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05-10-2019, 03:04 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Larry DeCamp
Why is it inappropriate for two fuses to feed one switch if all the wires and switch are protected by the fuses ?
Similarly, what is wrong with an avionics bus that is fed from adequate wiring and fusing for all loads /wires. ie, if you switch feed a bus from a 10A fuse with wires to all loads sized for 10Amps ?
I currently have all loads separately fused and switched on my flying -3, I am just trying to learn here.
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to be clear, I said don't splice two fused circuits into one single pole switch. (I apparently didn't state SP switch in my post) It is fine and normal to run them into a double pole switch. Don't have time to address the details, but if you want 10 amps coming out of the switch, run one 10 amp fused feed into it, not two 5 amp fused feeds. It's all downside with no upside. It is also fine to run multiple devices from a single pole switch/single feed, assuming switch, wire and fuse can handle the summed load. I never stated otherwise.
I should have been a bit clearer in my guidance.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 05-10-2019 at 03:13 PM.
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05-10-2019, 05:24 PM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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2 fused sources complicate the decision tree. Every wire would need the capacity to handle the sum current of the 2 fuses, and total load on the circuit should be less than the rating on 1 fuse.
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05-10-2019, 05:53 PM
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Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,573
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Quote:
Originally Posted by Larry DeCamp
Why is it inappropriate for two fuses to feed one switch if all the wires and switch are protected by the fuses ?
Similarly, what is wrong with an avionics bus that is fed from adequate wiring and fusing for all loads /wires. ie, if you switch feed a bus from a 10A fuse with wires to all loads sized for 10Amps ?
I currently have all loads separately fused and switched on my flying -3, I am just trying to learn here.
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Larry,
The other Larry may give a better answer, but if you have two fused leads going to a single-pole switch, and if you pull enough current to blow one of the fuses, the other fuse has not blown and so power is not interrupted. I suppose it might not be long before the second fuse blew too.
Now, on the other hand, if you were to have two fused circuits go FROM a single-pole switch to two separate devices, that would be better, but then those fuses do not protect the wire upstream from the switch if something should go wrong at the switch. You could use a toggle switch/breaker for the switch, rated at a little over the combined demands of the two devices, and a fuse for each device downstream of the switch.
__________________
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet"
Hobbs 625
LS6-15/18W sailplane SOLD
bought my old LS6-A back!! 
VAF donation Jan 2020
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05-10-2019, 05:59 PM
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Join Date: Apr 2008
Location: Riley TWP MI
Posts: 3,070
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Fuses should not be connected in parallel because current will not be divided equally between the two fuses due to resistance variations in the wire and terminals and in the fuses themselves. The fuse carrying the most current is likely to blow. When it does, the other fuse will be carrying all of the current and it will also blow.
If two loads are connected to two fuses in parallel, an overload or short in one load will blow both fuses. Now neither load has power.
__________________
Joe Gores
RV-12 Flying
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05-10-2019, 06:18 PM
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Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 1,558
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DPDT switch, keeping the two fused loads separate, or use a relay (available at Steinair), which allows modifications later by you or a future owner when you/he upgrades the panel - which WILL happen. Solid state surface mounted switches on many of our new avionics don?t corrode with disuse.
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SH
RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
Cincinnati, OH/KHAO
JAN2020
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05-10-2019, 06:26 PM
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Join Date: Apr 2005
Location: Indepenence, Oregon
Posts: 342
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I have a four pole single throw switch as my avionics master. Each device is individually fused and wired, intercom, radio 1, radio 2 and transponder. The transponder and radio come from the endurance buss and the others from the normal buss.
This places a lot of trust in a switch, however good quality switches should have a very long life. Relays will have lower reliability. I got it from Digi Key. It is much wider than the normal SPST and has a bigger handle.
Makes wiring easier. All of my power ships have the same arrangement.
__________________
Bruce Patton
Rans S-20 Raven 796S flying since 2019 (slowly)
RV-6A 596S flying since '99 (Sold)
HP-18 5596S flying since '89
RV-10 996S flying since 2014, quick build wing and slow build fues., - dual Skyviews with complete system, two radio and not much else. Interior completely finished with Zolatone. CF plenum. 1624 lbs, FLYING after a 21.5 month build.
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05-13-2019, 11:07 AM
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Join Date: Sep 2014
Location: IN
Posts: 254
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Probably "safer" to use a multi pole switch than to trust a single set of relay contacts powering the entire electronics buss...
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05-14-2019, 10:42 AM
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Join Date: Mar 2010
Location: Scappoose
Posts: 119
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If you think of electrical wire like they are water hoses, it can sometimes make it easier to understand.
If you have two water hoses connected to hose bibs on your house, and then spliced together to a single sprinkler for the garden, the water will flow through both hoses, if you run over one hose with your car and block it off, all of that water will flow through the other hose, Now the sprinkler has half the flow(current), and cant supply enough pressure(voltage) the garden doesn't get watered like it should and will die. Or more likely in this case, blow the fuse on the other side.
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