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messydeer
Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA
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 Posted: Tue Aug 23, 2011 3:19 pm Post subject: Magneto Grounding |
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I've got a Jabiru 3300, whose mags have fast on terminals. I know to connect them to the inner wire of the shielded ignition leads and to separate the outer braid, which connects to the engine ground. My #4 welding cable engine ground connects the firewall to the starter case. I'm planning on connecting both mag outer braids to the same spot, under the AN3 case bolt. So there'd be a #4 ring and two 20awg rings under this bolt. So far so good?
While reading about this I read that one person says he connects the mag grounds to each other at the switch. This would provide a redundant ground path in case of a failure of one mag ground. He said it wouldn't cause any ground loop noise problem. But isn't this why Bob says to "...attach the shield to one and only one switch terminal at the cockpit end..."?
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nuckolls.bob(at)aeroelect
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| Posted: Tue Aug 23, 2011 4:17 pm Post subject: Magneto Grounding |
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At 06:19 PM 8/23/2011, you wrote:
I've got a Jabiru 3300, whose mags have fast on terminals. I know to
connect them to the inner wire of the shielded ignition leads and to
separate the outer braid, which connects to the engine ground. My #4
welding cable engine ground connects the firewall to the starter
case. I'm planning on connecting both mag outer braids to the same
spot, under the AN3 case bolt. So there'd be a #4 ring and two 20awg
rings under this bolt. So far so good?
While reading about this I read that one person says he connects the
mag grounds to each other at the switch. This would provide a
redundant ground path in case of a failure of one mag ground. He said
it wouldn't cause any ground loop noise problem. But isn't this why
Bob says to "...attach the shield to one and only one switch terminal
at the cockpit end..."?
Failure of a magneto ground does not cause the engine
to stop . . . it only prevents the magneto control switch
from disabling that particular mag. This would be detected
during pre-flight . . . assuming that killing one mag
on a Jab3300 produces the classic 'mag-drop'.
If shutting down one of two mags does not produce a
tell-tale in the cockpit, then loss of a ground would
produce a latent failure. There's no good reason
to have multiple grounds but perhaps a good reason to
check ground integrity as part of a routine periodic
maintenance check.
I recommend each mag switch get grounded through the shield
of the p-lead where only the mag end of the p-lead shield
goes to the crankcase or other handy fastener on the engine.
Bob . . .
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