I dont quite get it, but theres two types of "ground". Theres the negative current, called a ground, and theres an earth ground. On a two prong plug, you get + and -. So the screw at the back of your cabinet should lead to an earth ground. I dont know about transformers, but anything that is plugged in with two prongs is not earth-grounded.
I just got it. The step down converter is probably not earth grounded, since it only uses 2 prongs. You probably have to put a ground wire to it.