Application Note

2 Fluke Corporation Clamp meter readings: problems and solutions
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. [phone] or
Fax [phone]
In Europe/M-East/Africa [phone] or
Fax [phone]
In Canada (800)-36-FLUKE or
Fax [phone]
From other countries [phone] or
Fax [phone]
Web access: http://www.fluke.com
©2010 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 10/2010 3949064A A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
Fluke. Keeping your world
up and running.
®
Safety is always a prime
consideration. Even when you
wear the appropriate shock and
arc-flash protection, having to
push and pull on live conductors
is not an enjoyable task and can
be dangerous. Conductors come
loose from underneath terminals.
Conductor insulation scraped off
during installation can come in
contact with the metal enclosure
when clamping around conduc-
tors. The phase-to-ground fault
can instantly escalate into a
phase-to-phase arcing fault with
potentially catastrophic arc-flash
results. There are no guarantees
anytime you are exposed to
energized conductors and circuit
parts. And, probing about those
conductors with the clamp meter
can only compound the problem.
The solution to the above
problems is a flexible clamp.
Weaving a flexible clamp around
a conductor means not pulling
and tugging on it to move it from
adjacent conductors. Whether
dealing with the 10AWG solid
conductor or a 2 AWG (or larger)
stranded conductor, installing
the flexible clamp is simpler
and unquestionably much safer.
The problem has always been
that flexible clamps were typi-
cally found only on equipment
such as data loggers and power
quality analyzers, and were not
available for the standard clamp
meter used by professionals in
the field. Not true anymore, as
Fluke now offers true-rms ac/
dc clamp meters that utilize the
iFlex
™ flexible current probe.
One model even offers remote
display.
Determining inrush
current
When an ac motor starts it will
draw more current than its full
load value on the nameplate.
How much more current depends
on the load on the motor when
it starts and the code letter on
the nameplate. Excessive start-
ing torque requirements or low
system voltages can result in
excessive inrush current during
When taking clamp meter
readings you should be ready for
problems. Remember that safety
is number one. And make sure
you have the right measurement
tool for the job—one that allows
for safe and accurate readings,
can measure the current range
you need, and even provide
remote display for both safety
and convenience.
start-up. Determining inrush
current can be tough with a
digital meter. Values change
rapidly and determining the
actual inrush value can be dif-
ficult. Min, max, average and
inrush recording features of the
Fluke models allow the meter
to capture variations automati-
cally. Also, electronics filter out
noise and capture motor start-
ing current exactly as the circuit
protection sees it. That means
what you see on the meter is
the same value sensed by the
overloads, time-delay fuses and
inverse-time circuit breakers.
Another problem is that inrush
current values may exceed the
maximum value read by the
meter. For example, a 460 volt,
250 horsepower motor should
draw no more than 302 amps
at full load. Inrush currents for
such a motor starting under full
load may be 1,500 to 2,000
amps. Determining the inrush
value can be important in diag-
nosing motor overload problems.
Obviously the meter must be
capable of reading such values.
Most clamp meters read a maxi-
mum value of 400 amps. Some
of the new Fluke clamp meters
have a 1000 amp limitation
with the fixed jaw and all have
a 2500 amp maximum current
measurement with the iFlex
™.
That means you can measure
the running current on a 460
volt, 500 horsepower motor!
Beyond motors
Certainly not all clamp-on
ammeter readings are taken on
motors. Current readings must
be taken on all types of non-
linear loads. Line side currents
on uninterrupted power supply
(UPS) systems and variable-
system drives (VFDs) must
be measured. Lighting loads
containing electronic bal-
lasts, branch circuits supplying
computers, printers and copy
machines must all be measured.
The new Fluke true-rms clamp
meters feature a low pass filter,
allowing for accurate readings
on such nonlinear equipment.