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Does a shielded cable provide any shielding if the shield isn't grounded? Would it be practically different than an unshielded cable?

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    \$\begingroup\$ Ungrounded shield can be worse than no shield at all. (This is for capacitive coupling.) \$\endgroup\$ – Nick Alexeev Jan 9 '17 at 17:03
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    \$\begingroup\$ There are situations where a shield is grounded at only one end. And sometimes the shield is actively driven by an amplifier circuit in an effort to reduce capacitance hot-to-shield. It is also possible that shield is connected to a local ground, rather than to earth. \$\endgroup\$ – glen_geek Jan 9 '17 at 17:21
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Acting as a faraday cage, an ungrounded shield covering a twisted-pair will be somewhat effective. When in the presense of an interfering electric field, both wires in the twisted-pair are influenced more similarly than without the ungrounded shield and, this means that if the input stage that receives the signal has a decent common mode rejection figure, then that shield has some effectivity but, there are much more gains to be made by grounding that shield at the input amplifier end (even if it means connecting to ground via a relatively low valued capacitor like 10 nF).

Grounding at both ends can make things worse in terms of earth fault currents being passed up the shield.

As with any shield used to reduce noise, balanced drive and termination resistors are vital to maintain lowest CMRR.

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  • \$\begingroup\$ What about wires that are not in a twisted pair? Does the ungrounded shield help a little or hurt any? \$\endgroup\$ – aswine Jan 9 '17 at 18:19
  • \$\begingroup\$ It will help a little for sure. \$\endgroup\$ – Andy aka Jan 9 '17 at 18:23

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