I am not sure I really agree with this, the double insulation mark 
indicates that there is two levels of insulator (case materials, fiberglass 
/ mylar sheeting, cable insulation etc) between the user and anything live. 
A double insulated appliance (such as most consumer electronics / 
appliances) should* not* need or indeed have an earth connection. For more 
info on that, see here : http://www.double-insulated.com/

Now, if you want an *electrically isolated output* the only true way of 
doing this is galvanomically - either an old style 50Hz transformer / 
rectifier (heavy power brick kind of thing) or a good quality switching 
supply with a guaranteed isolation level. You can then pass through the 
true earth to your chassis and, since it is isolated, peg the neutral 
output to earth as well - though I would argue this actually reduces the 
safety somewhat, I would always prefer to leave the isolated supply 
floating, that way you can contact either conductor and be safe.

The symbol you really want to see, is two overlapping circles like those 
seen on this page : http://sc3c.iec.ch/ap60417/TC96gif.html

Hopefully I have not spread any inaccuracies here, I recently did a large 
commercial project involving many 3kva isolating transformers so had to 
read up a bit on all this, but that was a few months ago now... 

- Alex


On Sunday, 18 September 2016 20:25:39 UTC+1, nixiebunny wrote:
>
> Nick, 
>
> The mark that indicates that the input and output are fully isolated 
> from each other is a square within another square, called "double 
> insulated". If you don't see that mark, and you see a UL or ETL mark, 
> then it's reasonable to assume that the barrel is grounded, because 
> otherwise the unit wouldn't pass UL or ETL testing. 
>
>
> On 9/18/16 12:06 PM, Nicholas Stock wrote: 
> > Hi Fellow lovers of neon... 
> > 
> > I have a question from a bit of a novice here (me..). I'm working on a 
> > project that involves a dekatron or two and want to make sure that 
> > there's some degree of safety when placed inside a metal case and HV is 
> > involved. Instead of an IEC inlet and transformer inside the device, I 
> > want to use a 12V DC supply with a ground connection or -ve side tied to 
> > Earth. I found the following picture in my PSU stash and checked the 
> > continuity of the barrel side of the supply and Earth and there's 
> > continuity. Can anyone tell me which symbol on the PSU denotes this 
> > (instead of it being a truly isolated supply) or if anyone has any other 
> > ideas about how to go about a 'safe' 12V DC supply (I looked at the 
> > medical grade PSU's with 4 pin DIN and earthed shell, but they're quite 
> > expensive). 
> > 
> > Apologies if this seems like a stupid question... 
> > 
> > Thanks, 
> > 
> > Nick 
> > 
> > image1.JPG 
> > 
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