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1. 08:47 AM - Switch Schemes for Reliability (wsimpso1)
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| Subject: | Switch Schemes for Reliability |
We all know we need backups on any circuit needed for comfortable completion of
a flight. I aimed for that. I have several circuits that fit that need, things
like pumps and ECU's and Ignition Coils. I am Installing Z-14 (IFR and Electrically
Dependant Engine). I would like your review and criticisms of the schemes
offered, and any alternatives you can offer too.
I have two of each important devices (pumps, coil power, ECU's, injector power)
and two always hot buses. I suspect we benefit from being able to connect either
pump to either bus, and have two independant switches/circuits to do so too.
For take-off and landing, many of us will want both pumps (or other paired
devices) running, so I am looking to that as well.
The clever way seemed to be two 700-2-10's for each device pair, in parallel, with
one connected to the Main Battery Bus, the other to the Aux Battery Bus. I
KNEW that gave me an error state that we would not do deliberately but which
was likely to occur: both buses running one pump. Upon first review with both
buses hot, this is a "so what?" with the only loss being only one pump on when
we were trying for both. But when one bus has gone cold, we are then trying
to power everything left On on that cold bus through the fuse and switch for the
Hot bus. Sure, if we REMEMBER to close the Bus-Tie first and both buses do
go hot, no problem, but if one bus is still cold, we lose access to pump. One
more out of order switch throw and we are out both pumps or coils or .... BAAAD.
Yes, I have run Fault Tree Analysis and then FMEA for my base scheme the way I
was taught by duPont using order of magnitude estimates with detectability, severity,
and failure probability. I plan to run the exercises on the alternatives
too. Fault Tree Analysis and Failure Modes and Effects Analysis are very useful
tools for estimating if one scheme is better or worse in total.
So, what are the alternatives?
Two SPST switches in parallel for each function, each connecting one bus to one
pump. That costs me half of the bus-pump possibilities, which sounds detrimental.
Series switch set that uses a 2-10 sequenced to a 2-1 that let's me connect either
bus to either pump and to run both pumps, without running one pump on both
buses simultaneously. This looks worse with two switches in series - instead
of having a SPOF, I have doubled it - UGH.
The scheme I like is paired 700-2-1's for each function with one switch per pump.
This allows either bus to power each pump. No cross bus connection is possible,
each pump (or other device) is separately powered by either bus, and there
is always the other pump.
Thoughts please on all of these schemes and my judgment here... If you have a better
scheme to be replicated several times in my panel to allow running both
devices from either bus, I am all ears.
Bill
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=513463#513463
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