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Placement of Switch within Simple Circuits - does it Matter?

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wsimpso1

Joined: 04 Nov 2018
Posts: 40
Location: Saline MI

Posted: Sat Jun 20, 2026 1:11 pm Post subject: Placement of Switch within Simple Circuits - does it Matter?

Is there any reason to put the switch close to the bus, close to the load, or even on the wire going to the ground bus? We have quite a few circuits in our airplanes where the switch is simply closing or opening the circuit. On this simplest of circuits, the function can be effective no matter where we put the switch.



We do have quite a few devices where circuit control is done on the ground side. EFII injectors and coils are commonly hot from the bus to the device and the return wire goes to the ECU where it is grounded. So that got me thinking about where we put our switches.



Taking this design philosophy question a step further, it occurs to me the choice to fuse the + volt side is because we traditionally have a metal airframe available for the ground path, and indeed is a spectrum of places where a hot wire will short back to the battery. In my non-conductive airframe, I am running a ground wire to a ground bus. It would seem that any place in my circuit could be shorted to another circuit of opposite polarity, and yet we do not fuse our ground wires. Is that really OK? 



Billski

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user9253

Joined: 28 Mar 2008
Posts: 1973
Location: Riley TWP Michigan

Posted: Sat Jun 20, 2026 5:51 pm Post subject: Re: Placement of Switch within Simple Circuits - does it Mat

 	  Quote: 	
		  and yet we do not fuse our ground wires. Is that really OK? 	


Yes, in most cases.  Suppose that a very large positive wire shorts to a small 

negative wire.  The fuse protecting the large positive wire might not blow.  But 

the chances of that happening are pretty small, especially if good workmanship 

is practiced.  Use lots of wire-ties to prevent relative motion between wires. 

Use grommets where wires pass through bulkheads.

  Installing a fuse in a negative wire introduces several new failure points.

Future troubleshooters will cuss you out.

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