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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.

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  • \$\begingroup\$ I'm fairly convinced now that for me, metal is better. I'm sticking stuff through the enclosure and it's really too difficult to try to maintain double insulated techniques. I'm reverting to the fridge model - metal box that's well earthed. \$\endgroup\$ – Paul Uszak Mar 18 '16 at 4:15
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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.

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  • \$\begingroup\$ So it is more complex than just "Anything that is connected to the AC Mains needs to have the equivalent of double insulation" \$\endgroup\$ – Paul Uszak Aug 18 '16 at 10:53
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For a plastic enclosure, you want to treat any metal stuff coming out the same way the DC output of a doubly insulated supply (e.g. wall wart) is, meaning it has to come from the side of board that's separate (to standards) from the mains. Separate to standards means clearance, creapage etc. You can look at some teardowns to see what's involved; here's one to medical standards: https://www.youtube.com/watch?v=_EMAggN5H-w

Honestly, I'm not sure why you want

a switch and some sort of optical indicator perhaps with metal bezels /lever

(emphasis mine) There are plastic alternatives for these, and even with insulation rated for mains [and above].

As for

there will be a mains transformer, with a bolted attachment through the case

This might be tougher to find alternatives for in DYI projects, but I've seen 50VA (mains) transformers held in place by nothing but plastic tabs, but this in specially/factory designed plastic cases where those tabs have the necessary thickness etc. and are part of the case body. So, how heavy/big is your transformer? If you can sacrifice screw thickness (or overbore the holes) without sacrificing mechanical endurance, you could use a solution similar to mounting TO-220 isolated on heatskins, i.e. insulating "shoulder" bushings also know as "nippels" in some parts of the world. Full-bodied nylon screws also exist, but you'll probably want to read the manufacturer's data carefully to make sure they are strong enough.

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As well as electrical considerations, you may wish to consider fire safety. Plastic that does not support combustion has fire retardant additives, some of which may or may not be prohibited (especially in the EU). One standard is UL94, with the V-0 classification a common one.

I'm pretty sure many of the cheap plastic project boxes that might be suitable for battery powered gadgets will not pass. Plain old polystrene molds more easily and costs less than a material with a pedigree.

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