Application Note

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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

Clamp Meter ABCs

With technological advances in

electrical equipment and circuits

come more challenges for elec-

tricians and technicians. These

advances not only require more

capability in today’s test equip-

ment, 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 tool-

boxes of electricians and techni-

cians alike.

A clamp meter is an electrical

tester that combines a voltme-

ter 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 single-

purpose 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 multi-

meter (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

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.

carrying ac current, that current

is coupled through the jaws,

similar to the iron core of a power

transformer, and into a second-

ary 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. Usu-

ally, the primary is represented

by the one conductor around

which the jaws or ﬂexible current

probe is clamped. If the second-

ary 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

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.

Summary of content (6 pages)

PAGE 1
Clamp Meter ABCs Application Note 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.
PAGE 2
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.
PAGE 3
Accuracy the crest factor would be 1.414. However, a signal that has a Accuracy is the largest allowvery sharp pulse would cause able error that will occur under the ratio, or crest factor, to be specific operating conditions. high. Depending on the width of In other words, it is an indicathe pulse and its frequency, you tion of how close the meter’s can see crest factors of 10:1 or displayed measurement is to the higher.
PAGE 4
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 bandwidth). 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.
PAGE 5
capacitance measurement function 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.
PAGE 6
Glossary Accuracy. How close the displayed measurement is to the actual value of the signal being measured. Expressed as a percentage of reading or a as a percentage of full scale. Analog Meter. An instrument that uses a needle movement to display the value of a measured signal. The user judges the reading based in the position of the needle on a scale. Annuciator. A symbol or icon that identifies a selected range or a function Average Responding Meter.