fast ethernet limits

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Date: Sun, 12 Jan 2003 19:59:08 +0100
Sender: owner-nanog@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.

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

2

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.

3

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.

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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.

Read the articles at www.compliance-club.com -- there is a two-piece
article on audio systems, by the technical manager at Cadac. It definitely
goes against all gut reactions we've forced ourselves into; especially in
the telco/audio business.

But it makes sense! It works! And, usually the audio folks were the last to
find out: Look at your average network component; the grounding scheme of a
big modern router. It is mesh grounds all over the place -- and because it
is the only way it will ever get the box approved by FCC/UL/CSA or any
similar agency.

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).

Which is why one must construct a low-resistance/impedance mesh, connecting
shields and grounds all over the place, so as to short out the potentials.
The steel frame could serve as one of these grids, but probably needs to be
augmented. And, I wrote "AWG way below 10"... If it is not enough, get a
thicker one, and get a new electrical contractor. (see below)

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

If you have potential differences like these the house needs a new
electrical grid. Wait, that means that the entire North American continent
needs rewiring :wink: Anyway, the mains PE MUST be made the best path home,
and in a fail-safe manner.

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

Look at the emissions that manage to get into a star grounded, one point
only system vs emissions bounced off a mesh grounded system with a decent
EMC testing rig and repeat that phrase afterwards :wink:

- --
M�ns Nilsson Systems Specialist
+46 70 681 7204 KTHNOC MN1334-RIPE

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

5

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

If you have potential differences like these the house needs a new
electrical grid. Wait, that means that the entire North American continent
needs rewiring :wink: Anyway, the mains PE MUST be made the best path home,
and in a fail-safe manner.

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

Look at the emissions that manage to get into a star grounded, one point
only system vs emissions bounced off a mesh grounded system with a decent
EMC testing rig and repeat that phrase afterwards :wink:

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 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..

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.

Steve

6

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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 :wink:

- --
M�ns Nilsson Systems Specialist
+46 70 681 7204 KTHNOC MN1334-RIPE

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