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mmayfield
Joined: 09 Oct 2009
Posts: 40
Location: NSW Central Coast, Australia
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 Posted: Tue Nov 02, 2010 2:56 pm Post subject: Battery contactor versus circuit breaker |
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OK, so the aircraft we're talking about has a very simple electrical system. Day VFR only, a comm, a transponder, a few miscellaneous widgets like boost pump, smoke pump, engine monitor, a couple of solenoid valves, and the engine is the venerable M14P which is an air start motor. So the battery is also correspondingly small, and recharged by the B&C SK10 alternator.
The traditional basic electrical schematic has an avionics master but I'm discarding this because, well, I just can't see why it is needed at all for this setup. Any issues there?
This diagram is also drawn with no battery contactor, but a main battery circuit breaker instead, and the master switch is 14AWG wire in one side and out the other to the bus, I believe.
How does this rate compared to using a battery contactor instead? I do recall Bob talking of this being a bit of an unconventional layout (regarding a B&C wiring diagram a while back). Would it be better to specify a battery contactor in a conventional arrangement, or for this system would it not matter too much?
Mike
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Wed Nov 03, 2010 5:12 am Post subject: Battery contactor versus circuit breaker |
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Without seeing your schematic it sounds like the main cable coming from your
battery pack has no protection on it until it gets to the panel where the
circuit breaker is. Protection of the main battery cable is the reason that
contactors are installed as close to the battery as possible, especially in
installations where the battery is in the back of the plane. That includes
the starter solenoid and master relay.
Noel
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Noel Loveys
Kitfox III-A
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Wed Nov 03, 2010 5:26 am Post subject: Re: Battery contactor versus circuit breaker |
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Mike,
Be aware that a battery disconnection device that can be operated with one hand is an FAA requirement. A battery contactor is an easy way to achieve this, but you can do it with a big switch too.
I have been recommending a race-car switch for battery disconnection. See "Flaming River" battery switch. Similar switches can be had from Harbor Freight. Removing the handle can offer anti-theft protection too.
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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
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nuckolls.bob(at)aeroelect
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| Posted: Wed Nov 03, 2010 7:37 am Post subject: Battery contactor versus circuit breaker |
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At 08:08 AM 11/3/2010, you wrote:
Without seeing your schematic it sounds like the main cable coming from your
battery pack has no protection on it until it gets to the panel where the
circuit breaker is. Protection of the main battery cable is the reason that
contactors are installed as close to the battery as possible, especially in
installations where the battery is in the back of the plane. That includes
the starter solenoid and master relay.
I'll offer the following expansion of the idea behind
local battery disconnect. It's not so much for "protection"
of any wires as for reduction in probability of post-crash
fire. A powered up electrical system could warm up your
day considerably without damaging a single wire.
A engineer friend of mine at Hawker-Beech used to investigate
accidents involving our products. He once shared an anecdotal
observation suggesting that if the airplane caught fire
after the crash, the battery was most likely still in the
airframe. If it didn't burn, he expected to find the battery
tossed into the woods somewhere.
Obviously not scientific but a valid recognition of the
energy that can be released when a battery is discharged
into the bent-metal products of a crash. Having a pre-crash
procedure that isolates the battery from as much of ship's
wiring as practical goes a long way toward reducing after-crash
hazard.
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 2:03 am Post subject: Re: Battery contactor versus circuit breaker |
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Noel,
on a couple of photos I've seen of what I believe is a similar setup (and on this same plane type), the batt c/b was located right next to the battery where one might otherwise find a battery contactor on other types, thus it would seem to offer supply wire protection.
The schematic is exceedingly simple: Batt, to batt c/b (right next to it), to master switch (panel), to bus (near panel). All in a straight line.
I guess I'm enquiring as to the relative wisdom of this type of main supply-wire protection versus using a contactor. Bear in mind that it's a very simple electrical system in either case.
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.
Mike
Eric - good point. Do you have a reference somewhere?
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Sun Nov 07, 2010 7:57 am Post subject: Battery contactor versus circuit breaker |
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To begin with regardless of the protective device protection is downstream
from the battery after the protection. So in fact the safest thing to do
would be to have the protection actually built into the battery. That not
being available the next best thing is to put the protection device next to
the battery. Cessna for one actually puts the protection right on the
outside of the battery box on the C-180, C-185. The advantage of using a
relay is that if you see a hard landing coming your way you can with the
flick of a switch (on the panel) you can isolate a remote installed battery.
With a circuit breaker in the same location it would require a short to
cause the breaker to open... That could be the spark you really wouldn't
want.
I've always seen the main cable for the starter come directly off the
battery to a very close solenoid ( another relay) then from the solenoid to
the starter. As Bob mentioned last week this doesn't give you any
protection from the solenoid jamming closed which aside leaving the heavy
cable to the starter energized but it would also cause the starter to run
continuously to destruction with no method of shut down. I have seen bendix
units with integral solenoids jam in cars but never the remote solenoid but
it's worth considering what bob mentioned and if you have a master relay
that can handle the current of the starter it would be a good idea to wire
the starter through it as an additional method of stopping the starter if
the starter solenoid jams closed.
So in short you are reading Bob correctly.
BTW you should also have a breaker or at least a switch between the
generator and the panel. In the event of an impending accident battery
isolation is a great thing to have.
Noel
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Noel Loveys
Kitfox III-A
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