Fluke 179 Fluke Current Clamp Meters - Why True RMS Non-Linear Loads Need A Tr
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- Fluke 179 | Fluke Current Clamp Meters - Why True RMS Non-Linear Loads Need A Tr - Page 1
current. The true-rms clamp on the right reads correctly but the average responding clamp reads low by 32 percent. Figure 2. A computer load. Introduction Troubleshooting the electrical service , the average responding meters typically read low. Worst case nonlinear loads include small - Fluke 179 | Fluke Current Clamp Meters - Why True RMS Non-Linear Loads Need A Tr - Page 2
336, and 337. Look for the words true-rms on the front panel. The second style consists of a current transformer (CT)type accessory which works with a digital multimeter. Examples include Fluke Models i200s, 80i-400, and 80i-600A. The jaws of the clamp enclose the conductor being measured which acts
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Application Note
Introduction
Troubleshooting the electrical
service feeding adjustable
speed motor loads can be diffi-
cult if you don’t have the right
tools. New solid state motor
drives and heating controls
often conduct non-sinusoidal
(distorted) current. In other
words, the current occurs in
short pulses rather than the
smooth sine wave drawn
by a standard induction motor.
The current wave shape can
have a drastic effect on a
current clamp reading.
Basically, there are two
types of current clamps
commonly available: “average
responding” and “true-rms.”
The average responding units
are widely used and are usually
Why true-rms?
Non-linear loads need a true-rms
current clamp for accurate readings
can usually be separated into
one of three categories:
1.
Too much current.
2. Too much heat in the
electrical enclosure.
3. Faulty circuit breaker
(or fuse).
Your first instinct will probably
be to measure the current with
a current clamp while the load
is on. If the current is within
the circuit rating, you may be
tempted to replace the circuit
breaker.
Figure 1.
One current—two readings. Which do you trust? The branch circuit above feeds
a non-linear load with distorted current. The true-rms clamp on the right reads correctly but
the average responding clamp reads low by 32 percent.
Figure 2.
A computer load.
Figure 3.
An adjustable speed motor load.
Non-linear loads that cause
measurement errors.
lower cost. They give correct
readings for linear loads such
as standard induction motors,
resistance heaters, and incan-
descent lights. But when loads
are non-linear, containing
semiconductors, the average
responding meters typically
read low. Worst case non-
linear loads include small
adjustable speed drives (5 hp
or less) connected line to line
across two phases of a 480 V,
three-phase system, solid state
heater controls connected
single phase to 240 V, or
computers connected to 120 V.
When troubleshooting a branch
circuit that suffers from circuit
breaker tripping (or fuse blow-
ing), the cause of the trouble