Matronics Email Lists :: View topic

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Fuse Sizing

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

jpx(at)Qenesis.com
Guest

Posted: Mon Jul 07, 2008 5:38 am Post subject: Fuse Sizing

I just looked through Chapter 10 and didn't find any guidance on  

selecting the size of a fuse.

I am quite sure that a 10A fuse is designed to blow at, or barely  

above, 10A and thus would be unsuitable for protecting a circuit with  

an actual current flow of 10A.



In my case, I have a landing light circuit totalling 9.2A.  Mostly it  

is 4.6A to each bulb, but there are some common wires at the switch  

that carry both loads.



Looking at Bob's Wire Size Selection chart with 10C Rise (which makes  

wire selection incredibly easy, compared to the graphs in 43-13-1B), I  

could use 18 gauge wire in the wings and 14 gauge wire where both  

currents are common.



I can't use a double pole switch to wire each light independently with  

separate fuses, because I am already using a double pole switch to  

give me off, on or wig wag.



So I need a single circuit with a single fuse.  The fuse needs to be  

sufficient for the total current.  This also means upgrading the 18  

gauge wire in the wings to 14 gauge, to avoid having a fuse too big  

for the wire.



So my gut feel is to use the 35C Rise part of the table to pick a  

current rating for the fuse, which would give me 15A fuse for 14 gauge  

wire. Correct ?



Also, the calculations quite clearly include the return to ground part  

of the current when calculating wire losses due to resistance.  What  

equivalent resistance value would be typical of the frame of aluminum  

aircraft when used for the ground return ?



Thanks for your advice,



Jeff Page

Dream Aircraft Tundra #10

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

nuckolls.bob(at)cox.net
Guest

Posted: Wed Jul 09, 2008 1:31 pm Post subject: Fuse Sizing

At 09:33 AM 7/7/2008 -0400, you wrote:

 	  Quote: 	
		  



I just looked through Chapter 10 and didn't find any guidance on

selecting the size of a fuse.

I am quite sure that a 10A fuse is designed to blow at, or barely

above, 10A and thus would be unsuitable for protecting a circuit with

an actual current flow of 10A.


	


   Fuse ratings can vary a lot from style to style

   and application to application for the device. There's

   no broad-brush statement one can make about "fuses" . . .

   the manufacturer's data is always the best place

   to go.



   In the case of the ATC plastic devices, the data shows



http://www.egauges.com/vdo_mult3.asp?Type=Voltmeter12&Series=Cyber_Red&Units=E



   . . . that you can probablyuse a 10A fuse in a circuit

   that draws 10A continuously.

 	  Quote: 	
		  In my case, I have a landing light circuit totalling 9.2A.  Mostly it

is 4.6A to each bulb, but there are some common wires at the switch

that carry both loads.


	


   Lamps also have bodacious inrush current. So look for 15A

   protection to reduce probability of nuisance tripping.

 	  Quote: 	
		  Looking at Bob's Wire Size Selection chart with 10C Rise (which makes

wire selection incredibly easy, compared to the graphs in 43-13-1B), I

could use 18 gauge wire in the wings and 14 gauge wire where both

currents are common.


	


  I can't use a double pole switch to wire each light independently with

 	  Quote: 	
		  separate fuses, because I am already using a double pole switch to

give me off, on or wig wag.



So I need a single circuit with a single fuse.  The fuse needs to be

sufficient for the total current.  This also means upgrading the 18

gauge wire in the wings to 14 gauge, to avoid having a fuse too big

for the wire.


	


   Likelihood of incrementally overloading a wire to the

   extent that the a 14 AWG wire 'burns' if it happens to

   be protected by say a 25A fuse is exceedingly low. The

   vast majority of overload faults in DC systems are pretty

   hard . . . 10-100 times the ratings of fuses or wires.

   Further, wires are quite robust. See:



http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf



   That little strand of 22AWG wire in the picture has

   be loaded to 20A for over thirty minutes. Its temperature

   rose to only 115C while the insulation was rated for

   150C.

 	  Quote: 	
		  So my gut feel is to use the 35C Rise part of the table to pick a

current rating for the fuse, which would give me 15A fuse for 14 gauge

wire. Correct ?



Also, the calculations quite clearly include the return to ground part

of the current when calculating wire losses due to resistance.  What

equivalent resistance value would be typical of the frame of aluminum

aircraft when used for the ground return ?


	


   For all practical purposes, your airplane as a ground conductor

   has zero ohms resistance. I've measured tip to tail on a Model

   400 Beechjet at 0.0012 ohms. I've measured crankcase to battery

   minus on a C-172 at 0.008 ohms . . . where virtually all the

   resistance came from poor joints in the connectors and wiring.

   The airframe itself was under 1 milliohm.



   Don't spend a lot of time on it. I'd go with 14AWG throughout

   the system and a 15A fuse. If you find that your landing lights

   nuisance trip the fuse after x-times of turning them on, then

   up-size to a 20A fuse and leave the wires alone. It will

   be just fine.



   Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

jpx(at)Qenesis.com
Guest

Posted: Thu Jul 10, 2008 5:20 am Post subject: Fuse Sizing

 	  Quote: 	
		   In the case of the ATC plastic devices, the data shows  

 http://www.egauges.com/vdo_mult3.asp?Type=Voltmeter12&Series=Cyber_Red&Units=E . . . that you can probablyuse a 10A fuse in a  

 circuit

that draws 10A continuously.
	




The above link points to a voltmeter, not a graph of fuse performance,  

which I would like to see.



The rest of the post answered my questions, thank you kindly, so I can  

continue with my design.



Do not archive



Jeff Page

Dream Aircraft Tundra #10

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

nuckolls.bob(at)cox.net
Guest

Posted: Thu Jul 10, 2008 8:00 am Post subject: Fuse Sizing

At 09:16 AM 7/10/2008 -0400, you wrote:

 	  Quote: 	

>In the case of the ATC plastic devices, the data shows

>http://www.egauges.com/vdo_mult3.asp?Type=Voltmeter12&Series=Cyber_Red&Units=E 

>. . . that you can probablyuse a 10A fuse in a

>circuit

that draws 10A continuously.

The above link points to a voltmeter, not a graph of fuse performance,

which I would like to see.

   Sorry 'bout that. Wrong link on the clipboard. Try

http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/

   Here you'll find data on a large number of Bussmann products

   including the ATC plastic fuses. In particular, note the

   characteristics of the ANL devices. Those critters are REALLY

   robust!

   A review of these data sheets will reinforce the notion that

   not all devices called "fuses" have the same or even similar

   performance. Each product was designed to meet the design goals

   for a particular task.

   Bob . . .

        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum