I've noticed that the output ground of most power supply are not quite the same as earth ground. The DC is nearly negligible, but there is usually an AC component between output ground and earth ground of magnitude 4-6 times the intended DC voltage (probably attributable to floating inductive spikes -- the voltage differential output may slightly float). Only the best power supplies, say a PC PSU, or linear regulated power supply can have output ground nearly equal to earth.
I haven't tested, but the current probably won't be that large. So why not use a Schottky rectifier or something to clamp the output ground to earth ground?