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Capacitive coupling: fixing
Capacitively coupled currents can be rendered harmless by shielding the signal wire as in Fig. 6.  Shielding means enveloping the signal wire by a conductor connected to common; this shield may be the return conductor or it may a separate shroud that is external to the return conductor.  Instead of flowing in the signal wire the capacitively coupled current flows to common through the shield and so does not interfere with the signal. The shield acts like a gutter.
Fig. 6 Shield surrounds signal wire. Capacitively coupled spurious current (red) drains through shield to common without contaminating signal (green).
Troubleshoot ground loops with ease
without disconnecting anything!
US Patent 7,791,353 B2