This may seem an obvious /silly question, but bear with me because I want you to consider it from a pragmatic view point.
I have a fledgling project that will be mains powered and fit in a space roughly 200mm in any dimension. Initially you'd think that a plastic box would be suitable. But consider: There will be penetrations in the form of a switch and some sort of optical indicator, both perhaps with metal bezels /lever. And there will be a mains transformer, with a bolted attachment through the case.
What initially seemed a simple example of a Double Insulated box, now has metallic (conductive) penetrations though it, compromising the insulation integrity. Commercial Double Insulated kit is quite complicated, with blind self tapping screws, multiple insulation layers, e.t.c that all seem a little out of reach for the typical electronics enthusiast.
It seems to me that it's safer to treat the whole thing like a washing machine, and put it in a metal box with a good earth …
P.S. Should I look for Aluminum or plastic enclosure for UPS touches upon these issues, but does not seem to address common enclosure penetrations.
Best Answer
The answer is mostly common sense. Anything that is connected to the AC Mains needs to have the equivalent of double insulation.
Use nylon or some other non-conducting hardware to mount the power transformer. If there is a power switch on the AC Mains, use an all-plastic switch such as a rocker switch.
You could use a metal enclosure but not only is metal much more difficult to machine, you now also require a grounded power cord.
I build projects in both metal and plastic enclosures. Metal when I need mechanical strength or EMI shielding, plastic when those attributes aren't needed.
One of my favorite plastic enclosures are intended for home electrical purposes. Thick, heavy-wall plastic that is really easy to machine both by hand and our little CNC machine.