Why are some X2 capacitors not for use "in series"

[ElCaron]

Hi, I build a mains voltage detection circuit with an H11AA1 and partially capacitive current limiting recently (resistive limit to max peak current, capacitive limit to 15mA - phase shift didn't matter).
I noticed that some X2 capacitors like this one are not for "in seires use". I would have expected that "in series" is LESS demanding than in parallel, because some voltage drops at the load. What is the reason for this limitation? Long- term stability of capacitance?

[Wawa]

Educated guess.
Not for use in "series with mains" type applications could mean "Do not use this cap in a capacitive supply".
I have replaced several gradually failing "self healing" caps in capacitive supplies.
Some loose 90% of their value in a few months.
Leo..

[TomGeorge]

Hi,
I agree with Wawa.
Your link says in full.

For worldwide use in electromagnetic interference (EMI) suppression in across-the-line applications requiring X2 safety classification. Intended for use in situations where capacitor failure would not result in exposure to electric shock. Not for use in "series with mains" type application

As a filter across the live and active they will be fine because in a properly constructed circuit you would have a circuit breaker of fuse in line to blow when the X2 finally broke through.
If the cap is in series with the active line as inbeing used to capacitively reduce voltage.
I the X2 capacitor breaks through, you will not have the capacitive impedance to drop the AC voltage, you  will have FULL AC current and voltage flowing, thus letting the smoke out of you low voltage device you were using the X2 to volt drop.
Tom....

[ElCaron]

Isn't that a general characteristic of the X2 rating and short-safe is Y2? AFAIK X2 is still allowed for use in series and Y2, which prevents full mains voltage in case of failure, is only mandatory in line-to-ground usage?
So your posting would be an explanation why NO X2 is suitable for the application (but according to regulations, and other X2 datasheets, they are, as far as I understand), but not why SOME X2 are not.

In my case, e.g, if the cap fails, the current of about 60mA will smoke the 2x 2kOhm/2W resistors and the H11A11 (slowly, because of allowed peak currents of 80mA), but since they are carbon resistors, they should die in a sufficiently safe manner, and if they don't, there is another 160mA fuse that will blow. In any case, my actual low voltage circuit is protected by the isolation of the H11AA1.

[TomGeorge]

Hi,
The manufacturer is covering his ass, he has no control over how his component is used.

I'd put a low voltage MOV in your circuit so it shorts if more than your running voltage that you are dropping to is exceeded by a cap fault.

Tom....

[ElCaron]

manufacturer is covering his ass

Well that cannot be the only thing to it. Why would a manufacturer drive away customers by ruling out certain uses so that no reputable device manufacturer would use it?

[MarkT]

Well that cannot be the only thing to it. Why would a manufacturer drive away customers by ruling out certain uses so that no reputable device manufacturer would use it?

Huh?  There are perfectly valid uses for a capacitor, you aren't required to connect it to the mains!!

Better not to get class-action lawsuit for starting fires don't you think?  Good business practice!

[ElCaron]

Then why would you even write X2 on your capacitor AND explicitly label it for use in parallel to mains?
Either your product can do it, then you can also sell it to people that want to put it in series, or it cannot do it, then you just put a label "terrible product" on your stuff. It just doesn't make any sense.

[DrAzzy]

I don't think X caps were ever supposed to be used in a series configuration, that's the whole idea, if they short, you'd have a hazard. An X cap is only for cases where a short circuit failure would not result in a safety hazard. If my interpretation is correct, the manufacturer is just being extra careful to warn people not to use them in places where they need Y caps.

[Wawa]

They are self-healing when connected across the mains.
But not with a small current through them as in a capacitive supply.

I use 630volt film caps for that, and still use extra protection behind it.
Leo..

[ElCaron]

I don't think X caps were ever supposed to be used in a series configuration, that's the whole idea, if they short, you'd have a hazard. An X cap is only for cases where a short circuit failure would not result in a safety hazard. If my interpretation is correct, the manufacturer is just being extra careful to warn people not to use them in places where they need Y caps.

Well here is the datasheet of the ones I finaly used
http://de.tdk.eu/inf/20/20/db/fc_2009/X2_B32921_928.pdf
Look at page 15.

[ElCaron]

They are self-healing when connected across the mains.
But not with a small current through them as in a capacitive supply.

I don't know what that would mean, physically. In both cases, no actual charge has to tunnel over the capacitor.

BTW, all datasheets I linked ARE for film capacitors. Not 630V, but that is only about the number, not the letter, AFAIK. (and for 1, we are taking about MUCH higher voltages).

[allanhurst]

I've used RIFA or WIMA paper X2 caps in series with mains eg to get a high dc voltage in a cockroft-walton multiplier.

never had a problem

regards

Allan

[ElCaron]

I've used RIFA or WIMA paper X2 caps in series with mains eg to get a high dc voltage in a cockroft-walton multiplier.

Well, as I said, I am pretty sure that X caps are fine for that, regulationwise. Y is mandatory for line-to-ground and that sort of stuff. The question it, what OTHER property except for the failure behavior that differentiates between X and Y determines if a cap is suitable for in-series use.

[Grumpy_Mike]

The question it, what OTHER property except for the failure behavior that differentiates between X and Y determines if a cap is suitable for in-series use.

Is there any other more important issue her than safety?

http://powerblog.vicorpower.com/2013/06/what-are-y-capacitors/