Application Note

5 Fluke Corporation Clamp Meter ABCs
capacitance measurement func-
tion is now being incorporated
into the feature set of many new
clamp meters. This function is
useful for checking motor start
capacitors or measuring values of
electrolytic capacitors contained
in controllers, power supplies
or motor drives.For electricians
who deal with motors in their
work, the ability to capture the
amount of current drawn by a
motor during its start up can tell a
lot about a motor’s condition and
loading. The Fluke 374, 375, 376,
and 381 clamp meters incorpo-
rate inrush current measurement
as part of their feature sets. After
clamping the jaws (or the flexible
current probe) around one of
the motor’s input leads, activate
the inrush mode. Next, turn on
the motor. The clamp meter’s
display will indicate the maxi-
mum current drawn by the motor
over the first 100 milliseconds
of its start cycle. This proprietary
inrush measurement technology
filters out noise and captures
motor starting current exactly as
the circuit protection sees it.
Clamp meter safety
Making measurements safely
starts with choosing the proper
meter for the environment in
which the meter will be used.
Once the proper meter has been
chosen, you should use it by
following good measurement
procedures.
The International Electrotechni-
cal Commission established new
safety standards for working on
electrical systems. Make sure you
are using a meter that meets the
IEC category and voltage rating
approved for the environment
where the measurement is to be
made. For instance, if a voltage
measurement needs to be made
in an electrical panel with 480V,
then a meter rated Category
III—600 V or higher should be
used. This means the input
circuitry of the meter has been
designed to withstand voltage
transients commonly found in this
environment without harming
the user.
1
Choosing a meter with
this rating, which also has a CSA
or TÜV certification, means the
meter not only has been designed
to IEC standards, but has been
independently tested and meets
those standards. (See indepen-
dent testing sidebar).
Many new clamp meters
now carry a Cat IV safety rating,
which means they can be
used in outdoor or underground
settings where lightning strikes
or transients can occur more
frequently and at higher levels.
Safety checklist
✓ Use a meter that meets
accepted safety standards for
the environment in which it
will be used.
✓ Inspect test leads or flexible
current probe for physical
damage before making a
measurement.
✓ Use the meter to check
continuity of the test leads or
flexible current probe.
✓ Use only test leads that have
shrouded connectors and
finger guards.
✓ Use only meters with recessed
input jacks.
✓ Be sure the meter is in good
operating order.
✓ Always disconnect the “hot”
(red) test lead first.
✓ Don’t work alone.
✓ Use a meter that has overload
protection on the ohms func-
tion.
1
See the ABCs of Multimeter Safety (literature
code 1263690) to learn more about IEC-1010
and how it applies to multimeter use.
Special features
The following special features
and functions may make it easier
to use your clamp meter.
• Annunciators (display icons)
show at a glance what is being
measured (volts, ohms, etc.)
• Data Hold allows you to freeze
the reading in the display.
• One-switch operation makes
it easy to select measurement
functions.
• Overload protection prevents
damage to both the meter and
the circuit, and protects the
user.
• Autoranging automatically
selects proper measurement
range. Manual ranging lets you
lock into a specific range for
repetitive measurements.
• Low battery indicator warns
you when the battery needs
changing.
• Display with backlight, easy-
to-read characters, and wide
viewing angle makes read-
ings easier to see in all sorts
of conditions. The backlight
display automatically sets the
correct measurement range so
you do not need to change the
switch positions while taking
a measurement.
• Integrated low pass filter and
state of the art signal process-
ing allows for use in noisy
electrical environments while
providing stable readings.