Ah, I think you have misunderstood me. Yes I did say that for a bridge rectifier:
The board's negative rail will normally be bouncing between the Neutral potential ... and approximately -340V ... with a 50Hz half wave rectified waveform.
but (assuming the presence of a sizeable reservoir capacitor), the positive rail will also be 'bouncing' with a similar waveform maintaining a difference between V+ and V- (i.e. across the cap) of about 340V (less a bit of ripple due to the load current).
If there's no reservoir cap, the positive rail waveform (with respect to Neutral) will be a similar, positive going half wave, with its peaks in the gaps in the negative going halfwave on V-. The difference between V+ and V- is then the full wave rectified waveform you would expect from a bridge rectifier.
in both cases the steady DC V (-ripple) or the full wave rectified sine DC V cannot be measured with ground referenced test equipment, and if you attempt to do so, *something* is certain to go *BANG*!