phillip.johnson(at)lmco.c
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 Posted: Wed May 27, 2009 6:15 am Post subject: AeroElectric-List Digest: 21 Msgs - 05/26/09 |
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Bob,
I read your message and went to your web link indicated below and I
can't accept that it is a good design. The problem that I see is that
your design does not create a single ground reference. Ideally you want
the engine block to be the ground reference with all currents flowing
through it. To this end you need to connect the battery -ve to the
crankcase and then make all grounds connect, either directly or
indirectly, to the crankcase as well. In doing this there will be no
ground potential differences to upset the instrumentation. If we look
at schematic given in http://www.aeroelectric.com/articles/grnding.pdf
there is no alternator shown but we all know that the alternator ground
is connected to the crankcase so all the current that the alternator
generates must pass through the engine block and then on to the battery
through the interconnections given in your circuit. If I now connect an
instrument such as an engine monitor to the instrument panel and ground
it as indicated in the diagram the ground voltage that the engine
monitor uses will be different to the ground reference seen by the
sensor located on the engine in particular when high currents are being
sourced from the alternator. Such a configuration will cause erroneous
readings unless a differential arrangement is utilised.
In addition to the grounding issues related to the instrumentation it is
likely that, at some point, someone will make some ground connections to
the engine block and some to the instrumentation ground. Somewhere , we
are all human, the two will interconnect (yes you could/should check for
all of these but people don't in particular when the owner is not the
builder). All will be well when the system is new but as the aircraft
ages and maybe the aircraft is sold to a new owner the ground between
the engine and instrumentation/firewall ground will be left off (maybe
after an annual inspection or maintenance session) or degraded through
loose connection. The pilot now tries to start the engine and all of
the current tries to flow through this instrumentation ground. This
causes a melt down of the wiring and a potential ground fire and risk to
those expensive electronics.
It is my opinion, and I believe that the automotive industry also adopts
this philosophy, that you connect the battery ground to the crankcase
and then strap the engine to the airframe/car body and system ground so
that the high current grounds pass only through the engine. Charging
ground currents, which can be extremely high, also only pass through the
engine block thereby preventing erroneous instrumentation and potential
fire risk.
OK I've now started to run and get my flame suit on.
Regards to all
Phillip Johnson
________________________________ Message 7
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Time: 01:56:29 PM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>
Subject: Re: Engine mount as starter ground path
At 10:14 AM 5/26/2009, you wrote:
| Quote: |
Does anyone have some scientific information (beyond the practical
description in the Connection) that I can use to convince someone that
running the engine ground strap to the engine mount is a poor choice ?
Thanks !
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The shortest, lowest resistance and minimum parts count path
in any high current conduction path is the road to Ground
System Nirvana. See:
http://www.aeroelectric.com/articles/grnding.pdf
Engine mounts are steel assembled with steel
hardware. The joints between mounts and other
components of the airframe are not treated for
the purpose of electrical "bonding". The engine
mount is intended to hold the engine on the
airplane and was not designed or installed to
be a part of the electrical system.
Note that Figure Z-15 and Chapter 5 of the
'Connection speak to and illustrate a notion
that engines are best tied to the single point
fire-wall ground block with a single-flexible-
fatwire. To avoid having the firewall sheet
participate in a PARALLEL ground path (read
ground loop) any bonding jumpers across the
engine mounts are best removed.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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