When was this last done?

[SimonH2]

However, if the 'good insulation' ceases to be intact, then an earthed screen would be a good thing if there was a fault just in the primary, but a bad thing if the fault were just in the secondary - and, as I said, the worry with the latter is that the user would be unaware of the hazard resulting from the output of the transformer no longer being isolated.

OK, so the insulation had failed, there's now a fault connection between secondary and primary. In the absence of a screen, how does the user know that there's a fault ? Answer - he is no wiser because eh still gets whatever voltage he should between line connections and so the "whatever" still works. However, that output is now referenced to the mains input - so potentially (in the worst case, one end of secondary winding touched one end of primary, with the right (or is that wrong ?) phasing) could now turn an isolated 240V output, into something that's 240 and 480V relative to supply neutral (near enough the same as earth for most installations for the purposes of this discussion).
With an earthed screen, the worst case is that the output is now 0V and 240V wrt earth.

A non-earthed (ideally non-conductive) 'screen' (in addition to the standard insulation) between primary and secondary ...

I believe we have a name for that, "double insulated".

 

[JohnW2]

OK, so the insulation had failed, there's now a fault connection between secondary and primary. In the absence of a screen, how does the user know that there's a fault ?

Fair enough, that's also true, and it underlines the fact that there are pros and cons. As you say, without an earthed screen, there could be a primary-secondary short without the user being aware. With an earthed screen, there could be a secondary-earth short without the user being aware. I suppose you are arguably right in going on to say that the former of those possibilities could be worse than the latter - but, at least qualitatively, it doesn't alter the fact that one is introducing one risk in order to remove another risk (albeit perhaps of a 'lesser hazard').

I believe we have a name for that, "double insulated".

That's obviously literally true, but it's not a term that we normally use for the insulation between two conductors - it normally relates to the insulation between live conductors and 'touchable' things.

Whatever, there is no doubt that the safest approach is to have separate bobbins - but one then has an almost equivalent debate to the above as to whether or not it is advisable for the core to be earthed.

Kind Regards, John

 

[SimonH2]

Indeed, there is always a tradeoff.