Application Note

Application Note
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y
Clamp meter readings:
problems and solutions
There is no smell of burnt insu-
lation, the load appears normal,
the coupling is intact, ambi-
ent temperature is not above
the nameplate specified value
of 40 °C (104 °F). As expected,
however, the motor is too hot to
touch. The next obvious trouble-
shooting tactic is to measure
the amperage the motor draws
while running. Since a small
increase in current flow to a
motor produces a proportionately
larger amount of heat, motor
amperage exceeding name-
plate values should be carefully
investigated as a possible cause.
These overload trips, though
often caused by motor loading
issues, can also indicate bearing
failure, insulation breakdown or
voltage unbalance.
After you ensure that all safety
requirements for working near
energized components are satis-
fied, open the fuse disconnect
door in preparation for using
your clamp meter to measure
current. Once again, a visual
inspection reveals no obvious
problems. You begin to clamp
around a motor phase conductor
at the bottom of a fuse—and now
the problems begin.
Clamping around a phase
conductor
When you use a clamp meter it
is necessary to clamp around
only one phase conductor at a
time. When current flows to a
conductor it produces a magnetic
field around the conductor. The
more current, the stronger the
magnetic field. The magnetic
field induces a voltage into the
clamp portion of the meter. The
meter reading will be propor-
tional to the amount of voltage
induced into the clamp. There-
fore, reading the current flow
through a conductor requires
that only the magnetic field
produced by that one conduc-
tor be induced into the clamp.
Clamping around two or more
conductors will cause the
magnetic fields to cancel each
other and the resultant reading,
if not “zero,” will be completely
erroneous.
So, what is the problem
when clamping around only
one conductor? Have you ever
tried getting a clamp around a
10 AWG (American wire gauge)
solid conductor with inadequate
conductor length inside the
box? Have you ever tried pulling
away one 2 AWG conductor from
two others in an attempt to get
that clamp to fit? Sometimes
two of the conductors can be
reached fairly easily, but clamp-
ing around the third conductor
may be difficult. In some cases,
the clamp you are using may
simply not be big enough to go
around the conductor you need
to measure. You may need to
clamp around two conductors in
parallel. Each set of measure-
ments will produce its own
unique problems.
A motor trips on overload.
Visual and physical inspection reveals no abnormalities.