fast ethernet limits

Mon Jan 13 18:35:14 UTC 2003

I find the same Kevin..I've done a lot of work in broadcast stations as 
well, and ground loops are a constant problem. Hum is introduced into audio 
lines, even in balanced pairs, and Cat5 is not much different.

In a high rise, I can see a neutral failing somewhere on a high floor, and 
that piece of #10 going incandescent, setting fire to anything combustible 
between the floor its terminated on and earth ground. (The resistance of an 
old steel framed building is NOT always lower than that piece of copper, 
especially old riveted buildings).

Maybe they do it differently in the EU, but fire safety is also a concern 
to me.

Thank you, but I'll ground nearest to the earth, common bonded point. Its 
served me well over the years.

> >
> > 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.
>
>R. Kevin Oberman, Network Engineer
>Energy Sciences Network (ESnet)
>Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
>E-mail: oberman at es.net                  Phone: +1 510 486-8634