One for the RFI gurus

[airportkid]

The shielding on both P-Lead sections from the noise filters to the Bendix mags were badly frayed and the soldered "buttons" had apparently been cold soldered aeons ago so were also deteriorating on a Cherokee 235, so off we went to the mag shop. There we learned that noise filters on P-Leads are only covering up for some deeper sin of electrical installation and shouldn't be necessary. So rather than invest $150 in new filters, I hustled back to the 235 to measure the wiring length from the switch to magneto so the shop could fabricate fresh shielded leads to run unbroken the full distance.

But back at the 235 I found the leads dangling from the firewall grommets were unshielded wire. The P-Lead run from the switch to the filter was unshielded. Now this airplane has never had any noise issues - we're just replacing deteriorated components, not chasing noise. If I don't have to muck about under the panel rethreading fully shielded P-Leads I'd rather not. But I also don't want to inadvertantly create a noise problem.

It started me wondering why the P-Leads are shielded in the first place. When the engine's running the P-Leads are just bits of open circuit wire with no current flow. Where's their RFI come from? Thank goodness for Google. The P-Leads act as antennas; they don't need to be grounded into a closed circuit to raise an electromagnetic ruckus. They have to be shielded, and as far as I can tell, they have to be shielded their entire length.

So now I've got a pile of questions.

1. How come there's no noise from the unshielded wires running from the switch to the filters?

2. Since there's no noise from that source, could I run shielded P-Leads from the mags to a firewall terminal block, grounding the shielding at the firewall, and leaving well enough alone from the terminal block back to the switch?

3. Or maybe NOT ground the shielding, since it's already grounded at the magneto end. Google perusal has revealed that it's apparently not a good idea to ground both ends of the shielding - but I can't find any good explanation for why not. The portion of wire running from a filter to the mag has its shielding securely grounded at both ends, so this injunction seems inconsistent.

4. What's best: buying new filters for $150 and restoring the existing installation to what it was before? Or biting the contortionist bullet and running unfiltered but fully shielded P-Leads from the switch to the mag? Or going whole hog and replacing not just the filters but also the switch to filter wires shielded - and which end (or both) of the shielding should get attached to ground?

Many thanks for any insights!

[RVDan]

What does the Piper maintenance manual say about the P lead arrangement? That might be a good starting place.

[jdeas]

Originally Posted by airportkid

1. How come there's no noise from the unshielded wires running from the switch to the filters?

Electronics have gotten much better at rejecting outside interference

Originally Posted by airportkid

2. Since there's no noise from that source, could I run shielded P-Leads from the mags to a firewall terminal block, grounding the shielding at the firewall, and leaving well enough alone from the terminal block back to the switch?

There is noise, it's just being rejected by the existing radios. I use the shield as the mag ground so for me, a single uninterrupted shielded wire to the mag switch is best

Originally Posted by airportkid

3. Or maybe NOT ground the shielding, since it's already grounded at the magneto end. Google perusal has revealed that it's apparently not a good idea to ground both ends of the shielding - but I can't find any good explanation for why not. The portion of wire running from a filter to the mag has its shielding securely grounded at both ends, so this injunction seems inconsistent.

Old rule of thumb, gnd both sides to keep noise in, gnd one side to keep noise out. In this case, gnd both. Usually you only need to ground one side on audio lines (like the mic) as grounding both sides on those connections can cause ground loops (noise) in the audio system.

Originally Posted by airportkid

4. What's best: buying new filters for $150 and restoring the existing installation to what it was before? Or biting the contortionist bullet and running unfiltered but fully shielded P-Leads from the switch to the mag? Or going whole hog and replacing not just the filters but also the switch to filter wires shielded - and which end (or both) of the shielding should get attached to ground?

Is this a certified aircraft? Follow the manual and see if there is a problem after the installation is done correctly.