Fluke 117 Application Note

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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.
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 Walstad, a journeyman electrician
with Rodgers Electric in Everett,
WA., handles all kinds of issues
related to electrical installation, maintenance and repair. A
frequent challenge is to determine why a device isn’t operating, 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 dangerous voltage, or merely “ghost
voltage” caused by capacitive
coupling between energized
wiring and adjacent unused
wiring?
“The other day I had a situation 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

Application Note

the wet, dirty plant environment, 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 timewasting wild goose chase, looking 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.

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

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.

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 measurement, it will have little impact
on circuit performance. This is
the desired effect for most voltage measurement applications,
and is especially important for
sensitive electronics or control
circuits.
Ghost voltage can
Older troubleshooting tools
look real
such as analog multimeters
Ghost voltages are caused when and solenoid testers generenergized circuits and nonally have low impedance input
energized wiring are located in circuitry around 10 kilohms or
close proximity to each other,
less. While these tools aren’t
such as in the same conduit or
fooled by ghost voltages, they
raceway. This condition forms a should only be used for testing
capacitor and allows capacitive power circuits or other circuits
coupling between the energized where the low impedance
wiring and the adjacent unused will not negatively impact or
wiring.
alter circuit performance. They
When you place your multirarely comply with the current
meter leads between the open
IEC 61010 safety standards
circuit and the neutral conducand North American regulatory
tor, you effectively complete
requirements.
the circuit through the input
of the multimeter. The capacitance 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 voltage 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 disconnected 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.

 Fluke Corporation

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

The best of both worlds
With dual impedance meters,
technicians can safely troubleshoot 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 regular Vac and Vdc switch positions are high impedance. Use
these switch positions for most
troubleshooting scenarios and
especially on sensitive electronic loads.
The Fluke low impedance
function is called Auto-V/LoZ.
•	 Auto-V stands for automatic
volts. This feature automatically determines whether the
measured signal is ac voltage or dc voltage, selects the
correct function and range,
and displays the correct information.
	
• LoZ stands for Low Impedance (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 presence of voltage.

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. 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.

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 troubleshooting and straightforward ‘go/no
go’ testing in the residential
electrical setting, and also is
designed for use in electric utility applications to test circuits
before turning power back on
at the meter. The 115 Multimeter is optimized for electrical
and electronic testing by field
service technicians. The 116
Multimeter features temperature
and microamps measurements
Non-contact voltage
useful for heating, ventilation,
detection
air conditioning and refrigeraThe Fluke 117 DMM also
tion (HVAC/R) technicians. And
includes the built-in VoltAlert™
the 117 Electrician’s Multimenon-contact voltage functer tested by Eldon Walstad is
tion. Located on the top of the
designed for commercial electrimeter above the LCD display,
cians. All are rated for use in
it’s another feature handy for
600 V CAT III environments,
detecting the invisible. The
and all except the 115 include
VoltAlert function detects the
the LoZ low impedance test
presence of ac voltage, sounds
capability.
an audible tone and activates
“I like it,” Walstad says of
a red LED indicator light on the the 117. “When you’re trying
top center of the LCD display.
to prove something, you’ve got
Using this feature before
to know whether you’ve got
making a contact measurement real power there or not. If you
can add an extra layer of safety. think you’ve got power but
Users should, of course, still
don’t, then you can go off in a
test for the presence of voltage different direction trying to find
using test leads with the meter problems when you really need
in the volts setting.
to be back here working on
something. You’re not wandering aimlessly, looking for it.”
And there are no ghosts
in sight.

Fluke.	Keeping your world
	
up and running.™
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

 Fluke Corporation

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



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