Class X/Class Y (Safety) Capacitor Parameters
“There are more capacitors in a distributor catalog, Horatio, than are dreamt of in your philosophy. “ – Shakespeare (I think? Close enough)
When you have the occasion to mix hazardous household voltage with capacitors (perhaps to keep noise from leaking out of your circuit, or for surge protection), special care needs to be taken in selecting the capacitors used.
Normal ceramic capacitors have the distressing tenancy of failing short. In the case of diagram to the right, such a capacitor in the “Cx” position would cause the mains to short through the capacitor, creating a risk of fire, (small) explosion, and a Bad Day. Should the failing capacitor be in the “Cy” position, the mains could be shorted to earth ground (risking fire, etc) or, if the case is not connected to earth, could just directly connect the case to mains, creating the risk of arcing, electrocution, and a Bad (hair?) Day for someone.
X-capacitors are designed to reduce flammability in the even of failure. Y-capacitors are designed to reduce flammability, fail open to reduce potential shock hazard, and to minimal values to reduce RMS through current to a safe level during normal (50/60Hz) use. In application, a Y-capacitor (insulation-bridging capacitor) can be used in place of an X-capacitor (cross-line capacitor), but an X-capacitor may NOT be used in place of a Y-capacitor. Capacitors can carry both an X and Y rating.
The different grades of X and Y capacitor, as summarized below:
|Type/Grade||Typ. val. range.||Insulation voltage||Peak impulse|
|Y1||<0.47uF||up to 500V ac||8kV peak min|
|Y2||2pF – 0.056uF||150-300V ac||5kV peak min|
|Y3||2pF – 0.056uF||up to 250V ac||none specified|
|Y4||up to 150V ac||2.5kV peak min|
|X1||2pF – 0.056uF||4kV peak*|
|X2||2pF – 0.056uF||2.5kV peak|
* Looks like there is a minimum tested guarante of 2.5kV with this rating, others may be rated by design and sample (destructive) demonstration
It must be said that if a rated capacitor claims a parameter on their datasheet that differs from the above, use the datasheet value. However, the grade is often given instead of the above parameters as an annoying short hand method.
Finally, note that if you are concerned about safety / certifications / performance (and give you are using safety capacitors, you probably are), make sure that the capacitors you select are UL/CE/etc certified. Relevant standards you might look for are below:
Europe IEC/EN 60384-14, USA UL 1414 and/or UL 1283, Canada CAN/CSA C22.2N°1 and CAN/CSA 384-14, China GB/T14472