Y capacitor is the ceramic capacitors respectively connected to the two input lines to ground. The power generally requires a bypass capacitor between the line and the earth. When the power is at rectified situation ,and the bypass capacitor is damaged, it may cause serious leakage to the base of the appliance, triggering a ground connection fault and an electric shock risk. So there needs a special type of capacitor (called Y capacitor). This capacitor has a built-in "self-healing" dielectric property to avoid excessive leakage. Y capacitor should meet EMC standard(IEC384-14, IEC132400 and UL1283). The Y capacitor and common mode inductor are used together to suppress common mode noise. In term of the EMI filter, the higher the Y capacitor value, the better, but from a safety point of view, the lower the Y capacitor's value, the better, because the value of the leakage current depending on the capacity of the Y capacitor for EMI filtering, the final capacity of the Y capacitor should be determined according to safety standards.