Just like the PROFIPUS(sy) the STR people designed their system with optoisolators and a master power supply. They designed radio systems for big ships where the distance between units never gets really long but on the other hand the ground loops may be a very big problem due to the ship material (steel).

If this would not be a BUS then I would recommend using optic fiber instead of copper cable. But since this IS a BUS ....

The shielding should, as Erik stated, be connected to REAL earth at one and ONLY one point. Having the signal ground running inside the mantle is also a good idea. Still, there may be loops introduced by capacitances of the mains transformers so the shielding of the cable should have enough current capacity.

For the same reason (transformer capacitances) there may be some inductively transferred spikes in the signal wires. Therefore the combination of haveing isolation and master PSU is propably the best for short distances. In this case the transferred voltages evatorate to nothing as their current does not have return path.