On 21 Jul 2019 23:39:57 +0100 (BST)
Theo wrote:

Suppose I have a variety of equipment which is variously powered from
one of the phases of a three phase supply. I'm concered about the
potential difference between two boxes on different phases. The
boxes have low voltage electrical connections between them (similar
to USB ports). Each box has a PC-style switching PSU. Leakage from
the mains side into the DC side could cause a high potential
difference between boxes on different phases, which would be enough
to zap sensitive inputs on the interconnections.

Sounds like a job for opto-isolators.