Purpose of grounding mag wire shield?

Dana said:
Yes, that's the whole point of shielding it.

Note that it should only be grounded at one end.
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Gets grounded at both the mag and the switch. From the Cessna 182 service manual:

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RF shielding is at both ends. AF shielding is at one end. Headphone jacks should be insulated from the panel. The headphone ground is taken all the way back to the audio panel ground.
 
Thanks. Didn’t think it had anything to do with hot-mag safety, but wasn’t sure (no expert).

Public service announcement for owner maintainers - don’t be so quick to recowl the plane to get it back in service after an oil change. Every single oil change I’ve found a few things that need attention. Now I try to find 1 thing to fix or improve; I usually find more. Especially on an older plane. Sometimes it takes me a few extra days to get it airborne again.
 
I suspect that’s the item he mentions finding on the oil change that he now has to repair.
 
Southpaw said:
Looks like a mouse took a bite out of one of the plug wires. What is the purpose of the ground connection with no wire connected ?
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Thats exactly the purpose. Like antenna coax, the mag ground wire has a braided shield on the outside to “keep” all the pulses from the mag from affecting radio and audio. That shield needs to be grounded on both ends from what I’ve learned from the gents above.

I think just the way the shield was crimped and its position, and annual maintenance, it eventually broke from metal fatigue.
 
Electromagnetic theory:

A magnetic field passed through a conductor will generate a current in that conductor.

A current passed through a conductor will generate a magnetic field around that conductor.

Radio waves are magnetic fields generated by electrostatic fields.

So the magneto's points opening and closing conduct and break current flows through the coil's primary winding. To shut that magneto off, we short across the points so that there is no abrupt cutoff of the current, so no abrupt and rapid collapse of the primary's field through the secondary, thing that generates the spark. That's a whole 'nother discussion. The P-leads that connect the mag to the switch therefore have pulsating current in them, sharply pulsating, and they generate electrostatic fields that make electromagnetic signals that your radios pick up.

The cure for that is the shield. The magnetic field generated by the pulse passes through that shielding, generating a current in it which is conducted to ground, thereby squashing the magnetic signal flat. It doesn't get past that shield. It's all used up making current in the shield.

Old cars were famous for AM radio noise from the ignition system, but it was less than that in airplanes. The battery was the source of the ignition primary power, and it absorbed a lot of the signal. Carmakers fixed a lot of the rest by securely grounding the engine to the body, making sure body panels were well-connected, and by using resistive sparkplug leads. Filters on the radio power supply also caught and suppressed noise picked up by the vehicle's wiring. Modern cars also don't carry ignition primary circuit signals into the car's interior. It's dealt with in shielded ECUs.
 
This EMI shielding prevents the mag from firing too early or too late. Any noise in this line feeds back to the ignition firing circuit.
 
NordicDave said:
Those P-Leads have seen better days, and likely the remainder of the ignition system as well.

Bogart makes a great cost effective P-Lead replacement. https://bogertaviation.com/search?type=product&q=p-leads
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Was going to replace mine with Bogert leads. They'red definitely 1963 original. After annual I found the left mag p lead disconnected but grounded. And the opposite on the right side. Got it corrected, but the shape of the wires is driving me nuts. Wasn't able to figure out what length for the pa28 180 I'd need and haven't gone out to measure. Any guesses?
 
Checkout_my_Six said:
This EMI shielding prevents the mag from firing too early or too late. Any noise in this line feeds back to the ignition firing circuit.
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So how did unshielded mags, like my old Case mags on my A-65, deal with early or late firing?
 
Checkout_my_Six said:
It probably never ran rough or misfired.... o_O
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P-lead noise isn't going to cause a mag to fire. The current in the primary is huge, several amps, and RF passing through the P-lead isn't going to generate anything like that. In all my training and experience I have never run across any ideas or problems like that. Electronic ignitions might be a different story, but mags are about as far as you can get, electrically, from electronics as is possible.
 
Dan Thomas said:
Gets grounded at both the mag and the switch. From the Cessna 182 service manual:

RF shielding is at both ends. AF shielding is at one end. Headphone jacks should be insulated from the panel. The headphone ground is taken all the way back to the audio panel ground.
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Audio panel manufacturers single point ground shields to prevent ground loops. Shouldn’t P lead and voltage regulator shields be single point grounded too?
 
Michael Denman said:
Audio panel manufacturers single point ground shields to prevent ground loops. Shouldn’t P lead and voltage regulator shields be single point grounded too?
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Ground looping through those doesn't cause audio noise.

P-leads are basically little broadband RF antennas, as are voltage regulator wires. To get maximum suppression, the shielding needs to be grounded at both ends so that the current generated in them gets shunted to ground. I have seen some audio shielding grounded at both ends, but grounded to the same point. The audio panel end is grounded to the audio panel, the other end has a wire that runs all the way back to that same point. That maximizes the shielding but avoids the ground loops that introduce noise.

Electronic voltage regulators (ACUs) are notoriously sensitive to stray RF. Their voltage-sensing circuit needs only a few microamps, in contrast to the electromechanical regulators that drew lots of milliamps to run the regulator relay. Such sensitivity can cause lots of frustration. I fought with our 185 over that. It was dropping the alternator offline constantly, requiring recycling of the alternator switch to get it back up. We finally noticed that it would drop offline when we keyed the mike. An obscure little paragraph in the service manual suggested a look at the antenna cable grounding at both ends, and sure enough, the antenna end was corroded and filthy. It was above the headliner, where condensation of the moisture from the occupants' breathing caused the corrosion. Cleaned it up and the alternator behaved after that. The RF escaping from the faulty shielding would cause voltage spikes in the sense line to the ACU, fooling it into thinking there was an overvolt situation.

ELT remote switch wiring has been famous for that, too. Sets off the ELT.
 
Dan, what about shielded strobe wires, should they be grounded at both ends?
 
Dan Thomas said:
Ground looping through those doesn't cause audio noise.

Electronic voltage regulators (ACUs) are notoriously sensitive to stray RF.
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I sent an email to Avidyne for their input.

Cessna never shielded their ACU or alternator wire after leaving the 4 wire automotive regulators (1980ish). Plane Power recommends unshielded wire for their R1224 regulator.
 
Shout-out to Michael Denman, if you’re the same guy that builds and sells high quality battery cable and other wiring related stuff.

You sent me a cable of exact length, the terminals clocked exactly like I needed, and heat shrink and terminal witness holes sealed. Top quality, from what I can tell.
 
Michael Denman said:
Cessna never shielded their ACU or alternator wire after leaving the 4 wire automotive regulators (1980ish).
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The old electromechanical regulators had a regulator relay in them that was buzzing constantly, opening and closing to control the field current. The inductance of the field winding leveled off the pulsation somewhat, but enough was left that it cause audio noise. Even some of the hybrid regulators, with a transistorized regulation section, had the on-off stuff, fired by a zener diode. The ACUs that showed up around 1980 use variable field current, not the "digital" on-off that causes noise.

The field noise was enough to upset the ADF. A capacitor on the field terminal of the alternator fixed that. Shunted the pulsations to ground.
 
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