fast ethernet limits

Mon Jan 13 18:17:40 UTC 2003

On Mon 13 Jan 2003 (08:33 -0800), Kevin Oberman wrote:
> 
> > Date: Sun, 12 Jan 2003 19:59:08 +0100
> > From: =?ISO-8859-1?Q?M=E5ns_Nilsson?= <mansaxel at sunet.se>
> > Sender: owner-nanog at merit.edu
> > 
> > Some hours reading the back issues of the journal found at
> > http://www.compliance-club.com will hopefully inform you why star grounding
> > is a thing of the past. Ground both ends. If you are afraid of ground
> > loops,  place a heavy (as in 10-16mm2 or AWG way below 10) ground conductor
> > alongside the signal cable, and ground it firmly in both ends. That will
> > take the current away from the shields.
> > 
> > Ungrounded shileds are inefficient for EMI and RF shielding, while at times
> > efficient AC hum blockers.
> > 
> > And, IANAEE, but I've played with big sound systems that exhibit all these
> > problems.
> 
> I find this simply frightening! Have you any idea how much potential
> of ground can vary in a large building? It's easy to have AMPS of
> current flowing through the shield of a cable and enough voltage
> offset to be dangerous to people. (802.3 allows enough breakdown
> potential that the equipment is unlikely to have a problem, though.)
> 
> Grounded at one end is better than floating for shielding, so this
> almost reasonable (as long as the wiring is all installed to spec) but
> grounding at both ends in the wrong environment can lead to serious
> problems.
> 
> Since the 802.3 sections on 10Base-T does not deal with shielded
> wire at all, there is nothing there on the subject. But other sections
> on 10Base5 and 10Base2 are explicit that 10Base5 MUST be grounded at
> exactly one point and 10Base2 recommends that one point be grounded
> when the cable extends out of a room. More than one ground is
> explicitly prohibited.
> 
> The archives of comp.dcom.lans.ethernet are full of people who have
> high error rates because of multiple grounds.

Do also consider that a one end grounded shield in these circumstances
is a health and possibly fire hazard.
If you are running cable in a building where ground has significant
potential differences between the two ends of the cable, you have a
serious shock hazard occurring in a place where people aren't likely
to expect or take precautions against it.

I don't know what the current carrying capacity of a shield is, so how
much of a fire risk you might run if your one end only sheild were to
become two ended.

My answer would be that if this situation exists, either get it fixed
or use optical fibre.

-- 
Jim Segrave           jes at nl.demon.net