Lets say mains wiring enters a metal enclosure, it is important that active is fused with an appropriate fuse and that any exposed connections are dressed with heat shrink sleeving.
In addition there needs to be a solid earth lug to the metal enclosure body, secured with a screw and lock washer and nut, ideally separate to the nut & lock washer that secures the earth lug connections onto that.
The earthing (wiring and connections) should be able to conduct at least 10A from the case body to the earth pin of the IEC connector or earth pin of the power cord's plug if there is no IEC connector. The reason for this is that if the active connection(wire) came loose in the case, or a failure that connected the active to the case, that the breaker box's circuit breaker trips . (You cannot rely that there might be a working RCD there that will trip off at a low unbalanced currents) Some authorities suggest the earthing should be 15A capable.
Never use block connectors on stranded wire where the screw end is applied directly to the wire, always use lugs or ferrules.
Just recently I did up a vintage computer project with a fair amount of discussion about a mains operated device inside a metal enclosure and the requirement of insulation and earthing that it would ideally meet, so have a read of this from about page 6 to the end with the electrical safety notes.
http://worldphaco.com/uploads/EXTERNAL_DUAL_5.pdf
A lot of cheap gear, or vintage gear comes nowhere near the standards expected today with the safety of the mains wiring. Nor unfortunately do many appliances coming out of the far east.
So you need to be sure yourself that if you do any of this mains wiring, it is beyond impeccable (and inspected) as what you make will likely fall into someone else's hands later.