Cable grounding question

[Lekro4]

Hello,

There is a router with plastic unshielded RJ-45 sockets. There is a PC with metal shielded RJ-45 socket. There is a CAT7 certified SFTP patch cable (Commscope).

Would the PC's shielded RJ-45 socket ground the cable properly?

I'm asking for scientific reasons, I know it would be an overkill in practical use.

Thanks in advance!

 

[bill001g]

That is the key issue with using shielded cables. People think the shield alone is some magic thing. If you do not properly ground the cable...on both ends..you take the risk the shield actually acts as antenna and introduces interference rather than preventing it.

The second issue is the ground on both ends must be a separate ground from the electrical ground. This is both for safety reasons as well as function. The electrical ground can get noise on it from faulty equipment, in worst case it can have actual electrical voltages on it.

In the data centers where they use this you will see a large copper wire running to each rack back to a ground bar. This is many times called a data center or telco ground. Although the wire likely runs back to the same grounding rod as the electrical ground uses it is still considered separate.

As you noticed the actual jack must have this special ground connector. Some commercial equipment has this. Since the case of the equipment is mounted to the rack which is connected to this special ground the equipment can then connect it internally to the jacks.

In practice it is not used much to connect end equipment. It is used on patch panels to patch panel connections where the whole panel can be connected to a ground. Then normal patch cable are used between the panel an the equipment. This protects the longer runs without the end equipment having to be special and since the cables are short it does not have much ability to get interference.

The need for shielded cable is fairly rare. It is used in industrial manufacturing applications where there are things like large motors that an introduce interference into cable running close by. The place I have seen it required are in application where they want to prevent the ethernet from causing interference to other equipment rather than the reverse. Things like airplanes and some medical equipment require it. It is also commonly used in cell towers but I am not sure why.

 

[Lekro4]

Thanks for the detailed response.

It is a 5 meters long patch cable. The PC's RJ-45 socket is grounded to the chassis which is grounded by the power supply.

Is the grounding still bad? I understand it would be better to ground at both ends but considering its only 5m long, I think its sufficient to ground only at the computer's socket.

Am I wrong?

 

[bill001g]

The first thing to remember is this a more of a cable manufacture marketing than a real problem. In all the reports I have seen where they tested cables installations in office buildings I have only seen 1 out of maybe a few hundred thousand jacks. In that case some idiot ran the wire through a florescent light fixture. All those were unsheilded cables and never has there been a problem that needed shielding.

Unless you wrap the ethernet around your refrigerator motor I doubt you could come up with a case that you could get interference induced in the wire in a home setting.

The twist in the wire alone are what is protecting the data.

The risk involved with your PC that only has grounding to the power supply is a ground loop. Say you would run a cable to your neighbor house and he too had a jack grounded via the power supply. The grounds on the buildings are just slightly different. You run the risk of passing current though this cable.

It will likely not cause you a problem it is just a improper design.

 

[Lekro4]

The ground loop would only be introduced when we ground both ends and to a different power source and possibly a different ground potential, is it not? That way we risk current passing through the shielding yes.

I'm not getting something here. The router has no shielded jack, only the PC has. That is 1 ground point.

 

[bill001g]

Yes it will be fine. In general cat7 is a waste of money. Cat6a will run 10g just fine and cat5e is good for 1g.