nuckolls.bob(at)aeroelect
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 Posted: Tue Jan 12, 2010 6:27 pm Post subject: fuses vs. breakers "aging"? |
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This seems to contradict my learning/memory but after 71 years,
either may be suspect. When asked, I had always advised that
fuses did 'not' suffer from memory whereas breakers did 'age'.
Can you point me in a direction to clarify my thoughts?
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Sure.
If you put a brand new breaker and brand new fuse
on the shelf for 20 years and then test them, you
wouldn't see a measurable shift in fuse performance
and you might see a tiny shift in the breaker. (This
presumes, of course, that you could "test" the fuse
before you stored it away!.)
The breaker is an electro-mechanical device and
therefore subject to some effects of age . . . but
I've never found an old breaker from my junk box
(some more than 40 years old!) that was not still
suited to task after being in service, removed and
stored. Fuses have no moving parts and as long as
they're not stored in a horribly corrosive environment,
there's nothing to "age".
However, there ARE effects of service stress that may
affect fuses and breakers differently . . .
Have you ever pulled a glass cartridge fuse and
seen a "sag" in the fusible element running down
the center? A fuse is a one-time, melting (fusing)
thing. When operated in a manner that bangs the
edge of its heating characteristics, the element
can become 'plastic' and change shape without going
all the way. Any change in shape during a transient
event ALWAYS works to reduce the I(squared)T energy
needed to trip the fuse . . . i.e. it drifts downward
with repeated service stresses over time.
This is why the fuse engineering catalogs suggest
that a fuse be operated at no more than 75% of
rating . . . less is even better if the system
whacks the fuse hard or often with transient
events. A typical 'fast' fuse is the glass cartridge
ACG series from Bussmann:
http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/AGC_Specs.pdf
Note on page 2 of the spec sheet that a 1A fuse will
carry 1A forever at room temperature but takes 200
seconds max at 2A. Suppose you whacked it with 5A
for say 10 milliseconds every time you turned on a
devise that draws only 1/2A but has some fat capacitors
across the input. Notice on page 2 of the spec that
on average, a 5A load will open the fuse in 50 milliseconds.
Suppose your electro-whizzie hits the fuse at 5 a for
10 milliseconds. Way too small to open it for one
hit or perhaps even dozens of hits. But each hit can
cause the fuse element to change shape slightly to
the point where it may ultimately fail to carry rated load
any more.
It's hard to do this on purpose and it's very rare
accidently but it has happened. This is why you
can buy fuses with "slow blow" or current limiters
with "long blow" characteristics.
This is a phenomenon that does not plague breakers
especially those neat little double break minatures
that we're all so fond of.
So when one speaks of the aging of these components,
you have to differentiate between effects of time
under normal conditions or the effects of stress
under transient overload.
Bob . . .
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