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Re: fast ethernet limits

  • From: Måns Nilsson
  • Date: Tue Jan 14 19:24:19 2003

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- --On Tuesday, January 14, 2003 12:31:24 +0000 "Stephen J. Wilcox"
<steve@telecomplete.co.uk> wrote:

> I thought earthing one side of a shielded cable (be it Cat5, or any other
> type) was actually part of electrical regulations. (maybe I just assumed
> that as a result of practice)

It varies with local regulations. In .uk, I'd say you need to ground the
screen, because a floating screen with potential difference is considered a
hazard at work. 

> It is a known fact that earth varies within buildings, and from place to
> place in the ground, this is nothing to do with faulty wiring in houses
> or fault north american continent, this is a simple chemical/physical
> phenomenon as a result of subtle changes in water tables, salt, minerals
> etc 
>
> So this means there is a difference in potential from point to point -
> thats voltage to you and me. So if you connect both of these together you
> get a flow of current - not good..

Current flow is inevitable. The key is to shunt it away ASAP, where "S"
means both "soon" and "safe", and to top those requirements, these
connections need to look like attractive paths to RF energy, in order to
cope with EM/RF interference rules. 
 
> Of course, when you ground one end it also means you should be careful
> when working at the other end not to touch the shied as you are earthed
> to local ground and it may therefore have a live voltage should you touch
> it.

See above. 

Summary: Ground often, ground well, mesh it, do away with long isolated
runs, and in general, avoid big potentials. Shunt them. 

This starts to look like the "peer locally" argument. Good to be back in
topic ;-) 

- -- 
Måns Nilsson            Systems Specialist
+46 70 681 7204         KTHNOC  MN1334-RIPE

We're sysadmins. To us, data is a protocol-overhead.
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