Fluke 323, 365, 373, 375, 376 Service Guide

Clamp Meter ABCs

What is a clamp meter and what can it do? What measurements
can be made with a clamp meter? How do you get the most
out of a clamp meter? Which clamp meter is best suited to the
environment the meter will be used in? The answers to these
questions can be found in this application note.

With technological advances in
electrical equipment and circuits
come more challenges for elec­tricians and technicians. These
advances not only require more
capability in today’s test equip­ment, but more skills on the part
of the people who use them.
An electrician who has a good
grounding in the fundamentals of
test equipment use will be better
prepared for today’s testing and
troubleshooting challenges. The
clamp meter is a important and
common tool found in the tool­boxes of electricians and techni­cians alike.

A clamp meter is an electrical
tester that combines a voltme­ter with a clamp-type current
meter. Like the multimeter, the
clamp meter has passed through
the analog period and into the
digital world of today. Originally
created primarily as a single­purpose test tool for electricians,
today’s models have incorporated
more measurement functions,
more accuracy, and in some
instruments, some very special
measurement features. Today’s
clamp meters have most of the
basic functions of a digital multi­meter (DMM), but with the added
feature of a current transformer
built into the product.

The transformer action

The ability of clamp meters to
measure large ac currents is
based on simple transformer
action. When you clamp the
instrument’s jaws or flexible
current probe around a conductor

Choose a clamp meter rated to meet the electrical environment you’ll be working in, as well as the resolution and
accuracy of measurement you’ll need for your testing.

carrying ac current, that current
is coupled through the jaws,
similar to the iron core of a power
transformer, and into a second­ary winding that is connected
across the shunt of the meter’s
input. A much smaller current
is delivered to the meter’s input
due to the ratio of the number
of secondary windings versus
the number of primary windings

wrapped around the core. Usu­ally, the primary is represented
by the one conductor around
which the jaws or flexible current
probe is clamped. If the second­ary has 1000 windings, then the
secondary current is 1/1000 the
current flowing in the primary, or
in this case the conductor being
measured. Thus, 1 amp of current
in the conductor being measured

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

would produce 0.001 amps or 1
milliamp of current at the input
of the meter. With this technique,
much larger currents can be
easily measured by increasing the
number of turns in the secondary.

Clamp meters measure any
combination of alternating and
direct current. This includes static
dc and charging dc as well as ac.
Clamp meters measure dc current
using Hall effect sensors. A Hall
effect sensor, basically a kind of
magnetometer, can sense the
strength of an applied magnetic
flux. Unlike a simple inductive
sensor, the Hall effect sensor will
work when the applied magnetic
flux is static, not changing. It will
work for alternating magnetic
fields as well. A clamp meter
contains a toroidal iron core that
clamps together with a Hall effect
chip in the gap between the
two halves, so that the induced
magnetic flux from the
current-carrying wire is
channeled through it.

Choosing your
clamp meter

Buying a clamp meter not
only requires looking at
specifications, but also looking
at features, functions, and the
overall value represented by a
meter’s design and the care taken
in its production.

Reliability, especially under
tough conditions, is more impor­tant than ever. Fluke’s design
engineers make a point of
building these test tools not only
electrically, but also mechanically,
robust. By the time Fluke clamp
meters are ready to be tossed into
toolcases, they’ve undergone a
rigorous testing and evaluation
program.

User safety should be a primary
consideration in choosing a
clamp meter—or any other piece
of electrical test equipment. Fluke
not only designs its clamp meters
to the latest electrical standards,
but each clamp meter is indepen­dently tested and then listed by
certified testing labs such as CSA,
TÜV, etc. Only with these certi­fications can you be assured an
electrical tester meets these new
safety standards.

Use a flexible current probe for situations like this, where large conductors make it difficult
to use the clamp meter jaws.

Using a clamp meter in
difficult situations

Electricians and technicians
often need to use clamp meters
in less-than-ideal situations.
The newest clamp meters use
the iFlex
to enable measuring where it’s
difficult to access—for example,
tight cabinets, bundled wires, or
awkward conductors.

remotely, a clamp meter with a
detachable display (such as the
Fluke 381) makes it possible to
see the display at a location other
than where the measurement
is being taken. This means one
person—not two—can take the
measurement.

™ flexible current probe

When it’s necessary to measure

Resolution, digits and
counts

Resolution refers to how fine a
measurement a meter can make.
By knowing the resolution of a
meter, you can determine if it’s
possible to see a small change
in the measured signal. For
example, if the clamp meter has
a resolution of 0.1 amp on a 600
amp range, it’s possible to see a
change of 0.1 amp while reading
100 amps.

You wouldn’t buy a ruler marked
in one-inch segments if you had
to measure down to one-quarter
inch. Similarly, you must choose a
meter that can display the
resolution you need to see in
your measurements.

2 Fluke Corporation Clamp Meter ABCs