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Old 04-05-2006, 03:03 AM   #1
gonzo0815
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Default Strange Voltage on long lan csbles and generall grounding praxis

Hi all,

i`ve noticed in many instalations, that if i tuch lan cables shild an ground that there are some Voltage on those lines. Now i examinated my computer and i have seen some sparcs arcing from the case to the speaker jack if i plug it in slowly. If i disconect the lan cable this arcing stops.
(my lan starts on the 1 flor an gos down to eart flor thru some cheap switches with build in cheap sps)
Now ive discussed this with a friend which is an professional in this area and he said that this is may be due to the stupid grounding praxis in germany, where we ground both sides of the cables. I ve read many about that currents in the internet, and i read several arcticles that those currents are derrived from switching power supplies without A-PFC or bad y caps (like those bad and unsafe ceramic in cheapo PSU´s), as those phase shift currents will somhow went to the neutral and earth lines. Now i have changed many PC suply with a-pfc ones, as they are IMHO in many ways better than the others. But the Volltage never disapeers. Even if i tried to ground the lan calbes close at my pc this arcing never stops I tried to cut the ground of the lan calbes, but this wou`t help either.

My friend told me, that if they have to do changes in building to building cables in large idustry complexes, they have realy high currents on the earth lines, so they have to use very very large calbes for it, if they will bypasse such calbes. No one cares where thos currents come from. I know in USA, shilds should only be put on one side tho ground. What is your experience with those things?

Last edited by gonzo0815; 04-05-2006 at 03:08 AM..
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Old 04-05-2006, 03:32 AM   #2
willawake
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Default Re: Strange Voltage on long lan csbles and generall grounding praxis

http://www.epanorama.net/documents/g...ck_wiring.html

interesting.

do you get shocked touching the back of the computer cases? you should test the mains power sockets, there may be a poor ground.
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Last edited by willawake; 04-05-2006 at 05:23 AM..
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Old 04-06-2006, 11:46 AM   #3
Per Hansson
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Default Re: Strange Voltage on long lan csbles and generall grounding praxis

I would say either your house has no proper grounding at all, or the grounding is in the electric grid behind only one of the computers, and therefore the other computer will use the LAN cable as grounding for itself through the other computers working ground.

This is very very common if you connect two buildings together, because since they do not share a common ground there will be ground potential differential; and it will try to level out through the LAN cable...

You will either have to fix the grounding problems or use fiber optic cable, there is no other way to fix it I am afraid
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Old 04-06-2006, 01:20 PM   #4
gonzo0815
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Default Re: Strange Voltage on long lan csbles and generall grounding praxis

Thx for answers.
No, iget not shocked if i touch the back of the pc case.
The socket of the used psu`s are all good and are properly grounded. even the cheep switches are properly grounded, i cheked this with an dvm. I have often seen such kind of strange voltage on lan cables, not only in my installation.
The grounding of the house is ok, at least what i have seen there are the normal grounding cables from the basement to all floors. All pc`s are now equipted with a-pfc psu`s from either Seasonic or Enermax.
The only theoretical explanation would be the resistance of the thik groundingcable running from the zentral grounding point. May be i should chek the whole way of the grounding calbes, may be is there some connectionoxidiced or coroded.
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Old 04-06-2006, 01:27 PM   #5
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Default Re: Strange Voltage on long lan csbles and generall grounding praxis

You are right Per but there is one thing more induced mains fields, long cable runs will most probably pick up more and result in bigger potential differences. Even equipments that are not safety grounded will have some potential differences against a real ground.
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Old 04-06-2006, 01:34 PM   #6
gonzo0815
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Default Re: Strange Voltage on long lan csbles and generall grounding praxis

@will

this article is exaclty describing what the problem is, i think so far. But never the les, the building is grounded es Germanies VDE suggest. I can`t install ne grounding wires only due to one lan cable going through the house. But ok, may be i disconect the shild of the installation cable and thus this will help. But as far as i think i am only using some unshilded cat 5 patchcables.
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