So, if I understood correctly, through a power supply it is not possible that the AC current arrives inside the case?
most cases are DC powered - similar to a laptop - in fact some power supplies actually use laptop bricks for converting AC mains power to DC +/- 12v and +5v
some such as doepfer are mains AC powered (using a 'kettle lead' for example - like a lot of synthesizers)
a few use AC converters to supply 12-15v AC (most likely with eurorack 12v to an internal power supply which will convert to +/-12v and possibly 5v) - as far as I know the only ones that do this are some Frequenct Central supplies)
For sure, tubes are more dangerous, but I don't plan to buy modules with tubes insides
usually the tubes won't be on the inside, they'll be on the outside - to show off... not many modules use regular tubes though! I suspect the newer korg nano-tubes are less dangerous - but probably better safe than sorry!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities