AC bypass capacitors

[K6JEK]

Can any of you impart wisdom on the caps  bypassing AC mains to ground?  I recall a thread on some reflector in which the importance of using AC rated caps was emphasized -- type Y2 for mains to ground and type X1 for across the mains, as I recall.   But I also recall a thread advising to rip them all out because these X and Y ratings were about not bursting into flames not about failing open not closed, but this I'm less sure of.  There are polyester X and Y rated caps and ceramics.  Is one more desirable than the other?  And finally what about values?  I see plenty of .01's and .001's in this use.  Do the .01's invite hum?  I read that somewhere too.

[Ian VK3KRI]

Y is for capacitors where failure may cause electric shock. eg line to ground/chassis  or to coupled circuits like antenna cables.  Typically they are lower capacitance so that their reactance at 50hz is higher , to stop 'tingles' and possibly tripping earth leakage detectors

X is capaticors whose failure wont cause a shock eg across the line.

There are muliple X and Y ratings indicating better construction HV test etc.

Heres a link I found on a quick search on "capacitor class X2 X3"
http://my.execpc.com/~endlr/line-filter.html.

                                                                             Ian VK3KRI

[K6JEK]

That is a very good reference.   Ripping out the old ceramic discs that most BA's have from each side of the AC mains to ground and replacing them with Y1's sounds like a really good idea.  The article also suggests that .01 is too big for that application, 4700 pf being the practical limit.  Many BA's have .001's but some have .01's. 

Jon