X and Y Capacitors

[skipjackrc4]

I've recently been running into designs calling for X and Y capacitors.  There does not seem to be much information about these online, and none of my classes have ever mentioned them. 

From what I can tell, they are very robust caps for use in direct-mains connected filtering that can withstand high voltage transients.  Can anybody please explain this a little better?  How are the two types different?

Thanks in advance!

[HackedFridgeMagnet]

Wikipedia is your friend.
http://en.wikipedia.org/wiki/Film_capacitor

[ejeffrey]

They are safety ratings for use on the line side of a power supply.  Most commonly they are part of the EMI suppression.  Class X capacitors are certified for use in line-neutral or line-line applications and need to handle spikes present on the AC line and not catch fire in case of a fault.  Class Y capacitors are used for line-ground connection, or connect primary to secondary over an isolation transformer.  They need to be self healing, as faults could cause electrocution.

[Amarbir[Lynx-India]]

I've recently been running into designs calling for X and Y capacitors.  There does not seem to be much information about these online, and none of my classes have ever mentioned them. 

From what I can tell, they are very robust caps for use in direct-mains connected filtering that can withstand high voltage transients.  Can anybody please explain this a little better?  How are the two types different?

Thanks in advance!

Hi ,
    When i was doing Vintage Repairs few months back i had access to a great resource of info on these .Cannot get the link ,Try registering in Antique radio forums and ask for help ,Someone imho there had a website dedicated to these  .

[PChi]

It's a while since I have worked on power supplies so I am not up to date with the details.
Class X capacitors can be connected between L and N and can survive transients and never fail in such a way to cause a fire.
Class Y capacitors can be connected between L , N and safety earth and are designed not to fail to cause current through earth (apart from the inevitable AC current).
Try Epcos at http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Capacitors/FilmCapacitors/PDF/PDF__EMIGeneralStandard,property=Data__en.pdf;/PDF_EMIGeneralStandard.pdf

[PChi]

Apologies, Oops all I have done is repeat the information in the post by ejeffrey.

[Kremmen]

Maybe a needless addition, but rated caps are marked so on the outside. So you must find one or more of the codes X1, X2, Y1, Y2 labeled on the component somewhere.
Also, Y rated caps tend to be relatively small capacitacnce since leakage current to PE wiring is strictly limited in the code.

[skipjackrc4]

So it seems that both types of caps are designed to prevent shorting?  X caps can't short or they risk a fire through over current, and Y caps can't short or they expose the user to mains voltages.  Is this correct?

[ejeffrey]

From what I understand, X caps are allowed to fail conducting, as long as they either melt and fuse without catching fire, or short out and blow the primary fuse.