Messing with electricity is not DIY unless you are 100% certain about how to test it and be safe with it. When in doubt, hire a professional - a service call fee is not worth risking your life or property over.
You couldāve saved everyone a lot of guesswork.
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By not stating that you connected according to that diagram
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Can someone with an electronic background explain why connecting 2 phases blows up the device?
Sure, not according to diagram, but electrically thereās only a phase difference.
- Why instaat āwrongā?
- Why does that make the device blow up?
Itās too easy to say ānot according to diagram, so your faultā.
Because the voltage between Phase and Neutral is 230V, but the voltage between two Phaseās is 400V.
This device is not designed to handle such high Voltage.
And yes, it is the user his own fault. Any electrician, working in a house, that is connected to a 3 Phase grid, would first check, that the wire going to a switch and the wire going to a lamp are both on the same āgroupā, secured by the same fuse. Thatās according to installation code.
So it was not wired according to diagram, the diagram nowhere states to connect the device to two Phaseās, it clearly says to connect between Phase and Neutral.
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Itās the answer that seems to be up to the level of the knowledge of OP since thatās exactly the reason that it went wrong.
If they had the right skills for wiring a device to the electrical system of his house, this would not have happened.
Of course itās OK to ask the for a more technical answer which @BebeMischa has provided.
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When I studied electrical engineering, I learned to visualize 3 phase power. Think of a Mercedes symbol. Neutral is in the center. Each phase is at the end of one of a āspikeā. If the distance between neutral and the outside is 230, then the distance between each spike is 400. Simple trigonometry.
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I wanted to edit, but accidentally deleted the post.
Thanks for the replies, should have realized that. Just rewired my whole fuse box, installed solar, heat pump, all works, 3 phase. No clue why I didnāt see the trees for the forest. Shouldnāt post after work.
Another thing: what colors are wires you connected shelly to? ( i donāt mean light output, but power input wires) - two blacks? Black and brown? ā¦ ? Just donāt say that one of them is blue (or God forbid even yellow-green) ā¦ if this is the case then DEFINITELY call a qualified electrician to re-wire your house as soon as possible, since itās only the matter of time when someone dies of electric shockā¦
Blue should be ONLY used for neutral, and yellow-green ONLY for ground.
The colors were black and brown.
Ahaā¦ so here is the answer alreadyā¦ if you connect anything to 230V one wire must always be blue.
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The fact that you make a statement like this means you should stay away from electricity. You will get yourself killed. This is not a joke.
Credentials: A masterās degree in electronic and electrical engineering with a additional major in computer science.
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Oh, I read your question wrong.
One wire for power supply of shelly was brown.
The other one connecting the switch to the shelly was turquoise. Power came from the black phase (Found out after investigation.)
I just assumed the whole floor was connected to one phase, which would have made sense since the upper floor is wired on a single phase (grey).
And neutral was connected on the N terminal on the shelly.
But youāre right, one must always be blue 
Assumptions are the mother of all fā¦k ups 
And when dealing with electricity, they are a serial killer.
Yes i realized that. Luckily it ājustā made boom and nothing worse happened.
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Maybe itās not bad to change your OP by stating that you did not install according the manual or at least leave that bit completely out.
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