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What can and can’t you do with an AC/DC clamp meter?

April 2011

By Chuck Newcombe


The hundred years before 1821 was a period during which many of the basic discoveries in electromagnetism took place. Benjamin Franklin, Luigi Galvani, Alesandro Volta, Hans Christian Oersted, and others paved the way for what would become the defining moment in electromagnetics.

In 1821 Michael Faraday first demonstrated the forerunner of a homopolar motor to the Royal Society. His experiment suggested that an electrical current flowing near a magnetic field might be harnessed to produce work. Check out a modern variation of Faraday’s 1821 homopolar motor experiment.

Just ten years later, James Clerk Maxwell was born. He published four famous equations, one of which was based on Faraday’s work.

…and now

I recently received an email from a long-time consultant friend regarding the use of current clamps. He was questioning the behavior of various types of clamps in the presence of dc current in an otherwise ac current circuit. He correctly suspected that a normal ac transformer-type current clamp might be difficult to open once closed around a conductor with significant dc current present.

An ac current clamp is, of course, nothing more than a transformer where the measured conductor is treated as a single turn primary. The secondary is wound around a split magnetic core with 1000 turns to produce a secondary current that is 1/1000 of the primary current. This much smaller current can be easily read on the milliamp ranges of a digital multimeter (DMM). The moveable core pieces of such a probe must close tightly, with no air gap between the core pieces, for maximum accuracy.

The effect of exposing this core to dc current is to saturate the core of the clamp, causing it to act like a permanent magnet, thus making it difficult to open. In addition, residual magnetism remaining in the core as a result of the saturation will cause errors in subsequent normal ac current readings. (We’ll discuss how to fix that a little later in this column.)

So how does a current clamp meter or current clamp accessory, which measures either ac or dc, work?

Two features allow such a current measuring device to measure either ac or dc. The first feature is to create an air gap in the magnetic circuit. The air gap limits the magnetic flux so that the core cannot saturate.

The second feature is a Hall Effect sensor, which is introduced into the air gap to measure the magnetic flux directly. The output voltage from the sensor is then amplified and scaled to represent the current flowing in a conductor placed in the jaws of the clamp.

You can generally tell the difference between the two types of clamps by inspection. The ac-only clamp will have bare metal core faces when you open the jaws, while the ac-dc clamp with a Hall Effect sensor will have the core ends covered by plastic jaw molding.

Care and feeding of Fluke current clamp jaws

Now that we’ve reviewed the operating details of the two types of clamps, what are the ins and outs of clamp care for a long and accurate service life?

For an ac-only laminated-steel transformer-type clamp, the jaw core faces must be clean and free of rust, dirt, and grease for accurate operation. The jaw faces of both ac and ac-dc clamps must also be properly aligned to complete the magnetic circuit.

The plastic shell halves of Fluke current clamps have a self-aligning feature to ensure proper orientation. For long life and accuracy, it is essential that you don’t ever use the clamp jaws as a pry bar to move stiff conductors apart in a panel.

What if an ac transformer type clamp is exposed to dc?

This can happen if you attempt to measure a dc current, or if a breaker or fuse opens at a peak of the ac current being measured. The effect in either case is to saturate the core and leave residual permanent magnetism in it.

The good news is that you can fix it by a process called “degaussing.” I use a variable autotransformer (such as a Variac) to provide a variable ac current to be measured. This current can be adjusted to be higher than the current that caused the saturation, which may require that you use a step-down transformer at the output of the Variac to drive the current loop conductor to be measured. After achieving the high ac current needed, you then slowly lower the output current to zero, using the Variac, before removing the clamp. This procedure should demagnetize the core and restore the clamp’s accuracy.

Fluke has recently introduced an iFlex™ ac clamp probe that does not use a steel core. See further discussion about the iFlex™ flexible current probe »

For more information about how a clamp meter works, go to Clamp Meter ABCs »

See Fluke’s newest clamp meters »