nuckolls.bob(at)aeroelect
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 Posted: Sun Sep 15, 2013 3:41 pm Post subject: Why does a master switch turn off devices? |
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Oops, sorry for the double posting of Part I. Here's
the rest of the story about energy management switches
in an airplane whether or not they are called "master
switches".
Consider this part of FAR23.1361
(b) Load circuits may be connected so that they remain energized when the
master switch is open, if the circuits are isolated, or physically shielded,
to prevent their igniting flammable fluids or vapors that might be liberated
by the leakage or rupture of any flammable fluid system; and
(1) The circuits are required for continued operation of the engine; or
(2) The circuits are protected by circuit protective devices with a rating
of five amperes or less adjacent to the electric power source.
(3) In addition, two or more circuits installed in accordance with the
requirements of paragraph (b)(2) of this section must not be used to supply a
load of more than five amperes.
This section goes to issues that took root in
OBAM aviation, the electrically dependent engine.
Some years back we began to consider and expanded role
for the BATTERY BUS in powering electronic ignition
systems and/or fuel pumps necessary for engine operation.
The combination of switches one might call 'masters'
is set apart from engine critical circuits. For crash
safety, the FAA has embraced a "5A max" always hot
feeders for a variety of tasks not the least of which
was engine support. Paragraph (b)(3) seems to suggest
that the sum total of power feeders taken directly
from the battery bus should not exceed 5A . . . not
sure of how this might be interpreted by the
regulatory guru's I've known.
Since FAR23 does not apply to our endeavors (at least
not today) then the significance of (b)(3) is moot.
Nevertheless, battery bus feeders greater than 5A
might best be fitted with a disconnect relay at the
bus . . . not unlike Figure Z-32
http://tinyurl.com/756ydec
where it is shown as an auxiliary feed path to the e-bus.
That same relay might power a fuel injection pump
and be fused at much greater than 5A.
The point of this semantic hat dance is to illustrate
the wide range of power control switches that might
find their way onto the panel of an airplane and to
emphasize the fact that they might be called something
other than a 'master switch'.
Irrespective of what the switch is called, legacy
design goals for making the electrical system go max-dark
with a minimum of effort/confusion at the pilot's
discretion is worthy of the builder's consideration
Bob . . . [quote][b]
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