Grounding STP cables

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 Question by Ken Lahonta posted 29 May 2007
 Grounding STP cables
I am running a shielded patch cable between a router and a switch (located in separate buildings), neither of which is grounded (ie - both are double insulated and connect to standard 2-prong 12 volt power supplies) I am running a shielded cable due to EMI/RFI issues. Is there another way to ground the shield in this circumstance short of running it through a shielded patch panel on both ends? I can convert to UTP once I'm in either building.

Thanks
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 Answer by Dmitri Abaimov posted 09 Jun 2007
Dear Ken,

If EMI/RFI is an issue, you have to sustain shielding all the way through between the devices. You will then need to use shielded jacks (which you can ground to local telecom ground bus) and shielded patch cables. You will therefore open yourself to all kinds of ground loop issues and I am not at a position to consult you over the Internet because people's and property safety may be at risk.

If you float the ground on one end, you will break the ground loop current but create a possible voltage differential between the building's local ground and the shield that's grounded in another building which may be substantial. In addition, by doing that you will turn the shield into an antenna which may be just good length to amplify whatever EMI you were supposed to shield from in the first place.

If you float ground on both ends, you have just wasted your money on bunch of aluminum foil that does absolutely nothing.

So, if I scared you enough, do yourself a favor and use all-dielectric fiber optic cable to connect the buildings.

If I did not, hire a professional electrical engineer (PE) to figure out the grounding situation and make him/her work in tandem with your telecom contractor (unless they are both electrical and telecom, of course).

I would not recommend saving money by not hiring a PE because in this case this would be your liability in case anything happens. In any case, DO NOT do it yourself. You may need to put some of the liability on your hired contractor's insurance in case something happens.

Sincerely,
Dmitri Abaimov, RCDD

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