Questions about earth grounding enclosures
Several occasions can lead to this.


-Imagine the power cable passing through a small hole of the metal enclosure to reach the transformer. The sharp edges around the hole are an easy way for the plastic insulation of the wire to start wearing out. Given some time and some rough handling the hot wire can touch the metal enclosure and transform it to a deadly trap for whoever touches it. 

-Also, not all devices have a transformer, who adds galvanic isolation. If, for example, the device uses a capacitor dropper, if ANY high voltage component touches the metal enclosure, the current can find a way from the outlet, through your body and to the ground. 

But how will a component come in touch with the enclosure? You may drop the device, or maybe shake it, or maybe some internal damage lead to some wire insulation to burn out, or to some component desoldering from its place. For all this cases, it is mandatory to ground the enclosure. 


If the enclosure is 100% plastic and is ZERO chance of someone touch any metal part that's inside, then there is really nothing to put the "ground" on.