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What are Class X capacitors and Class Y capacitors? How to select Class X capacitors and Class Y capacitors?

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AC/DC ConverterHow to use

Considering AC power supply, the capacitors which are located before rectification line are called as Class X capacitors. On the other hand, the capacitors between the line and the frame ground are called as Class Y capacitors.

If breakdown occurs by an electrical stress on the line before AC power supply's rectification, it may cause a hazordous electric shock or fire, therefore Class X capaitors and Class Y capacitors are mandatory, and the capacitors meet the standard, IEC 60384-14.  (Capacitors connected between line to line like Class X capacitors, the failure could result in fire. On the other hand, capacitors connected between line to ground like Class Y capacitors, the failure could result in electric shock.)

Typical characteristics of Class X capacitors and Class Y capacitors are a rated voltage, a capacity, and a surge voltage. 
The voltage rating and a capacity can be selected in the range of the specification required in your application as same as general capacitors. 

The difference is the sub-class definition of each capacitor. This is assigned depending on the surge voltage. 

The sub classes of Class X capacitors are X1 and X2.  The peak impulse voltage that Sub-class X2 can safely withstand is up to 2.5kV.  The peak impulse voltage that Sub-class X1 can safely withstand is more than 2.5kV and up to 4kV.

The sub classes of Class Y capacitors are Y1, Y2, and Y4. Sub-class Y1 is used to bridge double insulation or the equivalent reinforced insulation grades. The rated voltage is up to 500V, and the peak impulse voltage that they can safely withstand is 8000V.

Sub-class Y2 is used to bridge basic insulation or supplementary insulation grades. The rated voltage is equal or more than 150V and up to 500V, and the peak impulse voltage that they can safely withstand is different depending on the capacitance value.  In case the capacitance is up to 1uF, the peak impulse voltage that they can safely withstand is 5000V. In case the capacitance is beyond 1uF, the value is according to the next formula: 5000/√(C/10^-6)V 

Y4 is used to bridge basic insulation or supplementary insulation grades. The rated voltage is less than 150V, and the peak impulse voltage that they can safely withstand is 2500V.

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