Electrical – Sauna heater wiring help needed

You are missing something.

The schematic you drew is a fairly ordinary wiring arrangement where multiple thermostats and heaters are daisy-chained off of one electrical circuit.

In your drawing, the circuit feeding into the box might be shown at the bottom. The thermostat controlled circuit leaves on the top right to connect to the local heater, and the feed through to the next thermostat and heater in the chain leaves on the top left.

The next thermostat and heater in the chain may be in a different room entirely and have nothing in common with the local thermostat except it is fed from the same branch circuit.

The 2-6 volts is not leakage, it's being induced - picked up like an antenna from adjacent conductors which are hot. You're only seeing it because your DVM is very sensitive. If you put any kind of load on it, even a night light, the voltage will disappear because there's no force behind it. I've built passive AM radios that work on this principle, the airwaves itself powering a very tiny, delicate earpiece, doesn't work with heavier headphones.

RCBO, we call them GFCI in America, because they detect "ground faults". They work like this.

In the drawing, tying the RCBO's incoming AND outgoing neutral to your panel is a complete fiasco, because it gives neutral current a path around the RCBO, which utterly defeats the purpose. Here is how that will trip unnecessarily.

The smaller RCBO in your diagram is "less wrong" but still problematic, as neutral can go down the ground tie to your neighbor's ground tie, into his panel and back to the transformer that way. Assuming you share a transformer. And even worse, the ground bus is tied to the neutral output of the RCBO, which means the RCBO can't see actual ground faults.

You should have TWO neutrals - the rough-and-tumble base neutral prior to the RCBO, and the protected neutral after the RCBO, which serves the exact same circuits as the RCBO hots with no exceptions. IE something like this. (note that chaining RCBOs like this is purposeless and I assume you're doing it because of the panels you are stuck with.) I shaded the protected area blue, note the ground is outside of it!

And of course, the safety ground is also run to every outlet - that is the third wire. The safety ground must be tied to neutral before the RCBO - if it is tied downstream of an RCBO, then the RCBO won't be aware of ground faults. Fault current must bypass the RBCO.

This thing looks odd, because we're mixing European and American paradigms... Thailand. Having a whole-house RBCO is a European concept. In America you use individual GFCI breakers on certain circuits and not others - as a result, a trip will not plunge you into darkness or leave you a fridge full of spoiled food.

And lastly, I suspect you have a ground fault somewhere in the house - perhaps that's why you hooked up the RCBOs wrong (in the drawing) - because if you hook them up right, they trip. Well, they're supposed to trip.

Seeing 220V on your ground line is not right, unless you are measuring improperly somehow. That indicates a condition that the breaker or RCBO should be tripping on.