Fluke 117 User Manual
New Fluke 117 Digital
Multimeter drives the
ghosts out of the attic
(and the electric circuits)
As an electrician, Eldon
Walstad’s job can be like that
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
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.
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
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.
Ghost voltage can
look real
Ghost voltages are caused when
energized circuits and nonenergized 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 multimeter leads between the open
circuit and the neutral conductor, you effectively complete
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.
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.
Older troubleshooting tools
such as analog multimeters
and solenoid testers generally 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.
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 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 presence of voltage.
2 Fluke Corporation New Fluke 117 Digital Multimeter drives the ghosts out of the attic
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