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Thread: P Lead Wire Gauge

  1. #1
    Eddie Foy's Avatar
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    P Lead Wire Gauge

    What gauge should I use for my P leads?
    "Put out my hand and touched the face of God!"

  2. #2
    Darrel Starr's Avatar
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    I used shielded 16 gauge. This is the only shielded wire on the airplane in which the shield should be grounded at both ends.

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    skywagon8a's Avatar
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    Quote Originally Posted by Darrel Starr View Post
    I used shielded 16 gauge. This is the only shielded wire on the airplane in which the shield should be grounded at both ends.
    At the mag and on the switch. The circuit starts at the mag points through the center wire to the switch to the shield and back to the mag case.
    N1PA

  4. #4
    Eddie Foy's Avatar
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    Do you use 16 ga wire too?
    "Put out my hand and touched the face of God!"

  5. #5
    skywagon8a's Avatar
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    Never had to change it. There is no current flow as it's only purpose is to ground the primary winding on the coil to keep the mag from operating.
    N1PA

  6. #6
    wireweinie's Avatar
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    20 gauge is more than enough. The only reason I don't go smaller is the high vibration the wire has to withstand. But, electrically, there is no reason to go with a large wire as there is almost no current flow through it.

    But go with shielded wire, as stated above. Ground one end of the shield to the mag body and the other end to the ground terminal on the switch.

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  7. #7
    Eddie Foy's Avatar
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    Quote Originally Posted by wireweinie View Post
    20 gauge is more than enough. The only reason I don't go smaller is the high vibration the wire has to withstand. But, electrically, there is no reason to go with a large wire as there is almost no current flow through it.

    But go with shielded wire, as stated above. Ground one end of the shield to the mag body and the other end to the ground terminal on the switch.

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    One wire or two?
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  8. #8
    wireweinie's Avatar
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    One piece of single conductor, shielded wire for each mag.

    And don't bundle them with any other wires.

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  9. #9
    Darrel Starr's Avatar
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    I should say that I used 16 gauge for durability not for any electrical reason.

  10. #10
    algonquin's Avatar
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    You can also solder a peice of 20 ga to each end of the shield and a connector to attach to the mag and switch.

  11. #11
    skywagon8a's Avatar
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    Quote Originally Posted by algonquin View Post
    You can also solder a peice of 20 ga to each end of the shield and a connector to attach to the mag and switch.
    Just attach the connector to the shield. It will be more flexible and less likely to fail.
    N1PA

  12. #12
    Eddie Foy's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Just attach the connector to the shield. It will be more flexible and less likely to fail.
    I am using solder sleeves.
    https://youtu.be/uhRCUAYoSXg
    "Put out my hand and touched the face of God!"

  13. #13
    cubdriver2's Avatar
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    I thought a crimped connection held up better to vibration then a soldered connection?

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  14. #14
    algonquin's Avatar
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    I would think the solder sleves are the ticket, simple, neat,sealed and easy. That connection shouldn't move much being stabilized by the main wire.

  15. #15
    skywagon8a's Avatar
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    Quote Originally Posted by Lefoy84 View Post
    I am using solder sleeves.
    https://youtu.be/uhRCUAYoSXg
    Learn something new every day. Other than there now being two connectors to do one job, this should work. And as a reminder this ground connects to one of the screws on the mag.
    N1PA

  16. #16
    cubdriver2's Avatar
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    Can you use a 2 conductor shielded wire for both mags and share the ground sleeve?

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  17. #17
    wireweinie's Avatar
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    [QUOTE=cubdriver2;656881]Can you use a 2 conductor shielded wire for both mags and share the ground sleeve?

    Glenn[/QUOT

    I wouldn't. Each mag produces it's own power. When you turn a mag 'off', you simply short the P lead to ground. In this case, ground being the mag body. If you use a shared ground point you take a chance on a single point of failure, i.e. one bad ground connection and now both mags are hot.

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  18. #18
    Eddie Foy's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Learn something new every day. Other than there now being two connectors to do one job, this should work. And as a reminder this ground connects to one of the screws on the mag.
    Did I teach an old dog a new trick?

    I had never heard of them either till recently.
    "Put out my hand and touched the face of God!"

  19. #19

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    So, I have my shield wires going into one ring terminal and grounded "near" the switch. Should it be "on" the switch? Craig

  20. #20
    skywagon8a's Avatar
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    Quote Originally Posted by snowyriver View Post
    So, I have my shield wires going into one ring terminal and grounded "near" the switch. Should it be "on" the switch? Craig
    "on" the switch. Assume that the airplane is made of wood. The only components are the magneto, the switch, and the "two" wires connecting them both. Since there are two mags there are two switches and two wire pairs. The two wires are the center conductor and the outer shield. Two separate wires are used because the requirement is for two separate and independent ignition systems. The reason that a shielded wire is used is to reduce or eliminate the electrical "noise" in the radios. More common in low frequency older radios.

    Another thing to remember is that when the switch is in the "ON" position (particularly if it is a toggle as Eddie is using), the magneto is OFF. With the circuit closed the magneto is grounded and will not operate. Thus by the same token, if the wire is broken the magneto will always be ON or "HOT". The rotary magneto switches are labeled appropriately.
    N1PA

  21. #21
    wireweinie's Avatar
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    Quote Originally Posted by snowyriver View Post
    So, I have my shield wires going into one ring terminal and grounded "near" the switch. Should it be "on" the switch? Craig
    Read what Skywagon said.

    There are two ways of setting up mag switches; the 'all in one' switch (like a key switch or and old military switch) and a pair of toggles. If you use any key switch, you need to locate the terminals on the back that are marked for 'left', 'right', and 'ground'. The center conductor of each P lead is connected to it's own terminal, i.e. the lead from the right mag goes to the 'right' terminal and the lead from the left mag goes to the 'left' terminal. Now pigtail the shield from each lead and connect both pigtails to the 'ground' terminal. Go to the magneto and connect the center conductor to the p lead terminal and pigtail the shield and connect that pigtail to the mag body.

    If you choose to use two toggle switches, you need to start with two, single pole, single throw switches (or the equivalent). Pigtail both ends of the P leads and connect them to the mags, as above. But attach the center conductor and pigtail to a toggle switch so that when the toggle is pushed down, the center conductor and pigtail are connected through the switch. Do this for both switches and MARK THEM (please).

    Both of these techniques will work with any magnetos on any type of aircraft.

    Also note that Lycomings with one impulse coupling installed may need additional steps to insure that the non impulse coupled mag will not fire during start up.

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  22. #22

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    So, what I'm trying to learn is: is there any problem with having the shield wires at the switch end grounded near the switch but not on it? I don't think I have any noise issues. My mags are still grounded safely, right?

  23. #23
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by snowyriver View Post
    So, what I'm trying to learn is: is there any problem with having the shield wires at the switch end grounded near the switch but not on it? I don't think I have any noise issues. My mags are still grounded safely, right?
    it is best NOT TO GROUND AT SWITCH

    because if engine ground is/gets bad, starter and alternator will try to use the tinny switch ground path and MELT the plead plastic insulating coating and make the engine get quiet at a bad time... has happened

  24. #24
    Gordon Misch's Avatar
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    Hadn't thought of that, and it caught my eye since I found a broken engine to mount ground at my last annual. Fortunately I had installed the belt and suspenders arrangement using a redundant ground with a #8 wire from engine to firewall per your recommendation.

    However - - seems this same mag failure mode could happen if the switch end of the p-lead shielding is grounded directly to the airframe. What am I missing here??
    Gordon

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  25. #25
    skywagon8a's Avatar
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    Quote Originally Posted by snowyriver View Post
    So, what I'm trying to learn is: is there any problem with having the shield wires at the switch end grounded near the switch but not on it? I don't think I have any noise issues. My mags are still grounded safely, right?
    Quote Originally Posted by Gordon Misch View Post
    Hadn't thought of that, and it caught my eye since I found a broken engine to mount ground at my last annual. Fortunately I had installed the belt and suspenders arrangement using a redundant ground with a #8 wire from engine to firewall per your recommendation.

    However - - seems this same mag failure mode could happen if the switch end of the p-lead shielding is grounded directly to the airframe. What am I missing here??
    There seems to be a misunderstanding of the meaning of "grounding" as it applies to a magneto. This is NOT an electrical circuit which is connected to a battery with various electrical components which you turn on and off with the airframe being one of the conductors in the circuit.

    A magneto is a self contained electrical generator. It's ground path is it's case. It's electricity flow is generated by a rotating magnet which excites an electron flow in the primary winding of a coil. This flow passes from the coil winding through a switch (the Points) to the case and back to the primary winding. Thus a complete circuit. The P lead is connected to the coil side of the points. The P (Points) lead wire goes to a on/off switch in the cockpit. Another wire (the shield) is connected to the other pole of the cockpit switch. The other end of this shield is connected to the magneto case. Thus another complete circuit. When the cockpit switch is turned off, the switch contacts are closed. (This is just the opposite of what you normally think of as off. Normally OFF is OPEN and ON is CLOSED when you think of a switch). So when the magneto switch is closed (OFF) a circuit is completed from the coil side of the points around the points to the case side of the points. When the cockpit switch is closed the current flow from the primary coil bypasses the points taking them out of the circuit. When the points can no longer interrupt the current flow through the primary winding, the current flow in the secondary winding does not happen. The output from the magneto stops.

    The engine is connected to the airframe through rubber shock absorbing mounts. There is NO electrical connection between the engine and the airframe. The redundant ground with a #8 wire from engine to firewall is the ground return wire for the electric starter only which is connected to the battery in a totally separate circuit. This wire is NOT redundant as it completes the starter circuit. Think about a no electric J-3.

    Why do we normally use a shielded wire for the magneto P lead circuit? When the points in the mag open and close interrupting the current flow they generate an electrical pulse. This pulse travels along the P lead wire which acts like a radio transmitting antenna. IF you have a radio in the plane it will receive this signal as a tick,tick,tick sound. When the P lead wire has a shield, the shield prevents this tick from transmitting out of the wire. IF you don't have any radios in the plane this wire does not need to be shielded. Wireweinie may be able to come up with further reasons?
    N1PA
    Likes Eddie Foy liked this post

  26. #26
    RaisedByWolves's Avatar
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    Hey Eddie. Where did you get those connectors?

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    wireweinie's Avatar
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    Thank you Skywagon and Mike.

    Besides the noise and 'melty' issues, as above, I have seen cases where the mag did not connect to the ground path of the airframe. Between the heavy paint, gaskets, and rubber drive blocks, the mag body would not electrically read to ground. In cases like this the ONLY way to switch the magneto 'off' would be if the mag body, itself, was grounded back to the switch.

    Think of it another way; if you connect the ground terminal from your switch to air frame ground, how many riveted and bolted joints does that ground path take before it reaches the mag body? If you ground the switch to the mag body, directly, with the shield braid, there are NO joints for the ground path to cross. Better electrical path means the circuit works more reliably, means safer, means less chance of hot mags.

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  28. #28
    Gordon Misch's Avatar
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    The engine is connected to the airframe through rubber shock absorbing mounts. There is NO electrical connection between the engine and the airframe. The redundant ground with a #8 wire from engine to firewall is the ground return wire for the electric starter only which is connected to the battery in a totally separate circuit. This wire is NOT redundant as it completes the starter circuit. Think about a no electric J-3.
    Thanks for your well written post Pete. Actually I did know what you wrote. My redundant engine ground paths are from engine case to engine mount (flex strap), and engine case to firewall (#8 wire). As you stated, those have nothing to do with the mag circuit. My mag circuit is as you described, shield to the ground post on the switch and to the mag body.

    However if both of the engine ground paths were to fail, then the starter and alternator would try to ground via the p-lead shield (and primer line and carb control cables). My contention is that it wouldn't matter whether the P-lead were bonded to the airframe or the mag switch; it could be subject to a high current either way.
    Gordon

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  29. #29
    wireweinie's Avatar
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    The chances of TWO heavy grounds from engine to airframe, failing together is practically zero. It's about as safe as you can make it. Your primer lines, regulator lines, and P leads are safe, lol.

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  30. #30
    wireweinie's Avatar
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    Quote Originally Posted by RaisedByWolves View Post
    Hey Eddie. Where did you get those connectors?
    The ones I use are from Aviall. They don't have the wire lead pre installed on them as I usually use a couple of different techniques to terminate shields, depending on location. The part numbers I use are, from large to small, L-C-5, L-C-3, and L-C-2. Use one that fits snug over your shield/pig tail, before applying heat. Also, apply a dab of non corrosive flux before you install the sleeve.

    Use them carefully as they can be as much as $1.50 each depending on where you buy them.

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  31. #31
    skywagon8a's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    .....However if both of the engine ground paths were to fail, then the starter and alternator would try to ground via the p-lead shield (and primer line and carb control cables). My contention is that it wouldn't matter whether the P-lead were bonded to the airframe or the mag switch; it could be subject to a high current either way.
    Except that the P-lead shield is not airframe grounded at the switch end. Only at one end, the mag. Therefor would not "ground" the starter.
    N1PA

  32. #32
    cubdriver2's Avatar
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  33. #33
    wireweinie's Avatar
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    Test them out before you use them on a harness for install. Sometimes you get the quality that you paid for. So be cautious.

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  34. #34
    Gordon Misch's Avatar
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    Except that the P-lead shield is not airframe grounded at the switch end. Only at one end, the mag. Therefor would not "ground" the starter.

    In Mike's post it sounded like he was referring to the braid grounding to the airframe through the switch (specifically a mag switch, not toggles). Maybe I misinterpreted that. Again, if I read his post correctly, I think the hazard he was referring to would not be mitigated by grounding the shielding to the airframe (which would require bonding the ground post on the switch to the airframe).

    Oh wait - Maybe he meant bond the switch ground post to the airframe and leave the braid connected ONLY at the mag, then the potential hazard of overcurrent in the braid is eliminated. I think maybe that's what I did on my 12 - gotta go look. Interesting discussion.

    Gordon

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  35. #35
    Eddie Foy's Avatar
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    Quote Originally Posted by RaisedByWolves View Post
    Hey Eddie. Where did you get those connectors?
    I assume you mean the solder sleeves. Steinair dot com has them for sure. Spruce probably does too.

    The Ebay listing that Glenn posted is for butt splices. The ones for adding pigtails are a little different.
    Last edited by Eddie Foy; 04-03-2016 at 03:52 PM.
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  36. #36
    cubdriver2's Avatar
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    Quote Originally Posted by Lefoy84 View Post
    I assume you mean the solder sleeves. Steinair dot com has them for sure. Spruce probably does too.

    The Ebay listing that Glenn posted is for butt splices. The ones for adding pigtails are a little different.
    Sorry, I couldn't copy the link on my pad

    www.ebay.com/sch/i.html?_nkw=solder+sleeve



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  37. #37
    texmex's Avatar
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    Why do we normally use a shielded wire for the magneto P lead circuit? When the points in the mag open and close interrupting the current flow they generate an electrical pulse. This pulse travels along the P lead wire which acts like a radio transmitting antenna. IF you have a radio in the plane it will receive this signal as a tick,tick,tick sound. When the P lead wire has a shield, the shield prevents this tick from transmitting out of the wire.
    Not sure why it's copying in bold.

    But, if in normal operation, with the mag switch in the 'ON' position, the path between the wire from the primary coil, near the points to the mag switch, returning to the mag case is OPEN, how would the pulse travel along the P lead wire?
    Wouldn't current only flow along the P lead if the mag switch was 'OFF'?
    Thanks for the post Skywagon8a and wireweinie. I'm just trying to get it straight in my head. I've had that annoying tick before but was lead to believe it was an alternator noise not being suppressed. I'd love a separate thread on that please.

    Due to age, alcohol and or jet lag, the electrical currents in my head are not the sharpest. Could someone explain Mike's comments a little more? Is it if the engine earth straps are failed there will be significant current from the alternator looking for a pathway? One possible pathway is from the engine, mag case, P lead shield and an arch on the mag switch to the P lead?? This still would not complete the alternator circuit would it. I'm missing something.

    Also regarding the contecting of the P lead shield to the airframe. How would that complete the circuit of the P lead. That is from the Primary coil, out to the switch via the P lead core wire and back to the case via the mag case via the P lead wire shield. Wouldn't that circuit always be open, and therefore the mag 'HOT'??

    Electricity!!! What fun.

  38. #38
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by texmex View Post
    But, if in normal operation, with the mag switch in the 'ON' position, the path between the wire from the primary coil, near the points to the mag switch, returning to the mag case is OPEN, how would the pulse travel along the P lead wire?
    thats called an antenna

  39. #39
    wireweinie's Avatar
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    Quote Originally Posted by texmex View Post
    Not sure why it's copying in bold.

    But, if in normal operation, with the mag switch in the 'ON' position, the path between the wire from the primary coil, near the points to the mag switch, returning to the mag case is OPEN, how would the pulse travel along the P lead wire?
    Wouldn't current only flow along the P lead if the mag switch was 'OFF'?
    Thanks for the post Skywagon8a and wireweinie. I'm just trying to get it straight in my head. I've had that annoying tick before but was lead to believe it was an alternator noise not being suppressed. I'd love a separate thread on that please.

    Due to age, alcohol and or jet lag, the electrical currents in my head are not the sharpest. Could someone explain Mike's comments a little more? Is it if the engine earth straps are failed there will be significant current from the alternator looking for a pathway? One possible pathway is from the engine, mag case, P lead shield and an arch on the mag switch to the P lead?? This still would not complete the alternator circuit would it. I'm missing something.

    Also regarding the contecting of the P lead shield to the airframe. How would that complete the circuit of the P lead. That is from the Primary coil, out to the switch via the P lead core wire and back to the case via the mag case via the P lead wire shield. Wouldn't that circuit always be open, and therefore the mag 'HOT'??

    Electricity!!! What fun.
    For alternators, other than a diode failure, noise is mainly a whine that varies with engine RPM. Verify it by pulling the field breaker and then the output breaker. If the noise stops with one or the other, the charging system is the source. Mag noise, too, usually shows as a whine, except at low RPMs. Unless it's caused by both mags, you should be able to verify the source by doing your mag drop check. A 'tick, tick, tick...' is most likely a spark plug or lead. Check the plugs on a spark tester and inspect the leads. If you can get ahold of one, use an old school lead tester. It works kind of like a megger by putting a high voltage on the center conductor and seeing if it jumps to the shielding. Another common source of that ticking is the little pin/spring terminal end coming loose. Process of elimination.

    You are correct in pointing out that when the mags are 'on', the mag switch is in the open position and current will not be flowing. But it's trying hard, lol. Think of it like a water pipe when the pressure pulses for some reason. You can hear the pipe do the 'water hammer' thing. If you bundle P-leads with other wires, especially anything with audio signals, you can inject this electrical noise into your intercom or radios. Be aware than many systems now have audio lines that can be hooked up to your audio system. Think of the alarms built into some instruments. Your P-leads can be bundled with these and inject noise without you being aware of the source. Just play it safe and don't bundle them with anything else.

    I think skywagon expanded on Mike's comment pretty well in #25. If you ground the mag body directly to the switch, you are now dealing with two separate ground paths. One is for the mag only and the other is for the battery/charging system only. As long as they are not physically connected the current from one system cannot travel through the ground path of the other. Think about the current to drive the starter motor. It goes out through the starter cable, through the starter motor, and enters the ground path of the crank case. From the crank case it travels through the ground strap, to the fuselage frame, and back to the battery through the battery ground cable. If the engine/firewall ground strap is broken or never reinstalled (ahem) the current is going to attempt to return to the battery through any available means. And I mean any. Any conductive item attached, both to the engine and the fuselage will now become part of the ground path. Control cables, primer lines, etc. Now, if you have attached the shield of each P-lead to airframe ground, say up near the key switch, you'll have 100+ amps flowing through them from the engine to the airframe. For about three seconds. By the time you smell smoke, it's to late. But if the shields are only run from the mag body to the switch, it will be isolated from airframe ground, at the switch, and have no path to the airframe. The same explanation goes for the charging circuit. The alternator will seek a ground path just like the starter motor, just not quite as impressive as you're usually dealing with around 20 to 30 amps.

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  40. #40
    texmex's Avatar
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    1) Thanks for the noise elimination process there Wireweinie. I've copied that paragraph into some notes I keep. When you say
    Another common source of that ticking is the little pin/spring terminal end coming loose.
    , you mean the cigarette ends of the leads? Do you use DC4 on these?

    2) I can wrap my head around this. Even though there is no current flowing through the P leads when the mag switches are on (switch open) it's acting as antenna, effected by the primary coils creating and collapsing magnetic field as the points open and close. Hence we need Faraday's cage, or shielding around that wire. (who ever created electricity????)

    3) Done correctly, taking the shielding to the other pole in the mag switch, and assuming the switch is insulated/isolated from the frame, there is no path for the separate electrical or starter systems to use the p-lead path.

    If you where to join the p-lead shield to the earthed airframe, and not to the switch at all, obviously this would make the disable the switch, creating hot mags.

    Ok. The penny has dropped. What Mikes saying is if the shielded braid is joined to the switch pole, AND then to the ground it would operate normally BUT in this case if there was an engine ground wire issue a lot of current could flow down the shielded braid, causing it to get hot and melt the inner wirer insulation. The same as switching the mags off.

    I get it now. Crazy stuff.

    No winter for me. I'm in China and flying around the wrong hemisphere, east west daily. The 78 flies far too far. No Cub flying for the next six weeks also as a result!

    Thanks.

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