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nuckolls.bob(at)aeroelect
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 Posted: Thu Nov 04, 2010 5:49 am Post subject: Battery contactor versus circuit breaker |
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| Quote: |
If I'm reading what Bob wrote correctly, he is saying that a
contactor allows virtual complete isolation of the battery in an
emergency and consequent reduction of hazard/risk, and thus might be
preferable to a non-accessible battery circuit breaker in even such
a simple system with a low power alternator as this
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Correct. But if you're considering a pullable C/B as the
battery disconnect device, do you also intend to crank an
engine through this 'switch'? That takes a BIG breaker
and is generally pretty bulky and expensive and serves
so useful purpose for the 'protection' of wires.
Battery and cranking circuits on light aircraft do not
generally get protection from overload. Hence, you do
not find protective devices in this service on any
certified aircraft and exceedingly few OBAM aircraft.
In fact, the FARs speak specifically to this issue
and relieve the designer of adding any such 'protection'.
But crew accessible battery disconnect is another matter.
Many airplanes of yesteryear used the hand operated
switch. A TriPacer I took dual instruction in had the
battery under the passenger seat . . . batter switch
was on the closeout behind the pilots calves as was
a manual push button for the starter. Battery contactors
became useful when batteries were remotely located.
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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mmayfield
Joined: 09 Oct 2009
Posts: 40
Location: NSW Central Coast, Australia
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| Posted: Thu Nov 04, 2010 2:10 pm Post subject: Re: Battery contactor versus circuit breaker |
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No Bob, the engine is an air-start engine. The start sequence is nothing more than energising a booster coil and opening a solenoid valve for about 2 seconds to allow the compressed air in. Starter contactors and cranking currents don't exist on this plane.
It really sounds from what I'm reading that a battery contactor is the more desirable and "elegant" solution from a number of different aspects, rather than a circuit breaker purely serving as "automatic" battery wiring protection.
I know the diagram was only ever intended as a very rough guide for the builder, but I can only speculate why it was drawn with a breaker on the supply side instead of a battery contactor.
regards,
Mike
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nuckolls.bob(at)aeroelect
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| Posted: Fri Nov 05, 2010 5:25 am Post subject: Battery contactor versus circuit breaker |
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| Quote: |
It really sounds from what I'm reading that a battery contactor is
the more desirable and "elegant" solution from a number of different
aspects, rather than a circuit breaker purely serving as "automatic"
battery wiring protection.
I know the diagram was only ever intended as a very rough guide for
the builder, but I can only speculate why it was drawn with a
breaker on the supply side instead of a battery contactor.
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Most folks skilled at airframe and power plants
are not widely read or experienced in electrical
systems. While suggested architectures and diagrams
are always functional at some level, they almost
never embody the best we know how to do nor are
they tailored to the builder's mission profile and
design goals.
It's not uncommon to find 'suggestions' that really
stretch the imagination of the reader to deduce
the simple-ideas that support the feature's
incorporation.
Whether or not to go the contactor route is driven
by two major considerations. Location of the switching
device for convenience of operation and/or power
savings when the engine driven power source or
battery size is small. A 0.8A contactor current draw
is no big deal when a 60A alternator is keeping
the lights lit. But when the alternator is good
for say 8A or you're trying to extend battery-only
operations, elimination of a contactor has benefits.
So it isn't a one-size-fits-all thought exercise.
Bob . . .
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