Hi all,

I had an interesting incident with a fuse recently that I wanted to
contribute in case it helps anyone and to see if anyone else has ever seen
this.

I ordered qty 6 of Bussmann LPJ-2SP 2 Amp, 600VAC class J fuses from
McMaster-Carr several months ago. When they arrived, the bag contained qty
5 of the correct part as well as qty 1 of a fuse marked LPJ-30SP. I was
very upset that McMaster could (apparently) make such a mistake as to put a
30A fuse into a bag of 2A fuses, since it could cause quite the safety
hazard. I actually only needed 3 fuses and the other three were a backup in
case the first three blew so I just used 3 of the ones marked 2 Amp.

I was using these in a large motor testing machine which is powered by
480V, 3-phase AC. My design for the fixture taps off from the main 480V
input to provide low current, 240V 3-phase AC to a small blower motor for
cooling. I did this with a step-down transformer and to protect the
transformer, wire, and blower, I used a separate bank of 3 2 Amp fuses.

After running the blower a few times, it stopped working and I discovered
that it was because one of the 2 Amp fuses blew. I suspected that this was
caused by transformer inrush current. To measure the inrush current, I
replaced the single blown 2A fuse with the 30A fuse. I wanted to reduce the
chance of it blowing again right away and figured that nothing too bad
could go wrong because I had the other two 2A fuses still in place. Unless
I had a fault to ground, the 30A fuse should not blow.

I put a clamp-on scope current probe on one of the phases and switched the
power on and off a few times to capture the inrush current waveform when
the power was switched on at several random voltage points in the AC cycle.

After obtaining a few waveforms which showed an inrush peak current of
about 80 Amps worst case with an I^2*t integral of about 20 A^2*s, I tried
to capture a few more and the 30 Amp fuse blew. I was astounded and
concerned by how this could happen - especially since the other two fuses
(2 A) were still intact.

I became suspicious that perhaps the fuse was actually indeed a 2 Amp fuse
but was marked incorrectly by Bussmann. To confirm my theory, I cut it
open, along with the blown 2A fuse. Sure enough, inside each fuse were two
smaller fuse elements in their own fuse bodies with identical appearance
and part number markings. Of the two elements in each fuse, one had a
fiberglass body and the other a glass body filled with sand.

In both fuses, the elements were in series and it was the glass bodied one
which blew - the fiberglass body one was still intact.

I took out the fiberglass bodied elements and put them in a test circuit
which ran 10 Amps through them, one at a time. BOTH blew after about 10
seconds. Since there is no way that a 30 Amp fuse could have a series
element which opened at only 10 Amps, this further confirmed my suspicion
that the fuse was mis-marked and probably this happened at the factory.

Sean
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