Electrical grid - Awkward function

Today i noticed an Awkward function

i had designed a shield to work with an arduino mega 2560 .
i am using it to control something in an elevator (lift) so the elevator is connected to a 3 phase line

to power up my 12v transformer for arduino i am using one line of them for 220v with natural line

everything work fine

Today i wanted to check something in my arduino so i bring up with me my laptop in order to check the serial output with arduino ide

i was powering my laptop from the same line as the arduino's power transformer

when the elevator started running the Arduino IDE was always disconnected from the usb port

the Awkward is that when i removed my laptops power transformer from the line and leave it ti work with the battery , everything worked fine..

any explanation why this is happening?

I looks like you problem is due to earthing.
When the laptop is connected to mains the USB shield and probably the -5v rail is also connected to the building earth and to the arduino negative.
Somehow current is circulating through this path and "confuses" your PC usb hub. Its hard to figure out what exactly is happening without more data but I am pretty sure this is earth related.

Thanks for the reply