All are rated for use in

600 V CAT III environments,

and all except the 115 include

the LoZ low impedance test

capability.

“I like it,” Walstad says of

the 117. “When you’re trying

to prove something, you’ve got

to know whether you’ve got

real power there or not. If you

think you’ve got power but

don’t, then you can go off in a

different direction trying to find

problems when you really need

to be back here working on

something. You’re not wander-

ing aimlessly, looking for it.”

And there are no ghosts

in sight.

Fluke Corporation

PO 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) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2006 Fluke Corporation. All rights reserved.

Printed in U.S.A. 9/2006 2747091 A-EN-N Rev A

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up and running.™