Clamp meter ABCs

A back-to-basics explanation of how, when, why and what to measure.

By Chris Rayburn, Fluke

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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?

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 an 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. 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 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 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 1,000 windings, then the secondary current is 1/1000 the current flowing in the primary, or in this case the conductor being measured. Thus, 1 A of current in the conductor being measured would produce 0.001 Amps or 1 milliAmp of current at the input of the meter. With this technique, much larger currents can be 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.

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 A on a 600 A range, it’s possible to see a change of 0.1 A while reading 100 A.

You wouldn’t buy a ruler marked in 1-in. segments if you had to measure down to a quarter inch. Similarly, you must choose a meter that can display the resolution you need to see in your measurements.


Accuracy is the largest allowable error that will occur under specific operating conditions. In other words, it is an indication of how close the meter’s displayed measurement is to the actual value of the signal being measured.

Accuracy for a clamp meter is usually expressed as a percent of reading. An accuracy of 3% of reading means that for a displayed reading of 100 A, the actual value of the current could be anywhere between 97 and 103 A.

Specifications may also include a range of digits added to the basic accuracy specification. This indicates how many counts the digit to the extreme right of the display might vary. So the preceding accuracy example might be stated as ±(2% + 2). Therefore, for a displayed reading of 100 A, the actual current could then be estimated to be between 97.8 and 102.2 A.

Crest factor

With the growth of electronic power supplies, the current drawn from today’s electrical distribution system are no longer pure 60- or 50-cycle sine waves. These currents have become fairly distorted, due to the harmonic content these power supplies generate.

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