Re: Proper way to install shielded (STP) Ethernet
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Posted by Phil Schuman on August 11, 2006, 5:25 pm
Please log in for more thread options just thought I'd x-post this interesting thread to the cabling newsgroup > BJ wrote:
> > We're having our office rewired; old Cat3 and Cat5 wiring is being
replaced
> > with a shielded Cat6. (The reason for shielded is another subject
don't need
> > completely and is a done deal, so please, no "Why Shielded? You > > it" posts. I will say that having a shielded solution is of lesser
future.)
> > importance today, but will take on greater importance in the >
> (big snip) > > I first learned about the problems of grounding looking at a big > rack full of electronic equipment, all with the ground connection on > the power cords cut off... > > > - If I'm only concerned about EMI in my horizontal cabling
locations, and
> > think EMI will not be an issue in my rack or office locations at
present, is
> > it appropriate to use unshielded patch cables, as long as the
shielded
> > horizontal cabling is properly grounded to the patch panel (which is
itself
> > grounded thru a properly-grounded rack)?
>
> Are you asking about EMI from ethernet to other equipment, or EMI from > outside into ethernet? > > Shielding and grounding is a complicated subject once you have more > than one piece of equipment that is line powered. Electrically, > the best way is to have only one point connected to earth ground, > with everything else grounded through the shield. This often doesn't > agree with electrical codes requiring equipment to be grounded through > the power line. > > > - If the cable shield is ungrounded, does the shield simply provide
no
> > benefit, or does it create the potential to cause harm? I've read
actually
> > speculation that an ungrounded shield could act as an antenna that > > increases EMI exposure to the pairs inside.
>
> Ethernet is transformer coupled, and is pretty immune to outside EMI. > The possible exceptions are factories full of arc welders, or inside > a building with a high powered radio transmitter. There are possible > resonant situations where the shield length might be the appropriate > multiple of a half wavelength for a nearby EMI source, or for the > ethernet signal itself. In some cases a resistor between the shield > and the rest of the system might be needed. > > > Please understand, I'm not asking for an opinion about whether EMI
is likely
> > to pose a problem in my environment; I'm asking for sound ethernet
grounding
> > theory.
>
> Without knowing the possible EMI sources, it is hard to say. Sound > grounding theory says ground the cable to earth at only one point, > everything else grounds through the cable shield. There can only > be one grounding path between any two pieces of equipment, and that > path is through the shield. RJ45 connectors are not very good at > making ground contact, so you should connect from the shield to > a screw connection on each end of the cable. > > -- glen > | ||||||||||
