Magneto P-lead grounding
I cannot say enough about the proper grounding of the p-leads with a magneto. I've written this before but I'll repeat it since I've come across several examples of improper grounding.
The p-lead is connected to the breaker points in the magneto, and, in turn, connects to the primary of the ignition coil. Depending upon the load on the engine, the coil inductance and resistance, and the value of the capacitor, there can be as high as a 150V transient on the p-lead when the points open with the engine at high MAP. A shielded lead MUST be used from the magneto to the ignition switch. The shield MUST be connected to the magneto case and also at the magneto ground terminal at the switch. BUT, the ground at the switch MUST NOT be connected to the airframe. The high voltage transient on the p-lead couples to the shield, thus making the shielded wire a capacitor in parallel with the capacitor in the magneto. Since the coil is the source of this voltage, any current flowing from the coil, as from the p-lead to its shield, must get back to the coil. If you don't ground the shield at the magneto, but instead ground the shield ONLY at the switch and to the airframe, that high voltage, high frequency transient will, instead, flow through the airframe and radiate harmonic energy into the radios and also get into other low-level signals on its way back to the coil! Also, if you have one electronic ignition along with the magneto, and it is turned on from the same switch and connected to the same switch ground terminal, and the p-lead shield is NOT connected at the magneto, that transient will flow through your ignition unit's circuit board and then to its chassis and through the mounting screws to the airframe. This transient can do a lot of damage to sensitive electronic components as it flows through the EI!
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