Shields and ground

Marco Trapanese · 2011-12-01T12:10:55+00:00

Hello,let's talk about this setup:- electric cabinet that contains power supplies and a main board. It has a proper connection to ground earth.- several boxes placed around the cabinet, few meters far. They are connected to the main board (inside the cabinet) using a shielded cable.- each one of these peripheral boards has another shielded cable that connect to an external sensor.I'm wondering how manage the cables shields.My thoughts:- main board (inside the cabinet): provide a large connection to the earth of the cabinet itself. Connect the local earth to the shields of outgoing cables.- peripheral boards (inside external boxes): provide two pins to connect together the shields of both cables (the one from the main board and the one from the sensor).- sensor: leave the shield floating.Questions:a) is it correct?b) even if the sensor is placed very close to a metallic item already grounded, I must not connect the shields there. Otherwise I'll run into a ground loop, won't I?The goals are, as usual, reducing EMI and induced noise, protect the boards from high voltage spikes...ThanksMarco

Dec 01, 2011 26 Replies

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upsidedown 14 years ago

This current can really be quite large, several Amperes or even tens of Amperes if TN-C

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wiring convention is used, in which part of the neutral current of an unbalanced load (or harmonics even from a balanced load) flows through the signal cable shield.

If an R or RC network is used to ground the other end of the shield, typically a 100 ohm resistor with at least a few watts of power handling capacity is used. With a 10 Vrms potential difference (caused by unbalanced 3 phase load or 3rd harmonics from rectifier loads) will dissipate 1 W in the 100 ohm resistor.

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Charlie E. 14 years ago

I did a little audio system for our city council chamber a few years ago, and it seemed to be fine. Then, they had the first meeting, and there was all kinds of noise. The problem? They broadcast their meeting over the local cable system. When the guy who does the cameras came in, he hooked my audio feed to his equipment, which was also connected to the cable company's equipment. MASSIVE ground noise! I had to find some ground loop interrupters and put them into my audio feed before the next meeting.

Charlie

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John Larkin 14 years ago

I should have mentioned abandoning the facility as another technique for fixing ground loop problems.

John

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Joerg 14 years ago

"If it absolutely, positively has to be gone by tomorrow 10:00am, call the Marines" or something like that? I bet they can make a ground loop problem go away a well, except you may need a new building :-)

Regards, Joerg http://www.analogconsultants.com/

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Glen Walpert 14 years ago

I recall a communications facility grounding handbook which had complete instructions for quick installation of ground rods by making the holes with shaped charges, so the Marines could have a solution not requiring a new building :-).

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Fred Abse 14 years ago

Judging by your expertise in the Kablouie business, I should have thought you could do it all by yourself :-)

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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Joerg 14 years ago

Well, I have served, so ... :-)

Regards, Joerg http://www.analogconsultants.com/

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