One reason against the use of shielded twisted pairs in residential environment is the possibility to create a group loop between equipments, as discussed in Shielded Twisted Pair (STP) termination and grounding.
Some PCB mounted RJ45 ports are magnetically coupled through common mode chokes (Why Are Ethernet/RJ45 Sockets Magnetically Coupled?). However, non-isolated ports do seem to exist.
The common mode chokes should at least provide some degree of isolation for ground loop. What level of isolation can they provide? Assuming the ports on both ends have the chokes, until what point will they no longer be able handle the group loop and an isolation transformer is required?
