nuckolls.bob(at)aeroelect
Guest
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 Posted: Wed Mar 04, 2009 6:26 am Post subject: Diode across main contactor fat terminals |
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At 08:55 AM 3/1/2009, you wrote:
| Quote: |
On May 23, 2008, I posted a hypothetical question about Z-19 main
battery charging current back feeding through the endurance bus and
blowing fuses in the event that the main battery contactor fails and
the pilot closes the endurance bus alternate feed switch.
http://forums.matronics.com/viewtopic.php?p=184591&sid=86e2d2ac21d95a18ce267b57c4fcd85e
To prevent the above scenario, what if a diode were placed across
the main contactor fat terminals? If the contactor fails, this
diode would prevent the main battery from discharging. What are the
disadvantages?
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The only time you can back-feed the e-bus to the battery
with energy from the main bus is if the alternator is running.
The progressive transfer switch used to orchestrate main
battery and alternator operations precludes having the
alternator ON with the battery OFF.
| Quote: | If this diode shorted out (diode failures are rare), then the
pilot would lose the ability to shut off the master switch in case
of an electrical fire. However, current would be limited by the
lead size of the diode, similar to a fusible link. The chances of
an electrical fire and the diode failing on the same flight are
extremely remote. If one is concerned that the diode failure would
go unnoticed at engine shut down, then a warning chime could be
connected to terminal 3 of the master switch. I think the odds of
the main contactor failing and associated pilot error (with
dangerous consequences) are greater than the odds of a diode
failing (with minimal consequences). What are your thoughts?
Joe
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Adding a diode across the contactor defeats the purpose
of the contactor which is to provide complete disconnect
from the system when in the OFF position. Given that
your hypothesis for back-feeding the alternate feed path
is addressed by the configuration of the DC POWER MASTER
switch, I think your concerns are without foundation.
Bob . . .
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( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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