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Grounding Shielded Wire - One or Both Ends?

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antero

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Hanging out with lots of airplane people I have heard anecdotally time and time again that shielded wire should be used on sensitive equipment, especially things like audio wires, and that the shield should be grounded at ONE END ONLY. Usually this means grounding the shield at the source, i.e. grounding to the radio chassis for headphone connections and not at the headphone jack end. I have seen many homebuilts wired up this way. But I started to question this method after reading through my Garmin SL-30 radio installation manual. There, it shows in MOST cases that the shield should be grounded at BOTH ENDS - examples: the headphone connections (one to the radio chassis and one to aircraft ground) and radio/remote CDI signal connections (grounded to both chassis). However, it also shows that the radio / audio panel interconnections should be grounded at ONE END only, being the audio panel chassis.

Now, quoting scripture:

AC 43.13-1B, 11-89

With the increase in number of highly sensitive electronic devices found on modern aircraft, it has become very important to ensure proper shielding for many electric circuits. Shielding is the process of applying a metallic covering to wiring and equipment to eliminate interference caused by stray electromagnetic energy. Shielded wire or cable is typically connected to the aircraft’s ground at both ends of the wire, or at connectors in the cable. Electromagnetic Interference (EMI) is caused when electromagnetic fields (radio waves) induce high-frequency (HF) voltages in a wire or component. The induced voltage can cause system inaccuracies or even failure, therefore putting the aircraft and passengers at risk. Shielding helps to eliminate EMI by protecting the primary conductor with an outer conductor. Refer to MIL-DTL-27500, Cable, Power, Electrical and Cable Special Purpose, Electrical Shielded and Unshielded General Specifications.

and

AC 43.13-1B, 11-106

Wiring of sensitive circuits that may be affected by EMI must be routed away from other wiring interference, or provided with sufficient shielding to avoid system malfunctions under operating conditions. EMI between susceptible wiring and wiring which is a source of EMI increases in proportion to the length of parallel runs and decreases with greater separation. EMI should be limited to negligible levels in wiring related to critical systems, that is, the function of the critical system should not be affected by the EMI generated by the adjacent wire. Use of shielding with 85 percent coverage or greater is recommended. Coaxial, triaxial, twinaxial, or quadraxial cables should be used, wherever appropriate, with their shields connected to ground at a single point or multiple points, depending upon the purpose of the shielding. The airframe grounded structure may also be used as an EMI shield.

I am confused. For anyone who knows better than I, what is the proper method for grounding shielding - is it one end or both ends, or does it depend? Does it matter if it is an EMI source vs an EMI sink? What other considerations should I be thinking about?

Thanks in advance!
 
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