I have heard of Class I, II, III ratings for transformers. Here is a description and the link is at the bottom:
Class I transformer
A class I transformer is a transformer which protects against electric shock not only by means of basic insulation, but also by using an extra safety device such as an earthing terminal, i.e. in the event of a fault in the basic insulation, the exposed parts will not become dangerously live.
Class II transformer
A class II transformer is a transformer which protects against electric shock, not only by means of basic insulation but also using an extra safety device such as double or reinforced insulation. The transformer does not have to be equipped with a protective earth connection.
Class III transformer
A class III transformer is a transformer which protects against electric shock by means of a SELV power supply, with no voltages higher than SELV being generated (max. 50 VAC in and max. 50 VAC out).
* The transformer does not have to be equipped with a protective earth connection.
* Classification I, II or III has nothing to do with the insulation between the insulation between the primary and secondary windings.
"Transformer PDF Document(Transformer Classification)":en.noratel.ezpublish.no/content/download/5250/420446/file/16-2eng.pdf
