The UL Standard for toy transformers does not allow for grounded line cords. All exposed metal parts on a transformer are supposed to be "dead metal parts". This is tested by a dielectric strength test. Should the base be grounded and an internal failure allows line voltage to leak at the terminals you can have a lethal voltage differential. Hence the dielectric strength test.
When I did the RoW transformer I followed the UL Standard for sporting goods transformers which allows for a 3 wire plug and outputs greater than 190W. It also passed the dielectric strength test.
Just for fun one night we hooked up a ZW to the strength tester and it failed. Tying the inputs together and the outputs together, you apply 1000V + 2 x the highest rated [input] voltage for 1 minute to the grouped inputs and the grouped outputs. The ZW broke down (failed) at 600V. It should have held at 1240V.
Once again, more than you need to know.
Lou N
