X 1 Y2 Capacitor , Dual Purpose??

[HamishBoxer]

Have seen capacitors with X1 Y2 on the same cap, how can this be right when one fails open and the other is meant to fail short ?

[MeerKat]

Have a look at this, should answer your question.

https://www.allaboutcircuits.com/tec...-y-capacitors/

[Helder Crespo]

Originally Posted by MeerKat

Have a look at this, should answer your question.

https://www.allaboutcircuits.com/tec...-y-capacitors/

I had the same question some time ago, and was aware of that website, but the contradiction seems to be there anyway:

"
When a Class-X capacitor, also referred to as an "across the line capacitor"—the capacitor placed between line and neutral—fails because of an overvoltage event, it is likely to fail short. This failure, in turn, would cause an overcurrent protective device, like a fuse or circuit breaker, to open. Therefore, a capacitor failing in this fashion would not cause any electrical shock hazards.

If a Class-Y capacitor, also known as the "line to ground capacitor" or "the line bypass capacitor"—the capacitor placed between line and ground—fails short, this could lead to a fatal electric shock due to the loss of the ground connection. Class-Y safety capacitors are designed to fail open. A failure will cause your electronic device to be subjected to the noise and interference that the capacitor would normally filter out, but at least there will be no fatal electric shock hazard.
"

[paulsherwin]

I always thought that both X and Y caps were supposed to fail open. The specs are different in other respects of course.

The fact that lots of Xs fail short circuit wasn't a design objective, it's just that there are lots of lousy Xs out there!

[HamishBoxer]

My point was, there are caps being sold as rated at both (X 1 Y2). This is written on them. So I cannot see how one cap can be for two purposes. reading above Vishay do one that do both jobs, surely cannot be safe.

[Helder Crespo]

Originally Posted by HamishBoxer

My point was, there are caps being sold as rated at both (X 1 Y2). This is written on them. So I cannot see how one cap can be for two purposes.

From the point of view of peak voltages the caps can withstand, the Y specs are more demanding than the X specs, so it's not a surprise that something Y2 rated is also X1 rated. There is still the issue of failing short or open by design. I found it quite surprising that I couldn't find clear information on this, even though most manufacturers show a typical application sketch with an X cap across the line and an Y cap between line and ground, and the used arguments do imply that Xs fail short and Ys fail open. I really have no clue.

[paulsherwin]

I think you mean X2 Y1.

If I'm right that failing short circuit isn't part of the class X spec, then there's no reason why a manufacturer can't make a cap that exceeds both specs. Class X are normally much higher in capacity than class Y, so are physically bigger with a different construction, but the specs aren't contradictory.

[Helder Crespo]

Originally Posted by paulsherwin

I think you mean X2 Y1.

If I'm right that failing short circuit isn't part of the class X spec, then there's no reason why a manufacturer can't make a cap that exceeds both specs. Class X are normally much higher in capacity than class Y, so are physically bigger with a different construction, but the specs aren't contradictory.

It's really Y2, X1 (the smaller the number, the higher the voltage ratings):
https://www.powerelectronictips.com/...isolation-faq/
I agree that the voltage specs are not contradictory, but the failure mode is, and again this is mentioned in this page (X fails short; Y fails open).
- helder

[paulsherwin]

You live and learn.

[HamishBoxer]

It is the failure mode that bothers me, in my case I better buy a Y only as it isolates ac on an Eddystone. It is on an EB35 I think the model and uses the mains as an aerial, I will check again before I change the wax cap that is in that position now.

[Helder Crespo]

Originally Posted by HamishBoxer

It is the failure mode that bothers me, in my case I better buy a Y only as it isolates ac on an Eddystone.

In my case I installed a Y across the mains in my TV22. Even though according to most websites this should have been an X, I wanted it to fail open so that the consequence of a failure would be equivalent to clipping the suppression capacitor, which is usually done for testing purposes and to avoid a bang on a first plug-in (and sometimes also on a more permanent basis - I read several posts where members just leave the suppression cap out or their sets).
What had me think about this was the fact that Panrock used a Y cap in the Bush TV22 restoration detailed in his excellent website.
- Helder

[Ed_Dinning]

Hi Folks, Y caps are normally in values up to about 4n7 may and must not fail s/c . I don't think there is a mention in the srecs of them failing open

X caps can be had in values up to 2uF, and in failing short, the dielectric at the short melts, isolating the fault section of the cap. This reduces the total capacitance but allows the device to keep working in a reduced manner.

Ed

[Maarten]

Originally Posted by HamishBoxer

Have seen capacitors with X1 Y2 on the same cap, how can this be right when one fails open and the other is meant to fail short ?

X rated capactors may fail either way and are somehwat more likely to fail open in everyday use as Ed Dinning describes above (self healing effect, not guaranteed but works reasonably well), so the dual marking just means it is tested for that and more or less guaranteed to fail open.

I wrote 'more or less guaranteed', because sometimes even Y capacitors are known to fail short or leaky, especially paper capacitors. Somehow those are allowed to sport the Y marking.

[JonSnell]

Repairing brown goods, (VCR, TV, HI FI etc) for longer than I care to remember, I have always found a Class X to fail short circuit, however, I cannot speak for Class Y but assumed they usually fail open circuit allowing SMPSU noise to return to the mains supply.
There were not many but Grundig VCRs spring to mind for more failures than most others.
Maybe I had more Grundig devices brought to my bench than other makes ... who knows.
When they do fail, a Class X capacitor mutates into a charcoal looking blob as during the "self healing" process, little bits blow and build up under the epoxy skin awaiting the 'Frosted eyebrow feature', as I called it, for the poor engineer who was looking at it, at the point of failure.

[Helder Crespo]

Ok, time for a bad joke. The X-Y is a quantum capacitor. If fails both open and short. It's only when you look at it that its state collapses into one or another.

[paulsherwin]

I think the smoke and sizzling noises are a bit of a giveaway.

[Maarten]

Originally Posted by JonSnell

Repairing brown goods, (VCR, TV, HI FI etc) for longer than I care to remember, I have always found a Class X to fail short circuit, however, I cannot speak for Class Y but assumed they usually fail open circuit allowing SMPSU noise to return to the mains supply.
There were not many but Grundig VCRs spring to mind for more failures than most others.
Maybe I had more Grundig devices brought to my bench than other makes ... who knows.
When they do fail, a Class X capacitor mutates into a charcoal looking blob as during the "self healing" process, little bits blow and build up under the epoxy skin awaiting the 'Frosted eyebrow feature', as I called it, for the poor engineer who was looking at it, at the point of failure.

Both the charcoal and the failing short, fit in with the most used type of X capacitor used in Grundig equipment: the Wima MP3 paper capacitor. Only slightly better than the infamous Rifa types.

[GMB]

Y class are built in a way that makes them kind of like two capacitors in series. This is how the "fails open" works but what I do not like is that with checking for a rise in value you won't know it has "gone".
X class are just robust enough to live a life on the mains
So no reason that a Y built cap could not also meet the X spec. They both have an element of self repair.

[Roger Ramjet]

I would assume that most desktop computers use X-Y configuration on their power supplies. This is because alternative earth leakage protection (other than RCD) has to be considered when multiple computers are plugged into the same electrical sub circuit.

It would be interesting to know how much earth leakage current one computer spec. capacitor will create when connected across a mains supply.

Then again if both elements of the Y component are also connected across the phase & neutral I would expect that to null out any current via the earth connection (unless of course the two Y caps elements are not identical value i.e. inbalance) ?

Rog

[eddie_ce]

Originally Posted by Roger Ramjet

This is because alternative earth leakage protection (other than RCD) has to be considered when multiple computers are plugged into the same electrical sub circuit.

Do you mean an IT system? Is this what is used in data centres?

If I recall correctly total earth leakage current <=9mA is recommended for domestic circuits for each 30mA RCD. On an (outdated) split load board, this may mean swapping circuits between RCDs to prevent unwanted tripping.

Nowadays with modern electronics the earth leakage currents will soon add up.