Earth Loop Impedance Tester

[ppppenguin]

In the kitchen, he found twin light switches with two phases, the oven on another phase different to the hob next to it. etc. etc.

It would be unusual to have 3 phase in a domestic environment and it wouldn't be common to have multiple phases in one room. But the wiring regulations allow this, subject to appropriate labelling. The hazard is to an installer rather than a user. For example when working on those light switches they need to be aware that more than one circuit needs to be isolated and that there's 400V between terminals in one enclosure.