I'm looking for a list of ways that mains-powered ELV power supplies with touchable terminals can be made safe in the case of transformer failure with primary and secondary coils becoming connected within the transformer.
One method I'm aware of is a feedback control circuit that isolates the input to the transformer in the case of excessive voltage across the secondary winding, that in part uses an opto-isolator, as described here. However I'm not certain what failure conditions within the transformer that this method can/cannot protect against.
I'm primarily concerned with conformance with CE marking and other EU requirements but non-EU regulations and beyond would be a bonus. I'd like multiple options as I'd rather have overkill (no pun intended) than potentially mediocre protection. Cost is an issue but I'd like to know the options so appropriate optimisations can be made.
Last but not least, references to reputable sources are important.