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TECHNICAL TRAINING

The Amp Clamp: What is the Current Situation?

By Donald Dougherty Senior Technician

Another piece of test equipment in our toolbox is the amp clamp meter. This meter is used to measure current
flow in electrical circuits. Most machines have this type of test equipment on board. Today we will discuss how to
get the most out of this handy device.

Amp Clamps detect the generated magnetic flux in wires to measure
the current passing through them. The clamp portion of the meter is
the pickup which detects this. Additional circuitry inside the unit
converts this to the familiar “Amp” readings. Some larger meters
have a “rope” or auxiliary loop to measure large diameter cables or
bundles that make up a circuit.
Safety: Because current measurements are taken on live circuits,
remember to observe all Arc-Flash safety procedures and wear
Appropriate PPE while working on live electrical equipment!
Getting Started: Most units have a selector dial that is similar to
those found on other Fluke multimeters. To measure amps, select
the “A” position. There are typically two positions. One for AC~
and one for DC=. Once you have made the proper selection, simply
open the clamp and slip the jaws over the wire you wish to measure.
It is important the jaws close completely for the readings to take
place and be accurate. Also, make sure you clamp over just one of
the wires or phases at a time. Clamping over two or more phases gives a “null” reading and will be invalid.

When checking bundled cables (such as the output of a generator) make sure to loop the “rope” around each phase
bundle individually, and include all cables associated with that phase for accurate readings.

Taking AC current readings can be useful to troubleshooting a variety of issues. Voltage readings only tell you
part of the story. If for example you have a three-phase motor that nuisance trips, or is not performing well, check
the amp draw of each phase. All three phases should have equal current draw. If you see one phase with a signifi-
cantly lower draw, this could be due to a bad circuit breaker, motor starter, or the connections between the power
source and the motor.

DC Measurements: Most inexpensive amp clamp meters are not able to read DC (direct current). Fortunately,
the Fluke brand amp meters we use have this ability. Direct Current readings are useful to determine if alternators
are functioning correctly, valve coils are getting power all the way to them, and if current is flowing on the negative
side of devices such as motors, where a simple voltage reading might be misleading.
Although these types of meters are not used as often as the good old fluke, they are still very necessary and handy
to have around. Treat them with care and have them ready to go when needed. They will give you the rest of the
story when troubleshooting.

Page 15 MARCH 2021