I'm planning at restructuring my electrical panel. Therefore I want to disconnect the whole panel from the supply wires. The supply wires are 3*400V plus neutral and ground. What is the minimal distance between the supply wires which must be maintained to prevent arcing?
You need to de-energize the wiring
I realize you're very cocksure, but...
Really, really, you need to de-energize the wiring using the normal methods for doing that in your country.
Absolutely no one on this forum is going to counsel you to do that work "live", even with the much more docile American 120V-to-ground (2 or 3 phases).
Here are the real facts about arc flash.
I recognize this panel as being a common American industrial 480V board. (277V per leg). In America this is considered "industrial-scary" and is treated with grave respect.
However your voltage is only 18% less than this.
You may say "But that's so much more current than I deal with" - nope, when you do stuff like this, the current is limited only by the wires. US power companies guarantee that the wires will limit current to 25,000 amps or 10,000 amps. Even if we assume yours limits to 5000 amps, do you really want to mess with 5000 amps?
This just doesn't work. Pull the meter.
You're asking about the Dielectric strength of air. Among other things, it depends on the shape of the electrodes (the bare wires) and contaminants in the air (humidity, dust, etc). We can't give a precise answer - but at 400V the distance is quite small. If you can see a gap between the conductors then it's big enough that an arc won't form. Until an unexpected vibration causes the wires to move, close the gap, and strike an arc...
If you insulate the bare ends of the wires the dielectric strength will be improved and the chance for arcing will be reduced. It's a high-stakes game to count on a temporary insulator protecting you while live wires float about with potential of high fault current arcing.