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RJ45 ports by spec are magnetically coupled (Why Are Ethernet/RJ45 Sockets Magnetically Coupled?). However, one reason against the use of shielded twisted pairs is the possibility to create a group loop between equipments, as discussed in Shielded Twisted Pair (STP) termination and grounding.

The built-in isolation transformer should at least provide some degree of isolation for ground loop. Assuming there is a continuous connection from the shield to ground (patch bay, keystone, etc) Are the built-in isolation sufficient for this ground loop? Until what point will they no longer be able handle the group loop and an external one would be required?

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    \$\begingroup\$ Ethernet interface is transformer isolated already. The specs say so. If it's not, it can't be sold or called as Ethernet. Thus, I don't think non-isolated ports exist, they would not work. The transformers that isolate typically also include common-mode chokes to prevent common-mode interference. Common-mode chokes by themselves do not isolate at all. Can you be more specific what your question is, since Ethernet will always include isolation transformer by specifications. \$\endgroup\$
    – Justme
    Commented Jul 9, 2020 at 0:06
  • \$\begingroup\$ @Justme This question arises mostly from the inconsistency between the answer from Network Engineering and the ethernet spec. There are couple answers from NE that cable with grounding on only one side or an isolation transformer should be used. The non-isolated port is just a passing by mention in the magnetic coupling question. \$\endgroup\$
    – Mys_721tx
    Commented Jul 9, 2020 at 0:12
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    \$\begingroup\$ ti.com/lit/an/slla327/slla327.pdf \$\endgroup\$
    – Bruce Abbott
    Commented Jul 9, 2020 at 0:30
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    \$\begingroup\$ @Mys_721tx Typically in a building, you have cable shield grounded at a RJ45 patch bay, to the patch bay chassis which is grounded. The patch bay RJ45 connector shell can be plastic so there is no ground connection even if you used a STP cable to PC. The wall socket RJ45 connector can be plastic, so again there is no ground connection available. Yet, the cable shield is grounded. \$\endgroup\$
    – Justme
    Commented Jul 9, 2020 at 0:41
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    \$\begingroup\$ @Mys_721tx it depends on if the patch cables on both ends extend the ground connection from wall to devices, and if devices have connectors that extend the ground connection to device chassis, and if the device ground is grounded. If all of those points are yes, then, there is a ground loop, as a device (PC, whatever) gets ground via mains socket and ethernet socket. But you really have to try hard to make a ground loop, as it requires grounded RJ45 sockets, STP patch cables, etc. To avoid these, one must properly specify which kind of the patch bay, wall sockets and patch cables to install. \$\endgroup\$
    – Justme
    Commented Jul 9, 2020 at 0:49

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The ground connection does not go via the transformer isolation, so it has nothing to do with the formation of the ground loop.

So assume you have two PCs, both are connected to grounded mains outlets, so they both have grounded chassis, and both have transformer isolated Ethernet interface as usual.

If the Ethernet interface connectors have a metal shell that has a connection to PC chassis, and you have a STP cable that also provides the shield connection between the metal shells, then the grounds of the two PCs are also connected via the STP cable, and there is a ground loop.

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  • \$\begingroup\$ Since the transformer plays no role is the formation of the ground loop, can it be understood as the transformer will also not break the group loop? \$\endgroup\$
    – Mys_721tx
    Commented Jul 9, 2020 at 1:15
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    \$\begingroup\$ Transformer protects and isolates the data lines by making them float. With an UTP cable, you can have 1 or 2 kilovolt of potential difference between two devices. With some extra protective components the data pins can withstand surges caused by a lightning, though not a direct strike. But the transformer can't do anything about ground loops when someone comes and creates a ground loop by connecting two devices together from the grounded metal shells of the connectors with an STP cable. \$\endgroup\$
    – Justme
    Commented Jul 9, 2020 at 1:22

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