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Grounding Question

 
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Dennis Johnson



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

PostPosted: Tue Sep 26, 2006 7:49 pm    Post subject: Grounding Question Reply with 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?

Thanks,
Dennis Johnson
Lancair Legacy, wiring the panel
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PostPosted: Wed Sep 27, 2006 7:10 am    Post subject: Grounding Question Reply with quote

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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