Mag Grounding (One More Time)

[glenn654]

I am rewiring the P-leads to my mags because the original builder of my -4 put magneto filters in the leads and having read in the Sacramento Skyranch Mag manual that this is incompatible w/ Slick Mags. I have read thoroughly the posts on this site about grounding mags and there are two conflicting methods discussed and to an electrical novice like myself I am left confused. The two methods are:

First, ground the P-lead shielding w/ pigtails at the mag and switch and nowhere else (I use toggles)
OR
Second, ground the shielding only at the mag and ground the switch to the airframe.

There are two other methods I can think of:

One is to ground both ends of the shielding as mentioned above plus add another pigtail to ground the switch to the airframe as well.

And the last is to ground the shield only to the switch and ground the mag to the engine ground.

Curious to know what the Brain Trust thinks of my alternative methods as well as the first two.


Glenn Wilkinson

[breister]

There's probably a better answer on Aeroelectric Connection, but I do know that unless the shield of a shielded line is meant to carry power or signal then it should only be grounded at one end. This is because the purpose of the shield is to reduce electromagnetic interference (with other electronics), and grounding at both ends can result in a "ground loop" which can actually make electrical gremlins worse.

[jrs14855]

Post on this quoting Aeroelectric on 10-26 under p-lead splicing. Ground only the mag end. On the Slick I try to keep the unshielded portion as short as practical

[glenn654]

Finish rewiring the mags tonight, grounded the p-lead shielding w/ pigtails at the mags and the toggles to the airframe, no possibility of ground loop. Test run proved everything working perfectly, no EMI noise.....I think I was thinking too hard as usual, was not difficult with all the great help I got here.

Thanks,
Glenn Wilkinson

[Mel]

Standard procedure is to ground the shield at the mag.
"P" lead to one side of toggle switch, shield to other side of toggle switch.
Shield not grounded to airframe at switch.

[rvbuilder2002]

I have found broken shield leads on aircraft enough times throughout my years working on them, that the method I always use is ground the shield at the mag. and not at the switch. Then connect the other side of the switch to ground. Even a properly done pig tail on a P lead can break. If done the way I describe you at least reduce the danger of a live mag. because of a wire brakage in half.
This way if the shield (fragle) or pigtail wire ever brakes it doesn't leave your mag hot.
I also have never seen an ground loop issue...when the engine is operating all connections for the P lead are only on the mag. end.

[mquam99]

I have seen a number of options for this.. Let me know if this makes sense..

P lead attached to mag and shielding attached to the "GND" post on the MAG.

At the switch which side does the P-lead attach to on a SPST switch?

my best understanding is that the switch gets installed upside down and the P-Lead gets attached to the top connection..

am I totally confused??

[Walt]

It doesn't matter which switch post they go to. With the switch off you want the shield to connect to the 'P' lead so when the switch is off the 'P' lead is grounded (this would be the center and upper terminals).