Hi Doug,
In my days in reliability engineering, this discussion came up many times
when we were doing FMECA or fault tree analysis. Try posing the question
"what is the failure mode(s) of a fuse? " to a hardware designer. The
immediate answer you get is " When it goes open circuit". As you have
correctly noted in your posting, this is the wrong answer.
As we all know, a fuse is a protective device designed to go open circuit
under specified current conditions. A fuse fails when it doesn't go open
circuit under the specified current conditions. However the answer is not
is not as simple as it first looks. We have all seen cases where a fuse
blows when there is no apparent fault in the equipment, hence the
maintainer's comment "bad fuse". In my experience, this is usually caused
by a temporary overload condition or a power surge, not a "bad" fuse. Fuse
operation when there is no fault can also sometimes be caused by incorrect
specification at the design stage, i.e. the fuse is rated too low for the
actual current consumed. AIUI, there is a specified time it takes the fuse
to operate at 200% of the rated current, so for example a 1A normal blow
fuse requires 2A for approximately 10 seconds to go open circuit. This time
reduces exponentially as the current increases. There are probably some
fuse manufacturers on this forum who can correct me on this. At 135% of the
rated current, a fuse will not operate almost indefinitely, though there is
some heating and it will eventually go open due to metal fatigue of the
wire.
Field failure reports of "bad fuse" are very misleading, because you really
do not know what caused the fault. You can be almost certain that it was
not caused by a "bad" fuse.
Regards
Doug
[Doug's of the world unite]
"POWELL, DOUG"
<doug.pow...@aei.com> To: "EMC-PSTC (E-mail)"
<emc-p...@ieee.org>
Sent by: cc:
owner-emc-pstc@majordom Subject: Bad Fuse vs.
Good Fuse
o.ieee.org
05/23/03 06:56 PM
Please respond to
"POWELL, DOUG"
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
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