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  #51  
Old 06-07-2020, 11:36 AM
johnbright's Avatar
johnbright johnbright is offline
 
Join Date: Aug 2010
Location: Newport News, Va
Posts: 389
Default in the spirit of FAR 23.1361

Quote:
Originally Posted by mfleming View Post
My Final iteration DWG F105 ends up not having a battery bus.

Somewhere along the line Bob N. suggested a I put an inline fuse for the clearance delivery bus since it drew 3.2 amps. My thinking on the CD bus relay was similar. Since the CD bus amp draw was so low....simplify.

The SureFly will indeed be on the left side...I'll change that on the drawing. And thank you for the B.N. article on mag switches.
FYI Bob Nuckolls is tweaking Z101. As always, find the latest at http://www.aeroelectric.com/PPS/Adob...itecture_Pdfs/

Bob said you can delete the battery bus and use an in-line fuse holder for the CD bus but Bob would use a relay to switch the CD bus if the fuse is over 7.5A. Current draw does not matter, only fuse size, to limit energy delivered remote from the battery in case of accident or service preformed without disconnecting battery negative terminal.

Littlefuse advises fuse size should be, at minimum:
  • Operating current/(.75 x temperature factor).
  • 0.75 is an overhead factor to prevent nuisance blowing.
  • If we say 170F for your inline fuse on the firewall, that makes temp factor 0.92 for an ATOF blade fuse.
  • So fuse size should be minimum of 3.2/(.75*0.92) = 4.6A.
  • This is without considering any current pulses above 3.2A.
  • Therefore one could change the 15A CD bus fuse to 7.5A and have a robust fuse with no relay... except, as pointed out in the next post, the 7.5A fuse in the feed wire would be in series with the 7.5A fuse for the comm radio on the CD bus so that is not going to work.
  • Ref Littlefuse documents "Fuseology Selection Guide" and "littelfuse_atof_datasheet.pdf"

It's a great article Bob wrote "Magneto Switch Options", one of many great articles from him.
.
.
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John Bright, RV-6A, N1921R reserved, at FWF
O-360, 8.5:1, vert sump, dual SDS EFI EM-5-F
Z101 as a template, links

Last edited by johnbright : 06-07-2020 at 05:37 PM.
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  #52  
Old 06-07-2020, 05:24 PM
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mfleming mfleming is offline
 
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Location: Joseph, Oregon
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Default

Quote:
Originally Posted by johnbright View Post
FYI Bob Nuckolls is tweaking Z101. As always, find the latest at http://www.aeroelectric.com/PPS/Adob...itecture_Pdfs/

Bob said you can delete the battery bus and use an in-line fuse holder for the CD bus but Bob would use a relay to switch the CD bus if the fuse is over 7.5A. Current draw does not matter, only fuse size, to limit energy delivered remote from the battery in case of accident or service preformed without disconnecting battery negative terminal.

Littlefuse advises fuse size should be, at minimum:
  • Operating current/(.75 x temperature factor).
  • 0.75 is an overhead factor to prevent nuisance blowing.
  • If we say 170F for your inline fuse on the firewall, that makes temp factor 0.92 for an ATOF blade fuse.
  • So fuse size should be minimum of 3.2/(.75*0.92) = 4.6A.
  • This is without considering any current pulses above 3.2A.
  • Therefore you could change your 15A CD bus fuse to 7.5A and have a robust fuse with no relay.
  • Ref Littlefuse documents "Fuseology Selection Guide" and "littelfuse_atof_datasheet.pdf"
..........SNIP.........
.
Reading FAR 23.1361, a inline fuse of 5 amps would be the maximum allowable for a certified AC in this scenario. The 7.5 amp inline fuse would be more in the spirit of rule as opposed to a 15 amp fuse.

This fuse calculation would have a 7.5 amp inline fuse with a 7.5 amp fuse downstream for the comm. I'm not sure if thats workable?

Besides simplifying, the main reason I deleted the CD bus relay is that Bob shows wires to and from the relay of 6" or less. This would put the relay and CD bus mounted to the cockpit side of the firewall. This is not an option I'm interested in. My plans are to make a drop down shelf mounted to the sub-panel with all fuse blocks (busses) together for easy accessibility.

Now, personally, I think it's an error to show a 6" or less length of 16AWG wire from the CD relay to the bus on Z101BP3. There is a "B" next to the star and I can't find a legend that specifies what that means. BUT, if this relay from B&C is acceptable on the engine side of the firewall AND the 16AWG wire from the relay to the CD bus was not limited to 6" or less, that design would definitely comply with FAR 23.1361 and be acceptable to me.

My thinking on the 6" or less 16AWG from the relay to the bus:
The 6AWG wire from the battery contactor to the main bus is not similarly restricted in length.
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Joseph, OR
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RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring and Avionics harness completed 9/2/2021
FWF Started 9/3/2021

Donated for 2021 and so should you

Last edited by mfleming : 06-07-2020 at 05:28 PM. Reason: fix typos
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  #53  
Old 06-07-2020, 06:10 PM
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johnbright johnbright is offline
 
Join Date: Aug 2010
Location: Newport News, Va
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Quote:
Originally Posted by mfleming View Post
Reading FAR 23.1361, a inline fuse of 5 amps would be the maximum allowable for a certified AC in this scenario. The 7.5 amp inline fuse would be more in the spirit of rule as opposed to a 15 amp fuse.

This fuse calculation would have a 7.5 amp inline fuse with a 7.5 amp fuse downstream for the comm. I'm not sure if thats workable?

Besides simplifying, the main reason I deleted the CD bus relay is that Bob shows wires to and from the relay of 6" or less. This would put the relay and CD bus mounted to the cockpit side of the firewall. This is not an option I'm interested in. My plans are to make a drop down shelf mounted to the sub-panel with all fuse blocks (busses) together for easy accessibility.

Now, personally, I think it's an error to show a 6" or less length of 16AWG wire from the CD relay to the bus on Z101BP3. There is a "B" next to the star and I can't find a legend that specifies what that means. BUT, if this relay from B&C is acceptable on the engine side of the firewall AND the 16AWG wire from the relay to the CD bus was not limited to 6" or less, that design would definitely comply with FAR 23.1361 and be acceptable to me.

My thinking on the 6" or less 16AWG from the relay to the bus:
The 6AWG wire from the battery contactor to the main bus is not similarly restricted in length.
Bob follows what I refer to as the "spirit" of FAR 23.1361. He allows a CB of 5A and a fuse of 7.5A because fuses are faster, they take less energy IsquaredR to open.

Sorry, I didn't take fuse sizes on the CD bus into account. Two same-size fuses in series is a very bad idea.

The "B" next to the wire from CD bus relay to CD bus means there was a change with rev B of the dwg. In this case he changed 18 awg to 16 awg. I see at least two other changes he neglected to label with a "B". Iteration B is pending, BP3 is 3rd revision of B Pending.

It looks to me as though Z101BP3 has the CD bus adjacent the battery. No fuse or fuselink in the feeder. All Z dwgs are templates open to modification though.

The B&C relay you linked to, S8005-1, is the new stock number for the S704-1 Bob shows on Z101.
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O-360, 8.5:1, vert sump, dual SDS EFI EM-5-F
Z101 as a template, links

Last edited by johnbright : 06-07-2020 at 09:39 PM.
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  #54  
Old 06-07-2020, 08:59 PM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
Posts: 3,234
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Two of the same size fuses in series is a very bad idea. In fact, two of any size fuses in
series is not a good idea. Bob N is now recommending a fuselink for upstream bus protection.
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  #55  
Old 06-08-2020, 12:59 PM
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mfleming mfleming is offline
 
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Location: Joseph, Oregon
Posts: 798
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Quote:
Originally Posted by Mich48041 View Post
Two of the same size fuses in series is a very bad idea. In fact, two of any size fuses in
series is not a good idea. Bob N is now recommending a fuselink for upstream bus protection.
Joe, this response is not aimed at you. Many people have said the same thing and I'm interested in exploring this concept. Is it Dogma or is there sound reasons for not having fuses in series.

It seems to be common knowledge that having fuses in series is a bad thing but why? 43.13-1B doesn't say a thing pro or con about fuses in series or parallel, nor due any of the other FAA Advisory Circulars I searched.

I can't seem to find it now but Bob Nuckolls wrote an article on how fuses in series were a common thing and not detrimental to a electrical system. They just need to be well thought out. Just the other day Bob recommended I have an inline fuse for my clearance delivery bus that of course had fuses downstream of the inline one.

As far as two fuses of the same size in series...My question is why is this bad? I can see it as a nuisance maybe but I can't see something harmful happening.

As I said in the beginning, this is a open question and not directed at any individual.
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Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring and Avionics harness completed 9/2/2021
FWF Started 9/3/2021

Donated for 2021 and so should you

Last edited by mfleming : 06-08-2020 at 01:07 PM. Reason: Fix grammar
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  #56  
Old 06-08-2020, 06:52 PM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
Posts: 3,234
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The reason two fuses should not be connected in series is that you never
know which one will blow or both. If one of those fuses is in series with a
bus and that fuse blows, then the whole bus goes dead. What if the fuses
are two different sizes? Wouldn't only the smaller one blow? Not
necessarily. Suppose that a 20 amp fuse protects a bus. And a 5 amp fuse
protects a load on that bus. Suppose that the 5 amp circuit shorts hard to
ground. Sure the 5 amp fuse blows. But it takes time, milliseconds. During
that time the fuse element vaporizes but current still flows through that
vaporized metal, lots of it because it is a dead short. That current could be
enough to blow the upstream 20 amp fuse. The bus could have other loads
on it. Suppose that the normal current through the 20 amp fuse is 14 amps.
The 20 amp fuse is already warm from the normal load. It doesn't take a
whole lot more to blow it.
A properly installed bus and its feeder are unlikely to short to ground.
Protecting a bus and its feeder with a fuse increases the chances of that bus
losing power. That is why many, if not most, small aircraft only have a
battery contactor protecting the main power bus.
Have you ever noticed that some household 15 or 20 amp circuit breakers
have 10,000 amps printed on the label? How could that much current flow
through a 15 amp breaker? In case of a short circuit, during the short time
that it takes a circuit breaker to open, as much current will flow as the power
company can provide, limited of course by resistance of wires. That 15 amp
circuit breaker is guaranteed to open even if up to 10,000 amps flow.
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  #57  
Old 06-23-2020, 04:46 PM
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mfleming mfleming is offline
 
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Location: Joseph, Oregon
Posts: 798
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So here's the final version of my electrical system.

f106.PDF
__________________
Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring and Avionics harness completed 9/2/2021
FWF Started 9/3/2021

Donated for 2021 and so should you
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  #58  
Old 06-24-2020, 06:43 AM
Dave12 Dave12 is offline
 
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Location: Elkton, Md.
Posts: 1,744
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Quote:
Originally Posted by Mich48041 View Post
The reason two fuses should not be connected in series is that you never
know which one will blow or both. If one of those fuses is in series with a
bus and that fuse blows, then the whole bus goes dead. What if the fuses
are two different sizes? Wouldn't only the smaller one blow? Not
necessarily. Suppose that a 20 amp fuse protects a bus. And a 5 amp fuse
protects a load on that bus. Suppose that the 5 amp circuit shorts hard to
ground. Sure the 5 amp fuse blows. But it takes time, milliseconds. During
that time the fuse element vaporizes but current still flows through that
vaporized metal, lots of it because it is a dead short. That current could be
enough to blow the upstream 20 amp fuse. The bus could have other loads
on it. Suppose that the normal current through the 20 amp fuse is 14 amps.
The 20 amp fuse is already warm from the normal load. It doesn't take a
whole lot more to blow it.
A properly installed bus and its feeder are unlikely to short to ground.
Protecting a bus and its feeder with a fuse increases the chances of that bus
losing power. That is why many, if not most, small aircraft only have a
battery contactor protecting the main power bus.
Have you ever noticed that some household 15 or 20 amp circuit breakers
have 10,000 amps printed on the label? How could that much current flow
through a 15 amp breaker? In case of a short circuit, during the short time
that it takes a circuit breaker to open, as much current will flow as the power
company can provide, limited of course by resistance of wires. That 15 amp
circuit breaker is guaranteed to open even if up to 10,000 amps flow.
Joe really does see electricity!
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  #59  
Old 06-24-2020, 07:43 AM
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Walt Walt is offline
 
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Location: Dallas/Ft Worth, TX
Posts: 6,340
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Quote:
Originally Posted by mfleming View Post
So here's the final version of my electrical system.

f106.PDF
Personally I don't see what useful function the 'clearance' bus provides, even during a quick-turn I don't have any problem getting the ATIS and a clearance with the engine running.

I don't like running primary nav/com items thru a diode unless you plan on leaving the 'clearance' bus on all the time, extra failure point and voltage drop.

I don't see any back-up battery system for primary instruments like PFD/GSU25/EIS. If you want you can add the com to this BU bus.

General purpose relays are far more prone to failure than high quality switches, so I use switches with no relays for avionics/radio bus.
Todays panels generally draw under 8 amps so 30A relays are overkill.

I like to install 'avionics' and 'radio' bus switches these days as many of the units like the GTN have no on/off switch so that allows controlling those items with separate switches and it also splits the load/failure points.
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Last edited by Walt : 06-24-2020 at 07:53 AM.
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  #60  
Old 06-24-2020, 08:12 AM
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Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Quote:
Originally Posted by Walt View Post
Personally I don't see what useful function the 'clearance' bus provides, even during a quick-turn I don't have any problem getting the ATIS and a clearance with the engine running.

I don't like running primary nav/com items thru a diode unless you plan on leaving the 'clearance' bus on all the time, extra failure point and voltage drop.

I don't see any back-up battery system for primary instruments like PFD/GSU25/EIS. If you want you can add the com to this BU bus.

General purpose relays are far more prone to failure than high quality switches, so I use switches with no relays for avionics/radio bus.
Todays panels generally draw under 8 amps so 30A relays are overkill.

I like to install 'avionics' and 'radio' bus switches these days as many of the units like the GTN have no on/off switch so that allows controlling those items with separate switches and it also splits the load/failure points.
+1.
As you have now simplified this to a basic system, consider:
- Combining the Clearance Delivery and Avionics buses. Here you can run them from a relay that goes to the battery, not the output of the master solenoid (the main bus). Keep all non avionics loads on the main bus (e.g. fuel; pump, nav/strobe/landing lights, pitot heat, all the USB charging ports we now have, and anything else not basic to continued IFR flight). This make load shed and such easy.
- As this is now a basic system, you will need backup batteries on your avionics to be, in my opinion, adequate for IFR. Another option is to take the next step to have a backup battery for your avionics buss, that battery charged via a diode from your main buss. That way you have a battery for your required clearance deliver function and avoid the plethora of individual EFIS type batteries (as they tend to not last very long anyway, a double win).

I differ from Walt on the use of small relays for avionic type busses. I have four in each of the three planes I built. I like them as they are mounted at the battery so no long unprotected runs, the control switch is a simple DPDT toggle that needs only handle ~100ma, and it provides option to have each avionics buss powered from a different battery if needed.

Carl
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