Magneto grounding

vbpiper · #1 12-24-2007, 04:15 PM

Forgive me if this has been covered in the past, but several searches of the archives did not produce an answer.
I am in the process of wiring my Slick magnetos with 18 gauge shielded wire and I am using the ACS key switch. It appears that the center conductor is to be connected to the P-lead on the mag and the appropriate terminal on the switch ( "L" or "R"). I have the jumper installed on the switch as instructed for the impulse-coupled left mag. Then do I connect the shield to the ground terminal on the magneto and leave the shield unattached at the switch end? No attachment to the engine ground or the engine-side of the ground block on the firewall?
I am so confused!
Thanks in advance.

PS Have a Merry Christmas!!!!!
Thanks, Doug, for another great year of VAF.

decathlon737 · #2 12-24-2007, 06:43 PM

Ground shield to mag, leave ignition switch side open.
Merry Christmas

vbpiper · #3 12-24-2007, 07:03 PM

Thanks for the quick response, Bill!

vlittle · #4 12-24-2007, 10:42 PM

Actually, Bill's advice may be incomplete-- you may end up with always hot mags.

I connected the shields to the magnetos and to the GRD terminal on the mag switch.

Aternatively, you could just ground the GRD terminal to an airframe ground and not connect the shields at the switch.

Either way, the GRD terminal must be grounded somehow. I prefer the shields because its less wiring.

Vern

az_gila · #5 12-24-2007, 10:44 PM

Quote:

Originally Posted by decathlon737

Ground shield to mag, leave ignition switch side open.
Merry Christmas

Actually, it's a little safer if you connect the shields to the GND connection on the switch.

When the magnetos are ON (engine running) the switch is open, and the shields will work as shields.

When the magnetos are OFF, then the switch will connect the shield to the P-lead and the short in the switch makes the magnetos safe. No local ground at the switch is needed.

The reason this is a little safer is that this wiring method has the minimum number of connections from the magneto ground terminal to the P-lead.

The other system will work too (providing a local ground at the switch) but many more connections are involved...

AeroElectric Bob has a nice description here, showing shield connections....

http://www.aeroelectric.com/articles...tchOptions.pdf

gil A

Merry Christmas to all....

elippse · #6 12-25-2007, 10:04 AM

The "P", "primary" lead, is connected to the primary of the spark coil. Under high power conditions, the voltage pulses on that lead may be in excess of 100V. Any current which flows from a source must return to that source, by whatever path is available. The voltage pulses in the shielded wire will couple to the shield, electrostatically and electromagnetically, and so the current in the shield must get back to the return side of the spark coil primary. The shield should be connected at the magneto case and at the ground terminal of the switch, AND NO WHERE ELSE! If you connect the shield to the aircraft "ground", these currents will then use the aircraft skin to go to the engine and then back to the magneto, radiating electromagnetically all along the way. This is where you get spark noise into your radios. When the current only flows back through the shield, the current flowing in both directions will effectively cancel as far as radiation is concerned. And whatever you do, please don't install a "filter" in the "P" lead circuit. The reason you had ignition noise is because you didn't have a properly shielded and grounded circuit, and the filter just adds another alternate current path.

Norman CYYJ · #7 12-25-2007, 10:38 AM

I agree with the above statement. Ground only the shielded wire are the Mags. Then use the shield as the ground connection on the Mag switch. A whole lot less noise in the avionics.

vbpiper · #8 12-25-2007, 01:06 PM

Thanks for all of the input! Right now I have the ground terminal of the switch connected to the aircraft common ground. Should I disconnect this and connect both of the shields from the mag wires to the ground terminal of the switch, so that both ends of the shields are connected; one end to the mag ground terminal and the other end to the switch ground terminal, with no connection from the switch to the aircraft common ground? I assume that the ground terminal on the mag provides a ground to the engine?
Thanks, and Merry Christmas. Gotta get off the computer before the wife catches me!!!
Can you tell I'm no electrician?

Norman, you have really built 5 planes????

Norman CYYJ · #9 12-25-2007, 03:22 PM

Yes and on number 6 right now. Hope to have it done in the next month.
Yes take the shields tie them together and then fasten them to the mag switch ground. Do not use the air-frame common point ground. The ground on the mag switch should only come from the P lead shields, which are grounded only at the mags.

elippse · #10 12-26-2007, 10:00 AM

Quote:

Originally Posted by Norman CYYJ

Yes and on number 6 right now. Hope to have it done in the next month.
Yes take the shields tie them together and then fasten them to the mag switch ground. Do not use the air-frame common point ground. The ground on the mag switch should only come from the P lead shields, which are grounded only at the mags.

Right on, Norman!