Using chassis for -ve return is similar to earthing one pole (we have a similar concept on transportable units under BS 7671 where in many circumstances the vehicle in effect replaces the planet in terms of Earthing) e.g. for 'on board' TN-S systems. You can do that sort of thing for PELV - but not for SELV (hence Graham's first question). Insulation isn't just to protect people from the voltage of the SELV circuit itself - it's also there to prevent the SELV circuit picking up voltages from elsewhere (not just LV circuits but other SELV/PELV circuits too) - you'd effectively be relying on the cable sheath to do that - which is probably fine from a physics point of view, but doesn't tick the box from a standards/compliance perspective as the sheath isn't guaranteed to have any particular insulating properties. Yes you can have completely bare conductors - but they're usually on insulators, stretched across empty space in plain sight, well away from other circuits or earthed conductors - so the air effectively provides the necessary insulation from other potentials. Cables, running through the building structure, presumably either bunched in places or in contact with other cables or earthed structures, wouldn't really have the same margin of safety.

It may be that what you propose is no less safe than by-the-letter-of-BS 7671 compliance - especially given the details of your particular situation - e.g. it's the same pole of all the circuits that's "uninsulated" (so a fault between circuits can't start adding voltages), but then you're into the realms of having to note it as a deviation on the certificate and have to be prepared to justify your decision to anyone who asks. Or maybe PELV would be acceptable, or (as Graham notes) it might not be a purely BS 7671 matter anyway).

   - Andy.

I now understand that because I use a conductor which is bare then the supply must be PELV instead of SELV. Figures 9H and 9I of the regs describe what I want to use although not explicitly implying that the PEM conductor may be bare. The voltage limit will be below the limits stated in 414.4.5 so I can justify the use of single bare conductor.

Not sure if there is a voltage limit on the scope of the ESQC regs but these are not relevant as it is non UK project (will check for something local though) 

I now understand that because I use a conductor which is bare then the supply must be PELV instead of SELV. Figures 9H and 9I of the regs describe what I want to use although not explicitly implying that the PEM conductor may be bare. The voltage limit will be below the limits stated in 414.4.5 so I can justify the use of single bare conductor.

Not sure if there is a voltage limit on the scope of the ESQC regs but these are not relevant as it is non UK project (will check for something local though)