nuckolls.bob(at)cox.net Guest
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Posted: Fri Nov 07, 2008 5:51 am Post subject: ANL current limiter location on rear battery installatio |
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At 05:44 PM 11/6/2008 -0500, you wrote:
Quote: |
Ahhh, I see. I took your stance to be that the ANL fuse is a much required
item in our OBAM systems. I now take you to mean them to be fine to add, but
not wholly necessary to safety. Correct?
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It's not an easy thing to decide. Everything we do
in terms of refining a design go to the problems
of lowering failure rates (lowered risk and cost
of ownership), reducing cost to manufacture
(cost of ownership) and raising performance.
Quote: | So having no circuit protection in both/either the alternator and/or starter
circuit in a competently-designed system is ok? Hard to swallow, but that's
why I'm asking.
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This is an excellent example of the value of studying
the history of our craft and gleaning understanding of
how ideas and products evolved from the time Wallace,
Piper, Beech and others bolted their first batteries and
generators onto a production aircraft.
The question to be asked and answered is "protection from
what?" Obviously, shorted or overloaded wires represent some
hazard to overall functionality of the system. The idea
of protection is to limit risk by limiting loss of function
to the faulted pathway thus preventing the fault from
propagating to other parts of the system.
Quote: | I have an ANL fuse and holder, but in the interest of fewer connections
(hence less to go wrong), I may forego it.
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Given the very short transition between your alternator
b-lead terminal and the starter terminal, it's EASY to
make sure that wire is never faulted to ground. So the
risk to be mitigated is taking a system down due to shorted
diodes in the alternator. Let us assume you subscribe to the
idea of keeping a robust battery on board. Probability is
that shorted diodes will produce fault currents within
the alternator that will fuse internal stator wires
which are on the same order of robustness as the ANL
limiters. See:
http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ANL_Specs.pdf
These guys take a LOT of current to open them . . . the
kind of current that you can expect from a battery that
is maintained for the purposes of running your e-bus for
duration of fuel aboard. So leaving the ANL out of this
lead raises no specter of risk.
Now, run that battery until it doesn't crank the engine
any more and it's conceivable that a shorted set of
diodes could put a load on the system that the battery
cannot support and everything comes down. Having an
ANL in the b-lead probably wouldn't have made any difference.
The point is that if you procure, craft, and operate
a system from the perspective of meeting design goals
and understanding how that's accomplished, your risks
go WAY down compared to the vast majority of our
flying brothers who only pull levers and twist knobs.
Bob . . .
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