Dear Team:
I vaguely recall some fuse spec for some flavor of the little tubular
glass fuses, that said that an for N amp fuse, 50% of a population run at
exactly N amps at 25C into a resistive load would blow in 4 hours. And the
current rating goes up/down as a function of temperature.
And probably age. And when they used to use them in cars, they would
occasionally 'go bad' due to vibration and temperature cycling.
And we just had one 'go bad' due to the fact that the fuse holder contact
wasn't well made so it got hot.
A fuse is a cheap electronic component. Like resistors and capacitors,
most they are reliable when they are made. In application they have wear-out
mechanisms and accelerated failure conditions. But they're a whole lot better
than doing nothing.
Boris
-----Original Message-----
From: POWELL, DOUG [ mailto:doug.pow...@aei.com]
Sent: Friday, May 23, 2003 3:57 PM
To: EMC-PSTC (E-mail)
Subject: Bad Fuse vs. Good Fuse
Hello all,
Recently, in my company, we've been discussing what exactly constitutes a good
or bad fuse. In this industry we often hear that the trouble with a defective
product was, "the fuse was bad." I occurred to me that the fuse is not bad,
it performed exactly intended. In fact if the problem that caused the fuse to
"operate" is still present, then the fuse is still good even though it is now
an open circuit. The only time it can be a bad fuse is if it did not operate,
resulting in shock or a fire.
I have now have my ears tuned-in to this concept of a "bad fuse" and find it
is commonly used all over the industry. In fact you can go to any number of
websites that provide trouble shooting notes, and find instructions on how you
can measure a bad fuse from a good fuse using an Ohm meter, photos included.
And some of these instructions are from reputable manufacturers. Another term
often used is "defective fuse", which in some way sounds more scientific, but
is still fundamentally wrong.
I recently saw a newspaper article that gave the explaination why electrical
service was lost for over 100,000 people as a bad fuse. An investigation was
under way to determine why the fuse went bad. This is a little like hearing
the technologically uninitiated say "it must be a short somewhere", when the
television set stops working.
Maybe I am finicky, but this affects how companies view real product defects.
When the "defect" is the bad fuse, then the real problem may be covered up.
Often the answer is, increase the fuse size to prevent nuisance trips. The
risk, of course, is that for every incremental increase in fuse value, you
increase the risk of fire proportionally.
Any thoughts or experiences?
BTW - To all US citizens in the group, have a relaxing Memorial Day weekend.
-doug
Douglas E. Powell
Regulatory Compliance Engineer
Advanced Energy Industries, Inc.
Fort Collins, CO 80535 USA
Doug:
I would submit that a fuse can be "bad" if it opens at too low of a current,
thus creating an improper denial of service.
A fuse may also be "bad" if it does not follow the expected curve of energy
operation for both conductive and interrupted states. An "open" fuse may be
considered a successful application of protection, but, suppose it was a
one-amp rated fuse, was subjected to a 20-amp fault current, yet required 30
seconds to melt and create the interrupted condition. I suspect that most
engineers would expect a one-amp fuse to blow much faster than that.
As usual, it's always more complicated the closer you look at it.
Regards,
Ed Price
ed.pr...@cubic.com
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Is Our Specialty
