Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!ub!dsinc!netnews.upenn.edu!eniac.seas.upenn.edu!depolo
From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: Can fuses get "worn out"?
Message-ID: <34791@netnews.upenn.edu>
Date: 14 Dec 90 19:23:34 GMT
References: <1990Dec14.171020.3615@phri.nyu.edu>
Sender: news@netnews.upenn.edu
Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Organization: University of Pennsylvania
Lines: 28

Fuses can "wear out" for a couple of reasons.

If a fuse starts to blow, but never blows completely, it usually ends
up having a thin center portion of the fuse as compared to the ends.
This means that the temperature will tend to be higher in the center
of the fuse, so it will take a lower current than what it's rated
for to blow the next time full current is pulled through the fuse.

Also, as fuses get hotter and then cool, they suffer from thermal
expansion and contraction.  This can cause fractures and weakens
the fuse.

Also, if the cartridge isn't air-tight, the glowing phenomenon
that occurs when the current is high enough to cause the fuse
to glow bright red but not enough to make it melt will
tend to cause the fuse material to oxidize faster, making it's 
resistance increase, which in turn makes it hotter for any given
current flow so that eventually it will blow, even when drawing
under its rating.

There are probably other reasons that I didn't think of.

								--- Jeff
--
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
 depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
 University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104