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1. 09:49 AM - FUSIBLE LINKS & CURRENT LIMITERS (Robert L. Nuckolls, III)
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| Subject: | FUSIBLE LINKS & CURRENT LIMITERS |
Hi Bob,
Currently studying fusible links. It's straight forward circuit
protection but I'm confused about an example in your book. The third
paragraph of Note 4 on page Z7 references fusible links in the shunts
on figure Z14, and the fusible lengths are clearly shown in the
diagram. However, in the 5th paragraph you reference the
fusible-link in the AUX alternator B-lead feed but I don't see any
such link in the diagram. I do see what appears to be a fuse labeled
MAX30 at the aux batt contactor. Is this the fusible link?
While the MAX30 is fusible circuit protection it is not, by definition,
a fusible link. It is a member of a robust MAX family of plastic
fuses.
Note 10 says alternator B-lead protection is best done with current
limiters. Is there any difference between a current limiter and a
fusible link?
Only in their general time-to-function characteristics.
Fusible links are generally rather slow to operate and
are intended to stand off effects of a 'hard' (typically
battery driven) faults. They are quite robust and unlike
fuses, are not vulnerable to thermal "hammering".
Yes, a MAX30 fuse is shown in the b-lead of a 20A alternator
(de-rated by 30% or so). Fusible links are generally
preferred in this application. Stone simple, low parts
count, spliced into the feeder with reliability exceeded
only by the wire itself. But de-rated fuse serves the
same design goal even tho clumsy to implement
compared to the fusible link.
And finally, help me understand the fusible link between the buss and
master switch in Z11, but there is no fusible link in this circuit in Z12.
Are you talking about the link protecting the feeder
out to the alternator field circuit breaker? There's
a link in that feeder for both drawings.
A lot to absorb, trying to understand as best I can. Thanks again.
No problem.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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