arduino serial communication error


i have connected 8 relay circuit to arduino mega and have run lifa code on the same. I am controlling the relays through labview code. connections are as per attached image.some of the relay is connected to input (AC) of my function generator and other relays are connected to test points of my fixture. MY code works fine initially,but when i turn on the ac input to my board under test(despite not connected to arudino or relays) it causes my serial communcaiton of arduino to stop working. i had to restart the PC so that it work sagain. My ac input is given via Variac which supplies current through CT to my board. how to solve this issue?