Electronic – Shielded Ethernet Cables in Office Buildings – Ground Loops

emcethernetgroundinggroundloopsshielding

I have a question related to shielding and grounding of ethernet cabling in industrial environments, offices and datacentres.

What I assumed as correct so far (please correct me if I’m wrong):

  1. In Consumer PCs the ethernet shield is connected to the case via the
    IO-Shield. The case is grounded over the PE-conductor in the PSU.
  2. On a commercial server it is the same (only had one to check).
  3. The ethernet standard specifies signal transformers at each
    interface to achieve a galvanic isolation between endpoints.
  4. For a cable shield to be effective (at high frequencies) and not act as an antenna, it
    should be grounded at least at both ends. For very long cables more
    ground connections may be necessary. To break ground loops this
    connection may be capacitive at one end.
  5. The galvanic isolation on the signal line is, I presume, there to
    protect the sensitive electronics from a potential voltage
    difference between the two endpoints. But
    if you use shielded ethernet cables, then you will create a low
    impedance connection between the two endpoints and a current will flow
    through the shield.

Now to my question:
Industrial interfaces like EtherCat have a capacitive coupling of the shield to the case. And I assume in datacentres and the likes the grounding system is tightly controlled so no potential differences can arise. But how is this handled in office buildings? Is it just assumed that the grounding system is good enough so that there are no, or only very small, balancing currents on the shield between the two endpoints? Or are these currents just no problem in these practical applications?

Best Answer

In Consumer PCs the ethernet shield is connected to the case via the IO-Shield. The case is grounded over the PE-conductor in the PSU.

And it does NOT assume shielded Ethernet cable.

For a cable shield to be effective and not act as an antenna, it should be grounded at least at both ends. For very long cables more ground connections may be necessary. To break ground loops this connection may be capacitive at one end.

Not correct. This is not only a grounding issue from the protection point of view, it is also about integrity of the signal. Current flowing through the shield will cause problems with the signal in the wires inside it - as the current source is very close and can be of considerable value. Some time ago I was researching grounding and long transmission lines (but in audio), and there were a number of recommendations on how to minimize noise in the differential signals, I clearly recall one saying that if using grounded cable it must be connected to chassis only at one side. Unfortunately can not find this document right now.

But how is this handled in office buildings?

This is too broad question: every country may have its own requirements for the electricity in the buildings, and thus the grounding circuits for the devices being connected to the mains.

I assume in datacentres and the likes the grounding system is tightly controlled so no potential differences can arise.

Ideally it must be this way. In consumer non-datacenter environment variations are possible, but you must know one thing: if there's a rule for grounding, it must be strictly followed. As an example, the worst situation is you bringing non-grounded device (having virtual ground) to the fully grounded environment.

Or are these currents just no problem in these practical applications?

Yes in office environments. No shielded cables are required. Using shielded cables can actually worsen the environment.

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