Disconnect electrical panel - Can arc occur at 400V?
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 &quot;live&quot;, even with the much more docile American 120V-to-ground (2 or 3 phases).
Here are the real facts about arc flash.

https://www.youtube.com/watch?v=6hpE5LYj-CY
I recognize this panel as being a common American industrial 480V board. (277V per leg). In America this is considered &quot;industrial-scary&quot; and is treated with grave respect.
However your voltage is only 18% less than this.
You may say &quot;But that's so much more current than I deal with&quot; - 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.