Download You can guess which fuse is bad… or you can try this

You can guess
which fuse is bad…
or you can try this
You’ve got a bad fuse in your
three-phase disconnect or
combination motor starter.
With several to pick from, how
do you know which one is
the culprit? Use these simple
troubleshooting methods to
find out quickly.
Ideally circuits should be
de-energized to replace a fuse,
but initial troubleshooting is
sometimes done on live circuits.
Remember, always wear the
appropriate personal safety
gear. The first step is to open
the panel and visually inspect
it for arcing or burning – good
indications of a phase-toground fault that is popping
the fuse. A damaged enclosure
or component will require
replacement.
Checking Fuses
480 V three-phase system – one bad fuse
Line
L1
L2
A
Verify voltage
is present
Line
L3
B
L1
C
L2
A
B
L1
C
L2
A
L3
B
C
Load
Load
Load
480 V
480 V
480 V
Test Point 1
Test Point 2
Test Point 3
Line
Line
L1
Find the
bad fuse
Line
L3
L2
A
L1
L3
B
C
L2
A
Line
L3
B
L1
C
Open
Fuse
L2
A
L3
B
C
Load
Load
Load
480 V
0V
480 V
Test Point 4
Test Point 5
Test Point 6
Application Note
The first step, after opening
the enclosure, is always to test
the line side voltage, which
should be hot. After that, one of
several methods can be used to
isolate a bad fuse:
• Current measurement:
Use a clamp to read the
current on each phase: the
phase with an open fuse will
have zero current. Of course
this assumes that normal
loads are still running on
the good phases. On threephase induction motors, this
method is particularly useful.
That’s because a three-phase
induction motor will continue
to run even if one of the
phases is dead (unless
control circuit power is
drawn from that phase, in
which case the motor starter
will trip). Eventually, the
motor will overheat and the
overloads should trip. The
condition is called singlephasing. A clamp can easily
spot the open phase.
Of course, there is still
the question of why the fuse
opened. A clamp can also
be used to test if overloading
caused a fuse to open. Use
a Fluke 337 with a min/max
function, or a 180 Series
DMM in min/max mode with
a clamp accessory. Leave the
clamp attached for a normal
load cycle. You will be able
to capture the worst-case
load on the circuit.
From the Fluke Digital Library @ www.fluke.com/library
•
•
Same phase voltage
measurement: Use a DMM
or clamp meter in volts ac
mode to measure across each
fuse: one probe contacts the
line side, one probe contacts
the load side of the fuse.
A good fuse will have zero
voltage drop across it. There
might be a voltage drop in
the mV range due to the
resistance of the fuse clips,
but anything above this is
suspect.
Phase-to-phase voltage
measurement: Connect
one probe to the line side
of phase A. Test for fuses on
phases B and C by connecting the other probe to the
load side of each fuse in
turn. If fuses B and C are
good, you will read normal
phase-to-phase voltage
(the same as if you had
connected to the line side of
phase B and C). To complete
the test on the phase A fuse,
move the line side connection to another phase (B or C)
and connect the second
probe to the load side of
phase A.
If a fuse is bad, you will
read less than the phase-tophase voltage. For example,
if you were measuring from
the line side of phase A, and
there was a solid ground
fault on the load side of B
or C, you would read the
phase-to-ground voltage
(about 277 V in a 480 V
system).
De-energized circuits
Once a bad fuse is suspected,
the circuit should be de-energized:
• Open the safety switch or
combo starter (turn
the handle to off).
Open the enclosure,
but keep in mind
that the line side (top)
of the switch remains hot.
• Since the fuses are now
isolated from the line and
de-energized, it is possible
to use the ohms function of
a DMM or clamp meter to
check them. Read across
each individual fuse, line to
load side. A good fuse will
read close to zero ohms,
indicating continuity. An
open fuse will read infinite
ohms (OL). Modern fuses
will fail open, without partial
resistance.
• Take an additional reading
across the fuse clips, line to
load. If there is even a low
ohms reading, check the
condition of the fuse clip.
Old fuse clips that are losing
their spring tension may not
grip the fuse tightly enough.
This can show up as overheating and discoloration of
the fuse clip.
Fuses should always be pulled
and replaced under de-energized conditions. Use fuse
pullers designed for this job.
In the event that you are ever
required to pull a fuse in a
live circuit (where two of the
phases are still live), be sure
to wear the proper personnel
protective equipment. Remembering that the line side
remains hot, pull the line side
first and the load side second.
When replacing the fuse, insert
the load side first, line side
second.
Fluke. Keeping your world
up and running.
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
In Europe/M-East/Africa (31 40) 2 675 200 or
Fax (31 40) 2 675 222
In Canada (800)-36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.fluke.com/
©2002 Fluke Corporation. All rights reserved.
Printed in U.S.A. 9/2002 2042036 A-ENG-N Rev A