How Stray Voltage Affects
Multimeter Measurements
…and What You Can Do About It.
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For most electrical measurements
in the industrial environment, a
high impedance digital multime-
ter or electrical tester is the
appropriate tool. These test
instruments have a relatively high
input impedance (>1 megohm),
which means that when connect-
ed, they don’t load the circuit
under test. These test tools gener-
ally will not affect circuit opera-
tion or circuit measurements.
On the other hand, low imped-
ance test instruments can
seriously load down a circuit
under test and, in some cases,
can adversely affect circuit
operation and circuit measure-
ments. This can happen with
sensitive control circuits or in
many electronic circuits found
within industrial machinery.
However, even with a high
impedance multimeter, there’s
still one confusing measurement
situation that can occur in facili-
ties or manufacturing plants. It’s
called stray or ghost voltage, and
it happens specifically when
using a high impedance test
instrument for day to day
measurements.
Stray or ghost voltages occur
from capacitive coupling between
energized circuits and non en-
ergized, non connected adjacent
wiring. Because of this coupling
effect and the multimeter’s high
impedance, it’s not always pos-
sible to determine if the circuit
under test is energized or
de-energized, and this creates
confusion for the person
performing the test.
Where are stray voltages
encountered?
The most common place to en-
counter stray voltage is in unused
cable runs or electrical wiring in
existing conduit. When facilities
or buildings are built and wired,
it’s very common for electricians
to pull extra wire through the
conduit for future use.
These wires are typically left
unconnected until needed, but
are subject to capacitive coupling
from the powered wires.
Another example is an open
ground or neutral on a 120 V
branch circuit or in card cages
where 120 volt control circuits
are used to control assembly
line or conveyor functions.
Application Note
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
1.888.610.7664 sales@GlobalTestSupply.com
Fluke-Direct.com
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