As you know, Common mode noise returns to earth. (as you know, it obviously came in to the product via live or neutral)
If noise returns via the mains live or neutral than its not common mode noise. Any noise going through any of the y capacitors will end up returning via live or neutral, and so it will not be common mode noise.
True common mode noise, which returns to earth does not get “LC filtered”. Common mode noise couples out of the product, and then returns to earth, probably outside the product….it certainly does not traverse the y capacitors, because if it did, then it would be going back in to live or neutral, which means it would not be common mode noise any more.
Common mode noise is reduced by having a common mode choke…..the common mode choke “forces” noise which has coupled out of the product to come back into the live or neutral wires, via the common mode chokes other half……so it makes the noise into differential noise, which gets acted on by the differential filter.
So, Y capacitors do not filter common mode noise……Y capacitors actually transport noise back into the mains live or neutral from earth…..making it “differential mode noise”.
This is why I question why Cy1 (in schematic in post #5) is needed. Any noise which has coupled out of the product that will return back into live or neutral would choose the Cy2 route…because that takes it back through the other half of the common mode choke…where it sees the inductance of the common mode choke cancelled out…..if the noise were to return via Cy1, then it would miss out the other half of the common mode choke, and it would see a very high inductance, so therefore it would certainly not take that route…electricity generally takes the lowest impedance route, which in this case means returning back to mains via Cy2 and the other half of the common mode choke.
So why is Cy1 needed?