[TowerTalk] Total station grounding and ethernet

[Pete Soper]

In arranging for all conductors coming into my station building to pass
through a single point ground I've run into a snag. I've dealt with or have a 
sure plan for the feedlines, rotor and relay control, AC power and telephone 
but I'm not sure what to do about an ethernet line.

Can 10mbs ethernet performance be maintained with MOVs hanging off the twisted 
pairs of an ethernet cable? How about with fuses inline?

(By MOV I mean "Metal Oxide Varistor", a device that present an open circuit
until the potential difference across it reaches a certain value, typically a 
few tens or hundreds of volts, at which point it becomes a short circuit.
These are very common in power outlet boxes, surge suppressors for modems,
etc. CAT5 is short for "category 5", the common name for twisted pair cable 
used for network connections)

My plan is to strip the outer sheath off the CAT5 cable such that it can sit on
the the single point ground window copper sheet with copper straps holding it 
down and the twisted pairs flattened out. I'll then separate the two wires of 
each pair and attach MOVs to each, keeping the lead length short. The other 
lead of each MOV is connected to ground:

        (CAT5 CABLE from house)
        /\   /\
 .......\/...\/.................
 .      /\   /\                .
 .gnd   \/   \/     gnd (strap).
 .|...../\.../\.....|...........
  |     \/   \/     |
  |     /\   /\     |
  -MOV_/  \__\/__MOV-
       \  /  /\       
        \/   \/     (copper sheet well
        /\   /\       connected to earth gnd)
  -MOV__\/__/  \_MOV- 
  |     /\  \  /    |
  |     \/   \/     |
  |     /\   /\     |
 .|.....\/...\/.....|.....
 .|     /\   /\     |    .
 .gnd   \/   \/     gnd  .
 ......./\.../\...........
        \/   \/
        (CAT5 CABLE into station)


After looking at the punchdown connector of a typical RJ45 jack designed for
ethernet it's my belief that ethernet (at 10mbs at least) can tolerate an 
impedance "bump" like this. But you know how optimism goes when you're 
operating outside your area of expertise :-)

But if this is complete overkill with respect to controlling the impedance can 
I also put fuses into the path? I'd like to have fuses in each lead on the 
house side of the MOV connection to arrange an air gap to be present if the 
MOV is destroyed by the surge from an initial lightning strike.

What do you think?

Regards,
Pete
KS4XG

PS I'm also arranging to be able to do a total disconnect of the station
wiring. The ground system is really just a backup (so don't preach at me
about this please, I'm in the choir!)

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