Y capacitor connected between the AC line and the FG has the function of releasing the common mode noise current to the FG. Leakage current flows through the Y capacitor according to the frequency and voltage of the AC power supply. If the capacity of the Y capacitor is large, the leakage current will be large and there is a risk of electric shock, so safety standards such as UL limit it so that it does not exceed a certain value. Common mode noise and leakage current are inversely proportional.