Re: [SI-LIST] : Cable Shielding: How do you connect the drains?

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From: polus@lexmark.com
Date: Mon Apr 02 2001 - 12:43:52 PDT


It depends,

I speak from and SI/EMC point of view, not for safety etc.

From an EMC point of view you can get radiation with only 1 end connected -
that cable acts like a monopole, similar to a car antenna. It is general
practice to ground both ends, but I have attended seminars which offer other
opinions.

Also do not expect simply because one grounds the shield at both ends it will
make the cable either not radiate, or that it will not be susceptible to noise.
My experience is that the only real way to know is to test it in an EMC chamber.
Sometimes, grounding helps, does nothing, or makes the radiation worse.
Sometimes grounding one end helps, although theoretically it is not supposed to.

Certainly start with both ends grounded. Hope and pray it does not radiate.
Count your blessings if it works. Multipoint connections of the drain to ground
at the connectors is better than only 1 connection (although in most cases,
there is only 1 drain wire). A multipoint connection acts as like the shield on
a co-ax cable. Be sure to keep the drain crimps as short as possible. If the
cable has copper foil around it, again cut the foil back as little as possible
and have a neat ground crimp.

That's my two cents worth.

Harold Polus
Lexmark Laser Printer Development
SI, EMC

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