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I have a question about grounding.

I will have two short shielded cables with signal wires inside, located at the top of a tower. The cables have a grounding wire inside shorted to their foil shields to prevent the foil from building a charge.

I have two choices for terminating the grounding wire. (I only plan to terminate it at one end to avoid ground loops.)

A) I can attach it to the metal at the top of the tower. Let's say this metal body is ungrounded but it's decently big.

B) I can attach the grounding wires to signal ground which has to travel down long wires of 75 ft before it reaches its source.

Is there a clear better option for this? Does it even matter which I pick? I would appreciate any insight.

Thanks

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  • \$\begingroup\$ Please look at this article, especially the response by Howard Johnson. \$\endgroup\$ – bitsmack Mar 22 at 17:13
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I'll bet the additional 'grounding wire' is actually tied to the shield (ohm it out), if it's not, it is likely that its a signal ground. If it is a shield ground, then treat it like a shield, ideally the cables should be shielded all the way to the equipment.

If its a signal ground, it depends on what kind of signal it is. If it's differential (like ethernet, or RS485) then it needs to go down to the digital driver/receiver.

If your running the cable through a slip ring, then run the shield on an additional conductor if there is one available on the slip ring, then to the shield. The problem with running cables through slip rings is it allows for the signal to pick up noise because there is no shield (shield protect against electric fields). However, this is the best you can do.

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  • \$\begingroup\$ where is the most noise? \$\endgroup\$ – Jasen Mar 22 at 2:43
  • \$\begingroup\$ @Jasen What do you mean exactly? I expect there to possibly be some noise affecting the two cables, that's why I'm shielding them. But I don't really know how to answer the question if there's more noise in the metal at the top of the tower or in the signal ground. I think I misinterpreted your comment, could you clarify please? \$\endgroup\$ – user211492 Mar 22 at 13:16
  • \$\begingroup\$ @laptop2d The grounding wire is definitely a shield ground, which is shorted to the shielding inside the cable (I edited my question to clarify this). The thing is I have to bring my signals through a slip ring so the shielded cable can't go all the way, the wiring is shielded cable 1 -> slip ring -> shielded cable 2 -> DAQ \$\endgroup\$ – user211492 Mar 22 at 13:20

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