Dan,
So far there hasn't been mention of the mechanical stress aspect of fuse
failure related to the fuse wire breaking because of repeated cycling of
power. I have personally seen this happen at a commercial daytime radio
station. It took over a year for a properly rated fuse to blow because the
transmitter was turned on and off daily; the fuse wire was simply
mechanically flexed because of slight heating and finally went. Yes, it
blew again about a year after it was replaced. Maybe the transmitter
manufacturer should have specified a slow-blow fuse.
Frank de Vall
Manager Compliance Engineering
Assa Abloy ITG & HID Corporation
"Giblin, Dan"
<dgib...@buss.com> To:
"'emc-p...@majordomo.ieee.org'"
Sent by:
<emc-p...@majordomo.ieee.org>, "'POWELL, DOUG'"
owner-emc-pstc@majordo <doug.pow...@aei.com>
mo.ieee.org cc:
Subject: RE: Bad Fuse vs.
Good Fuse
05/27/2003 02:17 PM
Please respond to
"Giblin, Dan"
As a fuse manufacturer, I have read all the e-mail on this subject with
great interest. Unfortunately, many responses emphasize common
misunderstandings.
In the fuse industry, there is no such thing as a "Bad" fuse. The chain of
e-mails clearly indicate why we avoid this word due to its various
meanings. Similarly, we try to avoid the word 'failure'. A "bad" or
'failed' fuse can mean any number of things, but the most common is that
the fuse is open, (it operated). Thus, the fuse did what it was supposed
to do - operate - and yet it is deemed 'bad' or a 'failure'.
There are numerous occasions when a customer returns a 'bad' fuse. The
fuse was 'bad' because it operated properly. In many instances, the choice
of fuse type or amperage rating was flawed.
For being such a 'simple' device, there are many items that influence the
correct choice. I will address some of these by picking quotes out of the
various e-mail on the subject. Among these are:
Voltage rating - (This was addressed in Mr. Richmond's e-mail - I've
heard of a "bad" fuse, in this case, a fuse not adequate to protect the
circuit and user. A 30 volt fuse can't be counted on to interrupt a 408
volt circuit. This is absolutely correct. Choosing a voltage rating
below your application can be very dangerous.
Continuous Current rating - This was addressed in Mr. Beckwith's e-mail
- 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. Very true. One must look at
surges mentioned below and temperature.
Temporary Surges that you want the fuse to pass - A time delay fuse is
typically chosen for motor or transformer applications. This is
addressed a little in Mr. Beckwith's e-mail - 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.
Temporary Surges that you do not want the fuse to pass.
Ambient temperature rating - This was addressed in Mr. Hughes e-mail
below.
Interrupting Rating - This was addressed in Mr. Hughes e-mail - 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).
Shock/Vibration - Most fuses do not claim to be suitable for such
applications. From Mr. Schlentz - I have seen one instance where a fuse
opened during a vibration test. I think that was a BAD FUSE. I don't
know if the fuse in question was supposed to be suitable or not.
If chosen incorrectly, and 'nuisance' opening occurs, as Mr. Powell
mentions - 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. In doing this, you
may be defeating the purpose of having the protection there in the first
place. If a fuse nuisance opens, it is likely that one of the above
bulleted items was overlooked.
Additionally, as Mr. Richmond points out - Many fuses ARE mechanically
fragile; it's simple physics that says a low-current fuse will be a fragile
thread. Think of them as light bulbs. We do not call a light bulb
unreliable if it fails after being dropped on the floor; we call it broken.
This is particularly true for low amperage fuses.
As Mr. Beckwith points out - 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
different opening time requirements depending on the type of fuse, (Time
Delay versus Fast Acting), and its physical size. The 135%, 200% and/or
500% opening times are documented in the tri-national, harmonized 248
Series of fuse Standards, (248.1 through 248.16 - The standards are UL,
CSA, and ANCE).
If anyone has any specific questions, or would like more information, send
me a separate e-mail and I'll see what I can do.
Dan
Manager, Industry Standards & Certifications
Cooper Bussmann
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