Common mode Surge protection

Thread Starter


Joined Feb 6, 2018
I have a problem with EMC testing. The test that fails is IEC61000-4-5 (surge), 2kV L-PE level. The design is quite old, but I need to maintain it as it is.
In my application, I have two independent PCB boards. They are connected in parallel to 230V.

For differential mode surges (L-N), we have MOV varistors, and devices pass the EMC testing. But when we are applying the common mode Surge pulses +/-2kV, we have a fail, because the relay on the secondary side, which is switching the 230V power supply to AC motor, is switching on/sticking somehow and starting the motor.

I am trying to analyze the whole circuit, but I have no ideas why it is happening. Both boards have a common mode choke, and Y capacitors are also on both boars.
I am attaching the simplified scheme of the situation. Any ideas?