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Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Fri Jan 31, 2020 10:36 am Post subject: Protecting the fat wires

Hello all! This is my first time posting on this list so I'd like to introduce myself and say hello.  I also have a question that I posted on Facebook.  It was suggested that I post the query here.



How do you determine which fat wires need circuit protection of some sort? All of the branch circuits off of a bus get a fuse or a breaker, but what about the wires feeding the busses from the contactors? Mr. Nuckolls’ diagrams sometimes have protection for the larger wires but most often not. I read Aeroelectric Connection in the past and I think that he addressed this but I can’t find it. Ausman’s book also only talks specifically about protecting branch circuits. Can anybody school me on this?



Somebody pointed out, and I do remember reading, that the fat wires will ground to airframe and burn a hole rather than burning the wire itself.  It seems to me that the airframe is the fuse.  Why not just use an ANL?



Thanks

Randy

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Fri Jan 31, 2020 11:02 am Post subject: Re: Protecting the fat wires

This is my schematic based on Z-14 adapted for SDS with VPX. I'm looking for guidance on protecting the fat wires and any other suggestions on improvements or noted deficiencies.

C-GRPY Main Power.pdf

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user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Fri Jan 31, 2020 11:59 am Post subject: Re: Protecting the fat wires

Is the electrical system safer or less safe with the fat wire protected with an 

ANL fuse?  Count the ways that fuse can fail.  There are two crimps that 

connect the ring terminals to fat wires.  There are two bolts that connect the 

ring terminals to the fuse.  The fuse itself can blow.  And there are exposed 

terminals that can short to ground.  So installing an ANL fuse creates about 6 

more possible failure modes.  Why not just install the fat wire properly so 

that it is highly unlikely that it will short out?

 Use fuses instead of a VPX.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Fri Jan 31, 2020 12:31 pm Post subject: Re: Protecting the fat wires

The ANL fuse on the downstream side of the aux contactor could blow if the aux battery is used to crank the engine.

_________________
Joe Gores

ceengland7(at)gmail.com
Guest

Posted: Fri Jan 31, 2020 1:49 pm Post subject: Protecting the fat wires

On 1/31/2020 12:36 PM, Randy C-GRPY wrote:

 	  Quote: 	
		   



 Hello all! This is my first time posting on this list so I'd like to introduce myself and say hello.  I also have a question that I posted on Facebook.  It was suggested that I post the query here.



 How do you determine which fat wires need circuit protection of some sort? All of the branch circuits off of a bus get a fuse or a breaker, but what about the wires feeding the busses from the contactors? Mr. Nuckollsâ€™ diagrams sometimes have protection for the larger wires but most often not. I read Aeroelectric Connection in the past and I think that he addressed this but I canâ€™t find it. Ausmanâ€™s book also only talks specifically about protecting branch circuits. Can anybody school me on this?



 Somebody pointed out, and I do remember reading, that the fat wires will ground to airframe and burn a hole rather than burning the wire itself.  It seems to me that the airframe is the fuse.  Why not just use an ANL?



 Thanks

 Randy



As Joe pointed out, you're adding multiple failure points. It's worth 
	


mentioning that it's everyday practice for the primary power feeder to 

be protected only by the master contactor, which is (should be) next to 

the battery. Tens of thousands of cert. a/c are built that way. Just 

about all other wires do have some type of circuit protection.



I tend to agree with Joe about electronic distribution gadgets. The 

convenience looks attractive, but some of them have failure modes that 

can take the whole plane 'dark', and I'm not aware of any that publish 

every detail of their 'innards'. That means you don't know what you 

don't know. With a radio or other widget that's a 'black box', you can 

install a backup, but making an electronic power distribution system 

redundant would be a monumental task.



Charlie

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Fri Jan 31, 2020 2:03 pm Post subject: Re: Protecting the fat wires

 	  user9253 wrote: 	
		  The ANL fuse on the downstream side of the aux contactor could blow if the aux battery is used to crank the engine. 	




Ahhhh.....good point. 



The contactors are going to be next to the batteries.  With one battery in the tail there would be a large wire running to the firewall.  What I am understanding from you is that that wire is protected by the contactor and no further fusing is required.



How about the hot wires from the unswitched side of the contactors feeding the engine bus?  Would you protect that close to the battery?



I appreciate your input.

Randy

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Fri Jan 31, 2020 2:11 pm Post subject: Re: Protecting the fat wires

 	  ceengland7(at)gmail.com wrote: 	
		  On 1/31/2020 12:36 PM, Randy C-GRPY wrote:

 	  Quote: 	
		   



 

 Somebody pointed out, and I do remember reading, that the fat wires will ground to airframe and burn a hole rather than burning the wire itself.  It seems to me that the airframe is the fuse.  Why not just use an ANL?



 ThanksHello all! This is my first time posting on this list so I'd like to introduce myself and say hello.  I also have a question that I posted on Facebook.  It was suggested that I post the query here.



 How do you determine which fat wires need circuit protection of some sort? All of the branch circuits off of a bus get a fuse or a breaker, but what about the wires feeding the busses from the contactors? Mr. Nuckollsâ€™ diagrams sometimes have protection for the larger wires but most often not. I read Aeroelectric Connection in the past and I think that he addressed this but I canâ€™t find it. Ausmanâ€™s book also only talks specifically about protecting branch circuits. Can anybody school me on this?



 Randy



As Joe pointed out, you're adding multiple failure points. It's worth 

	




I tend to agree with Joe about electronic distribution gadgets. The 

convenience looks attractive, but some of them have failure modes that 

can take the whole plane 'dark', and I'm not aware of any that publish 

every detail of their 'innards'. That means you don't know what you 

don't know. With a radio or other widget that's a 'black box', you can 

install a backup, but making an electronic power distribution system 

redundant would be a monumental task.



Charlie 	




I appreciate your input. 



The VPX has a pretty good history so far and it serves as more than just a circuit breaker. I personally like it and it seems like a lot of users are choosing it these days.  I will rethink it though. If it were to go offline, say by turning the master off, my backup is one switch that will switch to an essential bus that served by the backup alternator.  At least that is what I was hoping for with the layout I designed. My panel does have a G5 with its own backup battery that will stay on even if Master and Aux power is turned off.

johnbright

Joined: 14 Dec 2011
Posts: 165
Location: Newport News, VA

Posted: Fri Jan 31, 2020 4:36 pm Post subject: Re: Protecting the fat wires

Fat wire burns thru elevator cable... one of Bob Nuckols' stories:



"Bottom line is that you're many, many times more likely to have a bad day in the cockpit for reasons far removed from a hard ground fault on your 6AWG bus feeder wire. . . and THAT because you didn't conduct due diligence in its installation.



In the case of the bus feeder, the risks are not so much to the wire as to the thing the wire touches.



Case in point: C90 twin turboprop on short final experiences disconnect of elevator cables. Pilot uses trim commands and power to execute go-around, assesses the condition and successfully lands the airplane with rudder, trim and power.



Pulling up floorboards in the cockpit revealed a 40A protected feeder to the windshield de-ice inverter had been mis-positioned against the elevator control cable during a maintenance operation. Over what had to be many hours of operation, motion of the cable wore through the insulation bringing the cable into contact with the hot wire. The arcing and sparking was of insufficient intensity to come to attention of crew in spite of the fact that it was going on virtually under their feet.



The copper wire was barely damaged. The breaker never popped while the elevator cable eventually eroded through and parted. Compare thermal properties of copper versus steel . . . this explains why the best steel safes have intermediate layers of copper in their construction. It's EASY to burn through steel . . . next to impossible on copper.



This narrative explains the high order probability that even if you DID get your 6AWG feeder faulted to ground, it's most likely to be a soft fault that burns a hole in your airplane while doing little damage to the wire . . . and certainly far short of getting it to smoke and/or open a fuse/breaker.



Adding 'protection' to this pathway doubles the number of joints in the pathway and adds nothing demonstrable in terms of fault response . . . which is why the spam-can builders don't do it either.



... Bob Nuckolls ..."

_________________
John Bright, RV-6A, at FWF, O-360
Z-101 single batt dual alt SDS EM-5-F.
john_s_bright@yahoo.com, Newport News, Va
N1921R links

ceengland7(at)gmail.com
Guest

Posted: Fri Jan 31, 2020 5:03 pm Post subject: Protecting the fat wires

On 1/31/2020 4:03 PM, Randy C-GRPY wrote:

 	  Quote: 	
		   

 user9253 wrote:

> The ANL fuse on the downstream side of the aux contactor could blow if the aux battery is used to crank the engine.



 Ahhhh.....good point.



 The contactors are going to be next to the batteries.  With one battery in the tail there would be a large wire running to the firewall.  What I am understanding from you is that that wire is protected by the contactor and no further fusing is required.



 How about the hot wires from the unswitched side of the contactors feeding the engine bus?  Would you protect that close to the battery?



 I appreciate your input.

 Randy

My choice is to use a fuselink soldered & heatshrunk onto the battery 
	


end of the wire feeding the engine bus. Fuselinks have been quite common 

in the automotive world to protect things like alternator B leads. An 

ANL would achieve the same thing, for more money & extra failure points. 

Protecting the wire makes good sense; I just wanted the minimum risk of 

a 'false positive' circuit interruption.



Charlie

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 31, 2020 7:50 pm Post subject: Protecting the fat wires

At 12:36 PM 1/31/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Randy C-GRPY" <rpulis(at)shaw.ca>



 Hello all! This is my first time posting on this list so I'd like to introduce myself and say hello.  I also have a question that I posted on Facebook.  It was suggested that I post the query here. 	


  Welcome aboard my friend. This is the intellectual

  watering hole for a useful contingent of experienced

  and contemplative thinkers.



  	  Quote: 	
		  How do you determine which fat wires need circuit protection of some sort? All of the branch circuits off of a bus get a fuse or a breaker, but what about the wires feeding the busses from the contactors? Mr. Nuckolls’ diagrams sometimes have protection for the larger wires but most often not. 	


  I think you're referring to b-lead protection

  on alternators . . . this is in recognition of

  solid state devices (rectifiers) in alternators

  with a known history of faults albeit a very

  old one. It's now a exceedingly rate occurrence.

  Some cars have fusible links in their alternator

  b-leads. I think my Kia mini-van has several

  mani-ANL limiters in the battery feeders. But

  as a general rule, you're on solid ground with

  the z-figures . . . all descendants of a century

  of practice in TC aircraft and 30 years 

  experience here on the List. 



 

  	  Quote: 	
		   I read Aeroelectric Connection in the past and I think that he addressed this but I can’t find it. Ausman’s book also only talks specifically about protecting branch circuits. Can anybody school me on this?



 Somebody pointed out, and I do remember reading, that the fat wires will ground to airframe and burn a hole rather than burning the wire itself.  It seems to me that the airframe is the fuse.  Why not just use an ANL? 	


   It simply adds parts count and cost that affords no

   practical value. A quick review of the power distribution

   on a couple hundred thousand TC light aircraft will confirm

   this design philosophy.



   You can add all the ANL devices that you wish, operationally

   they will add no risk beyond increased probability of

   failure due to un-warranted complexity.



 

   Bob . . .

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Sat Feb 01, 2020 8:22 am Post subject: Re: Protecting the fat wires

Thanks Bob. This simplifies things a lot with respect to wiring my aircraft.  



What I am understanding is that any of the fat wires, properly sized and installed on the switched side of the contactor feeding the busses will have an exceedingly low likelihood of causing problems and don't need any further protection.



What about the fat wires coming directly from the battery to feed an always hot bus? (The schematic is above) I have such wires for my fuel/ignition system from two batteries.  Does the same principle apply of sizing and installing the wire appropriately as being good in and of itself or should those be fused close to the battery?



Thanks!

Randy

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Sat Feb 01, 2020 8:32 am Post subject: Re: Protecting the fat wires

 	  ceengland7(at)gmail.com wrote: 	
		  On 1/31/2020 4:03 PM, Randy C-GRPY wrote:

 	  Quote: 	
		   

 user9253 wrote:

> The ANL fuse on the downstream side of the aux contactor could blow if the aux battery is used to crank the engine.



 Ahhhh.....good point.



 The contactors are going to be next to the batteries.  With one battery in the tail there would be a large wire running to the firewall.  What I am understanding from you is that that wire is protected by the contactor and no further fusing is required.



I'm grateful for your input.

Randy

 How about the hot wires from the unswitched side of the contactors feeding the engine bus?  Would you protect that close to the battery?



 I appreciate your input.

 Randy

My choice is to use a fuselink soldered & heatshrunk onto the battery 

	


end of the wire feeding the engine bus. Fuselinks have been quite common 

in the automotive world to protect things like alternator B leads. An 

ANL would achieve the same thing, for more money & extra failure points. 

Protecting the wire makes good sense; I just wanted the minimum risk of 

a 'false positive' circuit interruption.



Charlie 	




Initially I had those wires going to a breaker which I think would essentially accomplish what you suggest.  I liked the idea of actually seeing that there is a problem by witnessing the tripped breaker.  A fellow builder who is also an electrical engineer pointed out that if a short occurred somewhere between the breaker and battery in that setup, that there would be no way of turning off the electrons.  There are a lot of examples in Bob's diagrams where those hot wires are * as "6 in or less" presumably to guard against just such a scenario.  Does that not apply to the long runs supplying the bus which are much much longer than 6"?



I guess you could put the fusible link at the beginning of that run. Will fusible links have the predictable behaviour characteristics that a similarly fuse would have?



I'm grateful for your input



Randy

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Feb 01, 2020 10:23 am Post subject: Protecting the fat wires

At 10:22 AM 2/1/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Randy C-GRPY" <rpulis(at)shaw.ca>



 Thanks Bob. This simplifies things a lot with respect to wiring my aircraft.  



 What I am understanding is that any of the fat wires, properly sized and installed on the switched side of the contactor feeding the busses will have an exceedingly low likelihood of causing problems and don't need any further protection. 	


   Exactly . . .



 

  	  Quote: 	
		  What about the fat wires coming directly from the battery to feed an always hot bus? (The schematic is above) I have such wires for my fuel/ignition system from two batteries.  Does the same principle apply of sizing and installing the wire appropriately as being good in and of itself or should those be fused close to the battery?

 	
  The z-figures mark such feeders

   with an (*) meaning 6" or less . . .

   or at least as short as practical.



   There is nothing wrong with having

   battery bus fuse blocks located

   adjacent to battery contactors.



 

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Feb 01, 2020 10:31 am Post subject: Protecting the fat wires

 	  Quote: 	
		  

 Initially I had those wires going to a breaker which I think would essentially accomplish what you suggest.  I liked the idea of actually seeing that there is a problem by witnessing the tripped breaker. 	


   But what would be your FIRST clue? What

   event would prompt you to scan the breaker

   panel?



  	  Quote: 	
		    A fellow builder who is also an electrical engineer pointed out that if a short occurred somewhere between the breaker and battery in that setup, that there would be no way of turning off the electrons.  There are a lot of examples in Bob's diagrams where those hot wires are * as "6 in or less" presumably to guard against just such a scenario.  Does that not apply to the long runs supplying the bus which are much much longer than 6"? 	


   FAT wires capable of many hundreds of

   amps are (1) few in number and (2)

   installed with special attention to

   low risk for mechanical damage. Less-

   than-fat feeders to battery busses are

   recommended compliant with design goal (2) by

   keeping them short.



 

   Bob . . .

ceengland7(at)gmail.com
Guest

Posted: Sat Feb 01, 2020 10:56 am Post subject: Protecting the fat wires

On 2/1/2020 10:33 AM, Randy C-GRPY wrote:

 	  Quote: 	

 ceengland7(at)gmail.com wrote:

> On 1/31/2020 4:03 PM, Randy C-GRPY wrote:

>

>>   

>>

>>

>>   user9253 wrote:

>>> The ANL fuse on the downstream side of the aux contactor could blow if the aux battery is used to crank the engine.

>>   Ahhhh.....good point.

>>

>>   The contactors are going to be next to the batteries.  With one battery in the tail there would be a large wire running to the firewall.  What I am understanding from you is that that wire is protected by the contactor and no further fusing is required.

>>

>>   How about the hot wires from the unswitched side of the contactors feeding the engine bus?  Would you protect that close to the battery?

>>

>>   I appreciate your input.

>>   Randy

>> My choice is to use a fuselink soldered & heatshrunk onto the battery

>>

> end of the wire feeding the engine bus. Fuselinks have been quite common

> in the automotive world to protect things like alternator B leads. An

> ANL would achieve the same thing, for more money & extra failure points.

> Protecting the wire makes good sense; I just wanted the minimum risk of

> a 'false positive' circuit interruption.

>

> Charlie

 Initially I had those wires going to a breaker which I think would essentially accomplish what you suggest.  I liked the idea of actually seeing that there is a problem by witnessing the tripped breaker.  A fellow builder who is also an electrical engineer pointed out that if a short occurred somewhere between the breaker and battery in that setup, that there would be no way of turning off the electrons.  There are a lot of examples in Bob's diagrams where those hot wires are * as "6 in or less" presumably to guard against just such a scenario.  Does that not apply to the long runs supplying the bus which are much much longer than 6"?

My understanding of the '6" rule' (like the battery bus, etc) is that 

we're keeping the unprotected section of wire quite short, and enduring 

that it's in a protected area where it can't be inadvertently brought 

into contact with structure or objects that can short out the terminals. 

By doing this, we're driving the risk of having a problem with that 

unprotected wire to near zero. All wires leaving the bus would be 

protected by fuses/breakers.

I'd say that your engineer friend's thoughts were correct. The 

conventional alternative would be to place the bus near the battery (6" 

rule) and make fused runs to each switch from that bus. My choice for 

the engine bus was to mimic the the alternator B lead protection: I 

placed a fuse link (you could use an ANL, if desired) at the battery end 

of the wire that supplies power to the main ignition switch and 

subsequently to the engine bus. The time constant of fuse links & ANLs 

are so long that unlike a regular fuse, short-term current spikes from 

bringing the bus on line should never fatigue the protection into 

failure. The feeder is still protected from catastrophic fault.

My choice exceeds the fairly common practice of limiting always-hot 

feeds to around 3 amps. But the feed is only around 30" long, and the 

idea is not unprecedented;  many older certified a/c (with smaller 

electrical systems) had under-seat or baggage compartment batteries with 

an always-hot feeder all the way to the instrument panel mounted high 

current master switch (no contactor at the battery).

Charlie

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Sat Feb 01, 2020 11:00 am Post subject: Re: Protecting the fat wires

Mount 40 amp automotive relays (B&C S8009-1) within 3" of battery positive

posts.  Connect 14 AWG between battery positive and relay.  This 14 AWG 

wire will serve as a fuselink.  No fuses needed.   Connect the relay output to 

the engine bus with 10 AWG.  Diodes could be installed per your diagram, but 

they are not required.  The pilot can control which battery is connected to the 

engine bus by turning on one or both relays.

_________________
Joe Gores

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Sat Feb 01, 2020 12:49 pm Post subject: Re: Protecting the fat wires

 	  nuckolls.bob(at)aeroelect wrote: 	
		   	  Quote: 	
		  

 Initially I had those wires going to a breaker which I think would essentially accomplish what you suggest.  I liked the idea of actually seeing that there is a problem by witnessing the tripped breaker. 	


   But what would be your FIRST clue? What

   event would prompt you to scan the breaker

   panel?



  	  Quote: 	
		    A fellow builder who is also an electrical engineer pointed out that if a short occurred somewhere between the breaker and battery in that setup, that there would be no way of turning off the electrons.  There are a lot of examples in Bob's diagrams where those hot wires are * as "6 in or less" presumably to guard against just such a scenario.  Does that not apply to the long runs supplying the bus which are much much longer than 6"? 	


   FAT wires capable of many hundreds of

   amps are (1) few in number and (2)

   installed with special attention to

   low risk for mechanical damage. Less-

   than-fat feeders to battery busses are

   recommended compliant with design goal (2) by

   keeping them short.



 

   Bob . . . 	




There may not be a clue while in flight other than perhaps periodically scanning the breaker and seeing it popped. It would definitely be obvious during the preflight checklist if done correctly.  



Ok. I understand the difference between overweight wire (14AWG) and morbidly obese wire (#2 AWG) and that each has different needs. 



If it weren't practical to have the always battery bus immediately adjacent to the battery, is a fusible link or an inline fuse acceptable? Or alternatively, just use the fatter wire?



Randy

Last edited by Randy C-GRPY on Sat Feb 01, 2020 1:10 pm; edited 1 time in total

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Sat Feb 01, 2020 12:54 pm Post subject: Re: Protecting the fat wires

 	  user9253 wrote: 	
		  Mount 40 amp automotive relays (B&C S8009-1) within 3" of battery positive

posts.  Connect 14 AWG between battery positive and relay.  This 14 AWG 

wire will serve as a fuselink.  No fuses needed.   Connect the relay output to 

the engine bus with 10 AWG.  Diodes could be installed per your diagram, but 

they are not required.  The pilot can control which battery is connected to the 

engine bus by turning on one or both relays. 	




The diodes were suggested by an electrical engineer and fellow builder to mitigate a risk of high current flowing between the batteries if both feeds were on simultaneously and there for some reason was a significant difference in voltage state between the two batteries.  My understanding was that this could result in high current flow and cause problems with the fuses that I initially had on those lines, but am now going to omit based on the previous discussion.  



Thanks for the info and the ideas!  They are valued.



Randy

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Sat Feb 01, 2020 2:12 pm Post subject: Re: Protecting the fat wires

There could be low current between two batteries connected in parallel, but high current is very unlikely.

_________________
Joe Gores

Randy C-GRPY

Joined: 21 Apr 2018
Posts: 26

Posted: Sun Feb 02, 2020 5:20 pm Post subject: Re: Protecting the fat wires

 	  user9253 wrote: 	
		  Mount 40 amp automotive relays (B&C S8009-1) within 3" of battery positive

posts.  Connect 14 AWG between battery positive and relay.  This 14 AWG 

wire will serve as a fuselink.  No fuses needed.   Connect the relay output to 

the engine bus with 10 AWG.  Diodes could be installed per your diagram, but 

they are not required.  The pilot can control which battery is connected to the 

engine bus by turning on one or both relays. 	




Joe, would a contactor within 3” of the battery also be an option? Why the automotive relay?



Randy

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