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Grounding Question

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Tue Sep 26, 2006 7:49 pm Post subject: Grounding Question

Hi Bob,

  

 Thanks for the very helpful new drawing on grounding.  It motivated me  to ask a question that I've been wrestling with for some time.

  

 I'm installing a Grand Rapids Engine Information System (EIS) in my  composite OBAM aircraft.  Many of the engine sensors attach directly to the  engine itself and therefore ground to the engine.  The EIS instruction  manual says to ground the EIS module to the engine ground.  So far, so  good.

  

 But the oil pressure sensor is attached to a rubber hose, electrically  isolated from the engine, and I was planning to ground it to the engine side of  the firewall, to the "G2 FWL" ground block.  The manifold pressure sensor  is on the cockpit side of the firewall and I was planning to ground it to the  avionics ground block.  The outside air temperature sensor will ground to  the "G3 PNL" ground block.  (There are many more "electro-whizzies"  connected to the EIS module, but the ones mentioned illustrate the  point.)   

  

 Each individual sensor is grounded to a single spot. But will the EIS  module, to which they all connect, see the four separate grounds as a ground  loop?

  

 Thanks,

 Dennis Johnson

 Lancair Legacy, wiring the panel  

    [quote][b]

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nuckollsr(at)cox.net
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Posted: Wed Sep 27, 2006 7:10 am Post subject: Grounding Question

At 08:47 PM 9/26/2006 -0700, you wrote:



 	  Quote: 	
		  Hi Bob,



Thanks for the very helpful new drawing on grounding.  It motivated me to 

ask a question that I've been wrestling with for some time.



I'm installing a Grand Rapids Engine Information System (EIS) in my 

composite OBAM aircraft.  Many of the engine sensors attach directly to 

the engine itself and therefore ground to the engine.  The EIS instruction 

manual says to ground the EIS module to the engine ground.  So far, so good.



But the oil pressure sensor is attached to a rubber hose, electrically 

isolated from the engine, and I was planning to ground it to the engine 

side of the firewall, to the "G2 FWL" ground block.  The manifold pressure 

sensor is on the cockpit side of the firewall and I was planning to ground 

it to the avionics ground block.  The outside air temperature sensor will 

ground to the "G3 PNL" ground block.  (There are many more 

"electro-whizzies" connected to the EIS module, but the ones mentioned 

illustrate the point.)



Each individual sensor is grounded to a single spot. But will the EIS 

module, to which they all connect, see the four separate grounds as a 

ground loop?


	


    GREAT question! We tend to be unaware of inalterable

    system features that drive the astute system designer's

    grounding decisions.



    As we've noted, some engine sensors ground to the

    engine by design. No practical way to change it. This

    suggests that the engine or at least the firewall ground

    block is where EVERY ground associated with the engine

    instrumentation system should be tied down.



    Now, when one uses the a nice fat braided crankcase-

    to-firewall ground jumper, the electrical differences

    between crankcase, firewall-forward block, and cockpit-side

    ground blocks are insignificant.  This was the rationale

    for the original ground-block and crankcase-jumper concept

    cited back in Rev 9 or so.



    The idea was that EVERY grounded wire on a small aircraft

    would route to the single location tightly bonded to crankcase,

    battery (-) and airframe (via the firewall sheet). This single

    feature would absolutely eliminate the potential for ground

    loops known or unknown.



    As the airplane becomes more complex there's no practical

    way that EVERY wire can come to ground at the same location.

    If we tried that in a Beechjet, we would have a huge wire

    bundle of perhaps 100-200 wires trying to share the same

    grounding location in the airplane.



    So, the idea of distributed grounding as illustrated for

    small aircraft in Figure Z-15 offered a useful alternative

    to the monster-ground-wire-bundle.



    The same rules for grounding all wires common to a single

    system be tied to a single point apply with one caveat:

    Systems that talk to each other my experience ground loop

    issues in spite of the fact that a single ground is use for

    each system. One example, a FADEC computer at the tail of

    the airplane may have ground loop problems with small signal

    wires that run forward to a display system. Solution? Non-

    metallic (conducting) signal systems. These include fiber

    optics, transformer coupled (a-la Mil-STD-1553), capacitor

    coupled (radio frequency modems), etc.



    When the system designer picks out major components of

    the large airframe, an awareness of potential ground loop issues

    need to be part of the consideration. For our little airplanes,

    the firewall ground block augmented by the recently proposed

    avionics (panel) ground block are part of that thought process.



    It's good that you're exercising the gray-matter on this

    topic . . . know that taking all EIS grounds to either

    forward or aft firewall grounds is an good move.



    Bob . . .

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