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rockitdoc
Joined: 30 Mar 2021
Posts: 6
Location: FORT COLLINS
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 Posted: Tue Nov 22, 2022 6:37 am Post subject: Fuse or CB for largish wires into cabin? |
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I am designing my power distribution for my RV-14A. Single battery, dual alternators, two Dynon HDX1100's and two Pmags. I am using fuses for everything except Pmags and alternators.
I am planning an always hot bus coming off the master contactor before the battery for stuff that would be nice if the contactor fails. Should this wire (10AWG), and for that matter, the wire from the other side of the contactor to the main bus (6AWG), be protected by fuse or CBs since they enter the cabin?
Thanks, in advance.
Scott
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| This is the current iteration of the power distribution. |
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Wiring Diagram v2.5.pdf |
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_________________
RV14A
Began 7-23-20
Fuse and tail Completed 2-20-21
Wings 99%
Finish 90%
Engine 99%
Wiring in Process |
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user9253
Joined: 28 Mar 2008
Posts: 1913
Location: Riley TWP Michigan
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| Posted: Tue Nov 22, 2022 7:31 am Post subject: Re: Fuse or CB for largish wires into cabin? |
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The engine is more likely to quit than the battery contactor is to fail.
After a forced landing, that always hot #10 wire could make sparks which will ignite leaking gasoline.
Having fuses in series is not a good idea. A shorted load could blow both the load
fuse and the main fuse. Do not fuse the 6AWG main bus feeder.
If the battery contactor fails while flying, you might not even know it
because the alternator is supplying electrical power, not the battery.
If you are still concerned about the main contactor failing, see the attached circuit.
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Contactor Bypass.pdf |
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Joe Gores |
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bob.verwey(at)gmail.com
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| Posted: Tue Nov 22, 2022 8:35 am Post subject: Fuse or CB for largish wires into cabin? |
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Joe, slight thread drift..
My BD4 is wired with the battery in the back and a fat wire terminates at a firewall stud.
The Master Contactor is activated by a switched earth wire 20AWG (with in-line fuse as well) at the panel Master Switch.
The concern about unwanted electron flow refers...
In the event of a mishap which mashes wires, if the contactor earth wire is shorted out to the airframe by bent metal, then the fat cable gets energised...and being in close proximity to the earth wire, possibly would suffer a breach, and start "welding", the very situation we are trying to avoid.
Does one fuse the main cable? ...bearing in mind the horrendous amperage demand of an IO470 starter.
On Tue, 22 Nov 2022 at 17:34, user9253 <fransew(at)gmail.com (fransew(at)gmail.com)> wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>
The engine is more likely to quit than the battery contactor is to fail.
After a forced landing, that always hot #10 wire could make sparks which will ignite leaking gasoline.
Having fuses in series is not a good idea. A shorted load could blow both the load
fuse and the main fuse. Do not fuse the 6AWG main bus feeder.
If the battery contactor fails while flying, you might not even know it
because the alternator is supplying electrical power, not the battery.
If you are still concerned about the main contactor failing, see the attached circuit.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=508986#508986
Attachments:
http://forums.matronics.com//files/contactor_bypass_211.pdf
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user9253
Joined: 28 Mar 2008
Posts: 1913
Location: Riley TWP Michigan
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| Posted: Tue Nov 22, 2022 5:25 pm Post subject: Re: Fuse or CB for largish wires into cabin? |
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Don't put a fuse in series with the starter motor circuit.
A fuse has a certain amount of resistance which will reduce engine cranking speed.
As for the main power bus feeder, it is normally not fused, but some builder do install a fuse.
If that fuse fails, the panel goes dark. Good workmanship will minimize the danger.
Adding extra components complicates the electrical system and increases the danger.
If the bus feeder shorts out, the pilot can shut off the battery contactor.
Your concern about the battery contactor coil energizing wire accidentally
shorting to ground is valid. But I don't know of a safer way to wire it.
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