From: marv@lancair.net
Subject: Re: Reliability Question
To: lml
Date: Mon, 31 Dec 2007 14:37:03 -0500
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In-Reply-To: <39B0C804-6549-412C-A348-3885B8C13053@adelphia.net>
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 <39B0C804-6549-412C-A348-3885B8C13053@adelphia.net>
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Posted for Gary Casey <glcasey@adelphia.net>:

  A couple of opinions:
  If I were going to use an electronic device for a backup system  (that's not
quite a true statement:  I wouldn't) I would want to see  an instrument that
is completely independent.  It would have to be in  a metal box and be battery
powered with the battery inside the box.   I suppose that means it would have
to be normally off and then be  turned on only in case of failure of the
aircraft electrical system.   If there are any wires in common there could be
a problem, even if  they are not connected at the time of the over-voltage
event.  Wires  running parallel in a harness could induce voltages in each
other by  capacitive coupling, inductive couple or by arcing.  Even if the
  wires are heavily insulated a single pinhole could initiate an arc at  the
wrong time.  I agree with Brent - it's not the "reliability" of  each system
that counts, it's the "fault tolerance" and that is where  multiple systems
can fail from a common event.
  
  On the other question about dual electronic ignitions:
  I was thinking about building mine with a dual electronic ignition  and the
power source that I finally came up with (before I abandoned  the idea in
favor of using one mag) was to use a separate battery and  alternator for one
of the ignitions. One ignition would be powered by  the main battery (not
through the master contactor, but directly off  the battery) and the other
would be powered by its dedicated battery  and alternator.  There would be no
interconnect at all, ever.  If  either battery failed one ignition system
would fail, but there could  be no possibility of one failure leading to the
other side coming  down as well.  I've heard stories of complex electrical
system (in  airliners too) where systems were manually interconnected
following a  failure only to have the bad system pull the good one down with
it.
  
  Best regards to all for the new year,
  Gary Casey