---------------------------------------------------------- TeamGrumman-List Digest Archive --- Total Messages Posted Mon 02/27/06: 2 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 11:36 AM - Re: P-leads and grounding of shields (FLYaDIVE@aol.com) 2. 01:07 PM - Re: P-leads and grounding of shields (TeamGrumman@aol.com) ________________________________ Message 1 _____________________________________ Time: 11:36:07 AM PST US From: FLYaDIVE@aol.com Subject: Re: TeamGrumman-List: P-leads and grounding of shields --> TeamGrumman-List message posted by: FLYaDIVE@aol.com In a message dated 2/26/2006 6:49:55 PM Eastern Standard Time, gilalex@earthlink.net writes: --> TeamGrumman-List message posted by: Gil Alexander posted on the other list too.... I'm installing a new Horizon electronic tach. and replacing my p--leads at the same time, since the switch is out, and the cowling off for the Mitchell instrument installation - the old "while we are it" - and the magneto end does look ratty... However, I can't find the correct wiring for the shields of the p-leads at the ignition switch. My p-leads have been changed in the past (actually had a log book entry...: ) so I no longer know what the correct original configuration should be... Do the p-lead shields connect to the ground terminal at the ignition switch? This would seem to be a good way of connecting, and no extra ground would be needed at the switch. However, Gary's schematic shows a separate ground wire at the switch (going to the airframe - called 51JA3), and NO shield connections at the switch.... this would also seem correct. But, if I attach the P-leads shield to the ignition switch ground, and also take it to the airframe, I have made a large ground loop in a noisy circuit - I think. This does not seem correct, but that is how the present switch is wired. Does anyone know the correct wiring of shields at the ignition switch? ...thanks gil in Tucson 77 Tiger ==================================================== Gil: I usually agree with Gary but in this case the BASIC rules of electronics would over ride what Gary said. The basic rule of: On RF (Radio Frequencies) the Shield is grounded at BOTH ends and on AF (Audio Frequencies) the grounding is done at ONLY the Source. In a Mag situation there could be some discrepancy and here is why. The rotation speed of the Mag and the opening and closing of the contact points as well as the speed that the spark is sent down the ignition wires IS at Audio Frequencies. And that would indicate that the Shield should be grounded at the Mag (Source). YET! If the noise is being transmitted to free air and then to the radios via the ignition wires then the noise is RF and should be grounded at both ends. I would start by grounding at both ends, fly the plane and see what happens. IF you want to try shield grounding at one end then all you have to do is snip the shield ground at the far end. Maybe even install a temporary switch and see what happens. BUT REMEMBER you are breaking the ground at the far end away from the Mag. Ground loops do not happen at RF. They are a DC and AF problem. If there is a noise problem involving RF it is just an RF problem. There are three rules when dealing with RF problems: 1 - GROUND 2 - BLOCK & 3 - SHIELD Barry "Chop'd Liver" ________________________________ Message 2 _____________________________________ Time: 01:07:48 PM PST US From: TeamGrumman@aol.com Subject: Re: TeamGrumman-List: P-leads and grounding of shields --> TeamGrumman-List message posted by: TeamGrumman@aol.com In a message dated 2/26/06 9:34:55 PM, gilalex@earthlink.net writes: > I presume 18 gauge shielded wire from Aircraft Spruce is OK.... MIL-C-27500 > I think I used 20, but, 18 is ok.