[NTLUG:Discuss] Switch Question

[Kipton Moravec]

At 06:09 PM 11/13/04, you wrote:

> > On re-reading this, I could be wrong but I think the minimum that 10/100
> > Cat 5 or Cat 6 can cover is 500 feet on a "hop".  So a possibility is to
> > put a switch inside each building near the back wall, and then either
> > run the internal networks off of it, or connect it to the other switches
> > on the local network.  As mentioned in another post, you do want to try
> > to stay out from under the parking lot and sidewalks if possible, but do
> > make sure if you lay cable that it is deep enough not to be uprooted by
> > the Yard Maintenance crew.
>
>According to everything I have ever read and taught, this is a REALLY
>BAD thing to do.  There is something called the difference of potential
>which can very easily lead to severe electric shock.  Stick with the
>wireless, infrared, or fiber options.

The problem you are referring to is that the ground potential between two 
buildings could be several volts if they share the same power-pole 
transformer, or more if they have different power pole transformers.

This is a problem if you want to use RS-232 or RS-422 or RS-485 because 
usually one of the lines is a ground, so you are matching ground to ground 
through the relatively thin wire. Basically the whole building will want to 
equalize through the small ground wire. Not a good idea.

The way around it is not to use RS-232, but an isolated RS-422 (full 
duplex) or RS-485 (half duplex) if you are going with serial connections 
and wire.  This works by using a little transformer to isolate and prevent 
the grounds from connecting.  You end up with a "relative" system, 
independent of the building's power.

However, Ethernet is already an isolated system. The people who designed 
the standard were aware of the problem and the final component on all 
Ethernet cards and interfaces is a little transformer to provide the 
isolation, to prevent exactly what you are worried about.

Kip