On Oct 2, 12:14*pm, "
wrote:
On Sun, 2 Oct 2011 05:47:01 -0700 (PDT), "





wrote:
On Oct 1, 11:56*am, wrote:
On Sat, 1 Oct 2011 07:42:27 -0700 (PDT), "

wrote:
On Sep 30, 12:58*pm, DA wrote:
responding tohttp://www.homeownershub.com/maintenance/NEC-question-low-voltage-wir...

DA wrote:
Is it OK for the network cables to lie on the ceiling support members,
and thus beneath the Romex? Or should the network cables be above the
Romex? Or should one or both be in conduit?

Are you concerned with EMI (electromagnetic interference) or fire safety?

For fire safety NEC wants you to stay 6+ inches away from the power cables

You have a cite for that? *In addition to not believing it, it's hard
to see what difference 6 inches is going to make. I
regularly see low voltage cables of all kinds stapled near
Romex.

and that should also be far enough for EMI to be negligible. That shield
on the network cables is absolutely unnecessary in a residential home,

but
since you already go it, do not ground it on either end. I presume you
won't be able to ground it correctly at the outlet end anyhow, so just let
it float. It'll be useless as an EMI shield (which you don't need anyway)

Says who? *A Farraday cage is a Farraday cage
whether it's grounded or not.

*My experience has been an ungrounded sheild just becomes an antennaa

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A metal shield is just that. *It shields whatever is inside
it from an electric or varying magnetic fields outside it.
That is basic physics and requires no ground. *So, the
shield itself could be an antenna, but the conductor
within is shielded.

Not really true. *Whatever (common mode) is inside can couple to the shield,
making it the antenna. *

It most certainly is true. Go Google "Faraday Cage"
or look up Maxwell's Equations.




This stuff is symmetrical, so the same happens
outside-in. *

Yes, it's symmetrical, which is why surrounding an
antenna with a Faraday cage would render it ineffective.

The key is that the twists make the common mode cancel on the
differential signal, within limits. *

That's true but unrelated to shielding.


The shield keeps in any common-mode
radiation but it must be grounded to do so.

Following that logic, if run a microwave oven on
an extension cord without a ground wire, it will
spew microwaves all over the kitchen. Of course
it doesn't, because it's a Faraday cage, which
requires no ground.



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That is correct with regard to STATIC magnetic fields.
It most certainly does shield varying magnetic fields.