Sheilded Cat-5E Ground Loop - Myth or Reality?

Wed Apr 10 21:04:50 UTC 2002

I was simply pointing out that "ALL electrical installations" seemed a tad
too broad.

> Shielded/screened TP on utility poles? This is an insane thing to
> do. It violates more rules than I want to even start to think about!
> Telco wires are always connected to lightning arrestors at the
> termination of the long run. (This is NOT always the same as the
> demarc!) MGN is MANDITORY. This is not a do-it-yourself project.

The telco outside plant cables are all shielded. Generally one shield over
all, but there are special purpose cables (Z or D screen often in a
trademarked name) that have two seperate shielded compartments to eliminate
near end crosstalk on low pair count cables where opposite side of the cable
binder-group seperation in not sufficient for T1s or T1Cs.

The shield on outside plant telco cables is not just an electrostatic shield
(where one end grounding works) but is also part of a hum bucking strategy
where the shield shorted to ground at both ends is exposed to the same
powerline magnetic fields as the pairs within, and with the shield
intentionally shorted (however imperfectly) end to end via ground, the
resulting magnetic field bucks some of what would otherwise get into your
pairs.

Whether ISP or CLEC or both, I suspect a lot of us run into this kind of
wiring quite often.

----- Original Message -----
From: "Kevin Oberman" <oberman at es.net>
To: "Barton F Bruce" <barton at gnaps.com>
Cc: "Christopher K. Neitzert" <chris at neitzert.com>; <nanog at merit.edu>
Sent: Wednesday, April 10, 2002 4:37 PM
Subject: Re: Sheilded Cat-5E Ground Loop - Myth or Reality?

> > From: "Barton F Bruce" <barton at gnaps.com>
> > Date: Wed, 10 Apr 2002 16:14:01 -0400
> > Sender: owner-nanog at merit.edu
> >
> >
> > > None of his is specific to Cat-5e installations but is common to ALL
> > > electrical installations.
> >
> > This does NOT apply to telco cables run outside, often run on the same
poles
> > parallel to power wires for miles, grounded at many points in a MGN
(Multi
> > Grounded Neutral) environment where, like it or not, the earth carries
> > considerable current.
> >
> > Opening a shield ground in this environment is bad.
>
>
> Shielded/screened TP on utility poles? This is an insane thing to
> do. It violates more rules than I want to even start to think about!
> Telco wires are always connected to lightning arrestors at the
> termination of the long run. (This is NOT always the same as the
> demarc!) MGN is MANDITORY. This is not a do-it-yourself project.
>
> Similarly, you should not do this between buildings either! (It would
> probably not be a problem running the the garage, but if the next
> building is on a different transformer...You could easily melt the
> wires or burn something.
>
> Or, to put it in simpler term, "Don't try this at home, kids!". It
> could prove fatal. (Hint, power lines are always at the TOP of utility
> poles and gravity works.)
>
> 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
>