REFLECTOR: Switching Grounds

[jewen]

What is a live wire?  In an automotive application, convention dictates that the negative side of the battery is connected to the metal chassis.  The positive lead(s) are then usually considered live wires.  The fact is that both wires are live, by common usage the positive wire is considered live relative to the negative wire or chassis.  But the negative can be considered live as well when compared to a positive potential.  Many people assume that in a battery circuit, electricity flows from the positive to the negative, it actually flows from the side that has an abundance of electrons (-) to the side that has a deficit (+.)

With all the above in mind one might say that in a non conductive vehicle, it does not really matter whether the + or the - is switched.  IMO it does not make a difference which side is switched (exceptions noted below.)  What is important is that the Load can be switched off and that there is adequate overload protection to protect the load, switches and wiring in the event of an overload or short circuit condition.

++++++++ CB + + + + + SW +  +  +  +  +  +  +  +  LOAD  - - - - - - - - - - - - - 
- - - - - - - -  CB -  -  -  -  -  SW -  -  -  -  -  -  -  -  -  -  - LOAD + + + + + + + + + 

The above said, I would suggest to others and am planning on my aircraft to follow established conventions (Positive=Source, Negative=Common) for the following reasons:
1.  Standard practice is to use the negative as common, if someone works on the airplane other than I, he/she may not be aware of unconventional wiring which would increase the risk of smoking a component.
2.  I will be using a standard alternator, so anything touching the engine will have a negative potential, if I used positive as the common and it shorted to an engine component or anything electrically connected (intentionally or otherwise) to the engine I would run the risk of smoking the wiring and/or device (no protection since the CB is on the negative side.
3.  The radios normally have the negative electrically connected to the radio chassis/racks and are considered the common side.  The avionics equipment can be isolated from other Aircraft equipment, but confusion may result in that if the Aircraft's standard is to switch and overload protect the negative, then no overload protection would exist on the avionics since the overload could continue through the other avionics supply circuits.

While this Is Just My Opinion, I can say that following standard practices, normal conventions, and attention to detail has kept me from smoking anything electrical in over 15 years of commissioning, programming, and troubleshooting industrial electrical/electronic control systems.  Wish I could say that about program bugs.

Joe

  ----- Original Message ----- 
  From: Douglas Holub 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Sunday, September 12, 2004 9:08 PM
  Subject: Re: REFLECTOR: Switching Grounds


  If you switch the positive wires, when the switches are off, there are fewer live wires. That can only be good.
    ----- Original Message ----- 
    From: Randy Wharton 
    To: reflector at tvbf.org 
    Sent: Sunday, September 12, 2004 5:11 PM
    Subject: REFLECTOR: Switching Grounds


    Not too long ago, someone mentioned that they ran ground wires to their switches instead of switching the positive lead because they thought it was safer? Not sure if I recollect exactly. But I don't remember anyone contradicting the idea. Would there really be any advantage in doing that? Because the switch provides that break in a circuit so that current stops flowing across it. When the switch is open, on side of the switch will be at the positive potential, and the other side will be at ground potential (in a power circuit.) So I fail to see any difference from the switch's perspective whether you have the switch on the hot side or the ground side of the load. 

     

    Randy Wharton

    N248RB - Velocity XL-RG

    http://clik.to/velocity

     



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