Doug,
Linguistically, how can an inanimate object such as a fuse be "good" or
"bad"? Can it also be "angelic" or "evil"?
I may be able to train a dog by telling it that it is a "good boy" when it
does what I want it to do (plus offer a suitable reward, of course) or
prevent it from not doing what I want by telling it that it is a "bad boy"
at the right moment. However, as far as I know, it is not possible to
improve the reliability of a series of fuses by getting hold of each fuse
that has blown and telling it that it has been a "bad boy" - even if you
also tell all the fuses that haven't blown that they also "good boys".
Perhaps it is necessary to catch the fuse in the act of going "bad" (e.g.
blowing when its specification says it shouldn't?).
It seems to that others in this forum have many people have anecdotes
regarding incorrect collection of the fuse in the first case. A couple of
other common design drop-offs I would like to mention include failure to
select a fuse with an adequate prospective fault current (i.e. selecting a
5x20 mm glass fuse - rated at Isc=35A max and using it in a mains circuit
where the prospective fault is in the order of 1kA) and 'forgetting' to
de-rate the fuse for use in a high ambient temperature (although this has
already be mentioned re. pico-fuses).
Not in this forum, but I have been informed by other reliable sources that
it is possible to purchase counterfeited products that may look as though
they come from a reputable manufacturer, but in fact do not. Certainly, the
UK has had to withdraw mains plugs that were inadequate and I have also
heard from reputable sources of high rupture capacity fuses that do not
contain and sand and where the fusing element is of the wrong type and
configuration of metal - meaning that they could explode if subjected to the
prospective fault current that such fuses are supposed to handle. It could
be that the faulty components that John Woodgate mentioned fall into this
category.
So, we have fuses that do what they are intended to do and go open circuit
when an over-current is passed through them. Such components are well
behaved; good fuse, good designer.
We have fuses that behave as their manufacturer said they would, but
unfortunately the designer selected the part incorrectly; good fuse, bad
designer.
We have fuses that do not behave as expected because they are not purchased
>from the expected manufacture; bad fuse, good designer, {naughty purchasing
dept?}
We have fuses that fail due to excessive vibration {but are the fuses
specified to operate under those conditions?}; possibly naughty fuse,
possibly naughty designer.
Oh dear, I've fallen into the trap of ascribing fuses with some kind of
will. Oh well, at least I'm not alone...
Richard Hughes
Safety Answers Ltd.
From: ed.pr...@cubic.com [mailto:ed.pr...@cubic.com]
Sent: Friday, May 23, 2003 11:42 PM
To: emc-p...@ieee.org
Subject: RE: Bad Fuse vs. Good Fuse
-----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
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