Thank you very much.
P.S. And I forgot one more detail, it is not important:
There are 3 fans, they controlled by the following board:
Could be.
Make sure you 12V power supply can supply enough current for all 3 fans, the Nano and anything conneced to the Nano.
Well as long as everything draws leaa than 1.1A, should be OK. I say 1.1A because you don't want to run the supply right at it's maximum current.
Of course, once I've assembled everything, I'll take precise measurements at maximum fan speeds, and if the consumption is more than 50%, I'll think about replacing it with a more powerful power supply.
Again, thank you very much for your advice.
Have a nice day!
The 2 last pictures showed 5 volt relays. That tricked me.
I guess you did not read or understand the P.S.
Likely that. Too much information confuses more than it clearifies.
-
You are showing a GND connection from the lower power rail to the Arduino GND pin.
From the top GND terminal of the Arduino, to the top power rail, then to the GND terminal of the relay module. -
Do not run the relay module power GND through the Arduino.
You should have relay module GND terminal connected directly to the 12V power supply GND terminal. -
Your way has relay module's return current flowing through the Arduino PCB.
Practice is to switch Line, not Neutral - and make sure that’s ‘code’, too, so that you don’t end up with a surprise / “hot chassis”.
YES! switch the line.
Also an observation…
I am assuming, from the pictures and descriptions, this is 220 volt AC.
Not sure if it matters to your project, but in America (I can’t speak for Europe or anywhere else) 220 volt consists of two “hots”, not a hot and neutral. A neutral SHOULD NOT have power to it if the hot side is off (via the relay). In your picture, the plug is labeled 220V. That makes me think you will still feed 110V to the neutal side of your (power supply?? I am unsure what the ventilated component is) even with the first relay open.
At the very least it could damage the component, or worse be flat out dangerous.
Bebbington will know.
In NL (Europe) and NZ we do have hot and neutral.
There is 380-400volt between phases.
Leo..
I wonder where sciensys is from.
We have ACTIVE and NEUTRAL and EARTH.
Neutral is connected to GND at the junction/fuse box.
Tom.... 
Is that North or South America?
Hello, there are 4 pcs Q1-to Q4 mosfets or transistor on the relay driving part so its already separated by opto-couplers, be sure you supply input of optocouplers by 5V with the same Arduino (if necessary, replace opto-coupler drive resistances according to 5V. then be sure relay driving part will be 12V output of opto-couplers. if you need cut cupper on PCB or teke off jumper if exist.
Actually that is not necessary, a separate 5V supply could be used.
if necessary, replace opto-coupler drive resistances according to 5V.
Also not necessary.