Application Note

4 Fluke Corporation Clamp Meter ABCs
a meaningful reading. Due to
the high harmonic content in the
signal coming out of a VFD to the
motor, a DMM would measure
most of the voltage content
(depending on its input band-
width). Measuring the voltage
output of a VFD is now a common
measurement. A motor connected
to a VFD only responds to the
average value of the signal, and
to measure that power the input
bandwidth of the clamp meter
must be narrower than its DMM
counterpart. The Fluke 375, 376,
and 381 clamp meters have been
specifically designed for testing
and troubleshooting VFDs.
How to make voltage
measurements
1. Select Volts AC ( ) or Volts DC
(
), as desired.
2. Plug the black test probe into
the COM input jack. Plug the
red test probe into the V
input jack.
3. Touch the probe tips to the
circuit across a load or power
source (in parallel to the
circuit).
4. View the reading, being
sure to note the unit of
measurement.
5. (Optional) Press the HOLD
button to freeze the reading
in the display. Now you can
remove the meter from the
live circuit and then read the
display when you are safely
clear of the electrical hazard.
By taking a voltage measurement
at the circuit breaker and then
at the input of the load on that
breaker, you can determine the
voltage drop that occurs across
the wires connecting them. A
significant drop in voltage at the
load might affect how well the
load functions.
Measuring resistance
Resistance is measured in ohms
(Ω). Resistance values can vary
greatly, from a few milliohms
(mΩ) for contact resistance to
billions of ohms for insulators.
Most clamp meters measure down
to 0.1 Ω. When the measured
resistance is higher than the
upper limit of the meter, or the
circuit is open, “OL” appears in
the meter’s display.
Resistance measurements must
be made with the circuit power
off—otherwise, the meter or
circuit could be damaged. Some
clamp meters provide protec-
tion in the ohms mode in case of
accidental contact with voltages.
The level of protection may vary
greatly among different clamp
meter models.
How to make resistance
measurements
1. Turn off power to the
circuit.
2. Select resistance (W).
3. Plug the black test probe into
the COM input jack. Plug the
red test probe into the VW
input jack.
4. Connect the probe tips across
the component or portion of
the circuit for which you want
to determine resistance.
5. View the reading in the
meter’s display
Make sure the power is off
before making resistance
measurements.
Continuity
Continuity is a quick go/no-go
resistance test that distinguishes
between an open and a closed
circuit.
A clamp meter with a continuity
beeper allows you to complete
many continuity tests easily and
quickly. The meter beeps when
it detects a closed circuit, so you
don’t have to look at the meter as
you test. The level of resistance
required to trigger the beeper
varies from meter to meter. The
typical resistance setting to turn
on the beeper is a reading less
than between 20 ohms and
40 ohms.
Special functions
A fairly common measurement
function is reading the frequency
of an ac current waveform. With
the clamp meter’s jaws (or a
flexible current probe) wrapped
around a conductor carrying ac
current, switch on the Frequency
function and the meter’s display
will indicate the frequency of the
signal flowing in the conductor.
This is a very helpful measure-
ment when tracking down
harmonic problems in an electri-
cal distribution system.
Another feature that can be
found in some clamp meter
models is min, max, and aver-
age storage. When this feature is
activated, each reading the clamp
meter takes is compared against
any previously stored readings. If
the new reading is higher than
the reading in the high reading
memory, it replaces that reading
as the highest reading. The same
comparison is made against the
low reading memory, and the
new reading, if lower, replaces
the stored reading. The average
reading is updated accordingly.
As long as the min, max, and
average feature is active, all
readings are processed in this
way. Thus, after a period of time,
you can call up each of these
memory values to the display and
determine the highest, lowest,
and average reading over a
specific period of time.
In the past, not all clamp meters
could measure capacitance. The
Measuring current with a clamp meter.