Application Note

3 Fluke Corporation Clamp Meter ABCs
Accuracy
Accuracy is the largest allow-
able error that will occur under
specific operating conditions.
In other words, it is an indica-
tion 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 amps,
the actual value of the current
could be anywhere between 97.0
and 103.0 amps.
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 100.0 amps, the actual current
could then be estimated to be
between 97.8 and 102.2 amps.
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.
However, electrical power
system components such as fuses,
bus bars, conductors, and thermal
elements of circuit breakers are
rated in rms current because
their main limitation has to do
with heat dissipation. If we want
to check an electrical circuit for
overloading, we need to measure
the rms current and compare the
measured value to the rated value
for the component in question.
Therefore, today’s test equip-
ment must be able to accurately
measure the true-rms value of a
signal regardless of how distorted
the signal might be.
Crest factor is a simple ratio of
a signal’s peak value to its rms
value. For a pure ac sine wave,
the crest factor would be 1.414.
However, a signal that has a
very sharp pulse would cause
the ratio, or crest factor, to be
high. Depending on the width of
the pulse and its frequency, you
can see crest factors of 10:1 or
higher. In real power distribution
systems, crest factors of greater
than 3:1 are rarely seen. So as
you can see, crest factor is an
indication of a signal’s distortion.
A crest factor specification will
be found only in specifications for
meters that can make true-rms
measurements. It indicates how
much distortion a signal can have
and still be measured within
the meter’s accuracy specifica-
tion. Most true-rms reading
clamp meters have crest factor
specifications of 2:1 or 3:1. That
rating will handle most electrical
applications.
Measuring current
One of the most basic measure-
ments of a clamp meter is current.
Today’s clamp meters are capable
of measuring both ac and dc
current. Typical current measure-
ments are taken on various
branch circuits of an electrical
distribution system. Determining
how much current is flowing in
various branch circuits is a fairly
common task for the electrician.
How to make current
measurements
1. Select Amps ac or Amps
dc .
2. Open the jaws of the clamp
meter and close the jaws
around a single conductor.
(If you are measuring ac
current, you can switch to
the iFlex setting and use a
flexible current probe.)
3. View the reading in the
display.
By taking current measurements
along the run of a branch circuit,
you can easily tell how much
each load along the branch
circuit is drawing from the
distribution system.When a circuit
breaker or transformer appears
to be overheating, it’s best to
take a current measurement on
the branch circuit to determine
the load current. However, make
sure you are using a true-rms
responding meter so you can get
an accurate measurement of the
signal heating up these compo-
nents. The average responding
meter will not give a true reading
if the current and voltage are non-
sinusoidal due to non-linear loads.
Measuring voltage
Another common function for a
clamp meter is measuring voltage.
Today’s clamp meters are capable
of measuring both ac and dc
voltage. AC voltage is usually
created by a generator and then
distributed through an electrical
distribution system. An electri-
cian’s job is to be able to take
measurements throughout the
system to isolate and fix electri-
cal problems. Another common
voltage measurement would be
testing battery voltage. In this
case, you would be measuring
direct current or dc voltage.
Testing for proper supply voltage
is usually the first thing measured
when troubleshooting a circuit. If
there is no voltage present, or if
it is too high or too low, the volt-
age problem should be corrected
before investigating further.
A clamp meter’s ability to
measure ac voltage can be
affected by the frequency of the
signal. Most clamp meters can
accurately measure ac voltages
with frequencies from 50 Hz to
500 Hz, but a digital multimeter’s
ac measurement bandwidth
might be 100 kHz or higher.
This is why the reading of the
same voltage by a clamp meter
and digital multimeter can have
very different results. The digital
multimeter allows more of the
high frequency voltage through to
the measurement circuitry, while
the clamp meter filters out some
of the voltage contained in the
signal above the bandwidth of
the meter.
When troubleshooting a vari-
able frequency drive (VFD), the
input bandwidth of a meter can
become very important in getting