Some of those voltmeters aren’t floating - be careful or stuff will blow up.
Funny story
I needed a 13A PWM voltage limiter for my home brew boiler to calm it down a bit and stop it raining beer in the kitchen (literally!). I followed a guide on a forum that built a nice little box with air holes drilled in it and a small volt/amp meter on the front panel. All high grade 15A mains flex the whole heap.
The 40A rated PWM circuit works fine, huge heat sink, huge caps, but when I went to wire up the meter the instructions where not very clear. The connectors on the board where not labelled and it was a matter of assumption as to where they went. I googled it. Turns out there were two versions of the unit and the second batch were wired differently but shipped with the old instructions.
I was fairly sure I had a new model and wrong instructions as the 240V connected to something like a 9V battery wire, but a voice in my head said, "If you ignore the instructions that came with it and follow something off google and it blows up you will be right embarrassed." So I followed the instructions that came with it.
I have a rule. When I switch on something dodgy that is connected to the mains I make sure I am not holding or touching it and the only thing that comes near me is the mains switch. I also have my fingers position so I can quickly turn it off again.
PUFFFF!! Big cloud of magic smoke.... and 4 lovely little circular burn marks in my carpet later and I realised I should have followed the instructions off google. I'm pretty sure I put 240V through low voltage sense side and from the colour of the debris I think a capacity exploded spraying flames and cooper coloured goo all over the inside of the box and the carpet.
Surprisingly the 40A "dimmer" still works fine, though I treat it like it's got small pox when it's plugged in. Still smells of magic smoke though. The hole where the meter was adds cooling, but could also provide a way to put your fingers in and get zapped.
Do I get a "Dodgy cowboy" badge? If that doesn't get me the badge, when I was 14 I took a CRT monitor apart and wondered what was under the sucker on the back of the tube, so I tried to pry it off with my fingers. BANG! I was launched clean across the room and slid down the wall shaking like a leaf. Of course I now know that is the 50KV pull up and the capacity it was connected to was nearly the size of a coke can! At least I lived to tell the tale.