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PROBLEM. Electrical Grounding with a primed airframe.

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psm(at)ATT.NET
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Posted: Fri Nov 23, 2007 7:46 pm Post subject: PROBLEM. Electrical Grounding with a primed airframe.

Hi Geoff & Ed,



 I guess it was me you refer to, Ed . . .  . . . 



 No, I would not suggest twisting the battery cables.  These are the extra heavy gauge wires needed to supply the 100 or so amps to the starter for a few seconds at a time.  Twisting would be appropriate if they were high frequence signal wires, but this is a high current DC application.



 You can probably run the wires together to the engine compartment if you like.  The only caveat is you need to make sure the insulation is not broken so a short can occur.  You will probably ground the aluminum in the airplane anyway (connect to the negative battery contact) so the only real critical insulation is on the positive lead cable.



 If you prefer to run them separately then that is fine too.



 One more comment.  Several guys seem to think the engine mount is a good place to form your single ground point for the whole plane.  This means the place the negative battery lead is attached and as many other negative lead wires as you connect to ground.  Actually, I would prefer some piece of copper or aluminum (bus bar?) for this purpose.  The steel engine mount is probably the worst electrical conductor in the whole plane, and it would be better to use a good conductor for this purpose.



 Good luck,



 Paul

 XL fuselage



 

 At 10:56 AM 11/23/2007, you wrote:

 [quote]Might be good to run them as a twisted pair. Electronic gurus on the list can confirm or correct.

  

 Dred

   ---

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planecrazydld(at)yahoo.co
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Posted: Sat Nov 24, 2007 3:50 am Post subject: PROBLEM. Electrical Grounding with a primed airframe.

running them separately is desirable to minimize the potential for a battery direct short in the case of wear or an incident. Of course with an aluminum airframe, that may be a moot point!



Paul Mulwitz <psm(at)att.net> wrote:[quote]   Hi Geoff & Ed,



 I guess it was me you refer to, Ed . . .  . . . 



 No, I would not suggest twisting the battery cables.  These are the extra heavy gauge wires needed to supply the 100 or so amps to the starter for a few seconds at a time.  Twisting would be appropriate if they were high frequence signal wires, but this is a high current DC application.



 You can probably run the wires together to the engine compartment if you like.  The only caveat is you need to make sure the insulation is not broken so a short can occur.  You will probably ground the aluminum in the  airplane anyway (connect to the negative battery contact) so the only real critical insulation is on the positive lead cable.



 If you prefer to run them separately then that is fine too.



 One more comment.  Several guys seem to think the engine mount is a good place to form your single ground point for the whole plane.  This means the place the negative battery lead is attached and as many other negative lead wires as you connect to ground.  Actually, I would prefer some piece of copper or aluminum (bus bar?) for this purpose.  The steel engine mount is probably the worst electrical conductor in the whole plane, and it would be better to use a good conductor for this purpose.



 Good luck,



 Paul

 XL fuselage



 

 At 10:56 AM 11/23/2007, you wrote:

 [quote]Might be good to run them as a twisted pair. Electronic gurus on the list can confirm or correct.

   

 Dred

   ---

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psm(at)ATT.NET
Guest

Posted: Sat Nov 24, 2007 4:32 pm Post subject: PROBLEM. Electrical Grounding with a primed airframe.

Hi Steve,



I'm sure you are right about the resistance of the aluminum airframe 

compared to the much smaller cross section copper cable.  However, I 

don't think this is the only issue.



It seems to me that it is much safer to use the ground cable to get 

from a remote battery to the engine compartment.  If you rely on the 

airframe for this purpose, I would certainly install an avionics 

master switch and make sure the avionics are isolated when the 

starter is operated.



I realize there is a weight penalty using the copper cable for 

grounding the battery.  However, moving the weight around to get the 

CG in order is the whole point of moving the battery anyway.  OK, 

this is a lame argument . . .



Paul

XL fuselage

At 10:40 AM 11/24/2007, you wrote:

 	  Quote: 	
		  I don't know if you follow Bob Nucholls stuff in the Aeroelectric 

section, but this topic has come up there numerous times. I'm no 

electrical expert so I can only say what I have done and what 

experts like Bob have recommended. Basically with an aluminum 

airframe there is no reason to run a ground cable forward to the 

firewall. There is enough contact with the rivets, even if 

everything is primed, to provide an ample path for the electrons to 

flow. You'll find more resistance in a 2 G cable than you'll see in 

the airframe. You can also safely ground pitot heat, landing lights, 

and position lights locally.

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