Application Note

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.