Questions about the dual mode (X/Y) safety capacitors - Page 1
Forum section: electronics / projects

author: jjd
author_rank: Contributor
timestamp: June 23, 2023, 11:39:06 pm
content: I need to find a replacement for a Y class and searches for Y1 or Y2 produce these X1/Y* type caps and this classification has me confused.I understand the difference between the two different classes of safety caps, but don't understand how the combination (X1/Y1 or X1/Y2) caps work when they fail. Given X class are used line-to-line and fails short, and Y class are line-to-ground and fail open, how do the dual/mode caps fail?I understand Y class can be used in place of X because they Y fail open, but X cannot be used in place of Y because X fail short. Does this also mean that the X1/Y1 or X1/Y2 caps can only be used line-to-line or can they be used line-to-ground too? If both, how do these caps fail? For example, this Kemet R413I21000000K (https://connect.kemet.com:7667/gateway/IntelliData-ComponentDocumentation/1.0/download/datasheet/R413I21000000K) says it's suitable for these line-to-ground and line-to-line but also says:"Not for use in "series with mains" type applications."What exactly does in "series with mains" mean, line-to-... line, ground or something else?Can the X1/Y1 or X1/Y2 caps be used in place of a straight Y class cap?

author: coppercone2
author_rank: Super Contributor
timestamp: June 24, 2023, 02:43:04 am
content: I am suspiciousBut series mains means as a voltage divider in a low current device like a light bulb, the old capacitive dropper.

author: TimNJ
author_rank: Super Contributor
timestamp: June 24, 2023, 04:22:27 am
content: If it has both ratings printed, then you can use it in either application.X-caps are not "designed to fail short". It would not be a desirable situation to short out your AC mains supply! Both X-caps and Y-caps are pretty much designed tonot fail at allwhen used in the normative application of each.Nevertheless, the requirements for "robustness" are a little different for each, because the nature of the hypothetical failure modes are different. If a Y-cap fails, it's a shock hazard. If an X-cap fails, it's a fire hazard. If an X-cap fails, the best case scenario is that the fuse or breaker trips and the hazard is eliminated (although it's not necessarily always an immediate BANG failure.) If a Y-cap fails, it's very hard to detect, and can result in (previously) isolated parts becoming energized. It does not take much current to cause human injury, so this type of failure won't cause a breaker to trip or anything like that. Well, I should say that it *could* cause GFCI to trip if there is one.So, arguably, Y-cap robustness is more important than X-cap robustness. Under this proposition, a Y-cap is usually also rated for line-to-line usage (as an X-cap), but an X-cap is usually not rated for line-to-earth or primary-to-secondary application (as a Y-cap). X-cap failure = bad, but other bad things have to happen as a result to risk human injury. Y-cap failure = very bad because it directly risks human injury.

author: TimNJ
author_rank: Super Contributor
timestamp: June 24, 2023, 04:25:49 am
content: As for Y1 vs Y2. Check the capacitor you are replacing for the type used. There are no plain "Y" caps (without a subsequent number e.g. 1, 2, 3).When in doubt, replace with Y1.Also...you have failed Y caps in your equipment? What happened? Be careful. If they were seriously that overstressed, who know what else was overstressed (e.g. transformer insulation).

author: jjd
author_rank: Contributor
timestamp: June 24, 2023, 02:28:45 pm
content: Quote from: TimNJ on June 24, 2023, 04:22:27 amWas that a bad choice to say designed? I agree they are designed to not fail, but they do I've replaced plenty of both in consumer grade electronics. It's also frequent in vintage computers where ancient, metalized-paper RIFA caps (both X and Y) fail.It's my understanding when X fail they fail short which should then trigger another component (fuse or breaker) to handle the failure. Understanding that, I can't see why placing this behavior between line and ground would be desirable even tho these Y designs should also include over-power protections like fuses/breakers. Shock from electrifying ground would be the primary concern since many devices have cases connected to ground.

author: jjd
author_rank: Contributor
timestamp: June 24, 2023, 02:40:28 pm
content: Quote from: TimNJ on June 24, 2023, 04:25:49 amThanks, yes I understand the difference between Y1 and Y2. This is household voltage so 2 (X2/Y2) is fine.As to what happened... these are filters in vintage computer power supplies. The cause is almost always failure due to age, and when X fails the fuse usually blows too which appears to be the designed behavior.I have not seen a Y failure, most look to be in good shape. But given the similar materials used in construction and their age I prefer to refresh those to prevent something bad from happening since these are wired between line and ground.

author: jjd
author_rank: Contributor
timestamp: June 24, 2023, 02:55:10 pm
content: Quote from: coppercone2 on June 24, 2023, 02:43:04 amLOLso you think "in series" literally mean inline?Do you think that's included just to cover their butt? Because I don't recall ever seeing a design where either class is used like that.

author: coppercone2
author_rank: Super Contributor
timestamp: June 24, 2023, 03:23:04 pm
content: I am pretty sure I saw maybe big clive or someone take apart those small gadgets that plug into a wall and there was a capactiive dropper supply with those filter capacitors in it. Or maybe a LED power supply or something.I don't see how it could not get into mains. It says mains on it some where in the name or first paragraph description of the data sheet. That is all the back story a cheap designer needs to do something with it.I imagine it being something like "well it might be a little better then just a regular foil capacitor". The thing is I am pretty sure I saw it with a X cap, but you would think they would want a Y cap there.

author: TimNJ
author_rank: Super Contributor
timestamp: June 24, 2023, 03:45:50 pm
content: Quote from: jjd on June 24, 2023, 02:28:45 pmI think your perception of what youthinkthe X and Y designations mean is throwing off your ability to understand this. Neither type ismeantto fail open or short by design. To pass IEC 60384-14 testing, the capacitor must survive voltage impulse, long term endurance, mechanical, and environmental testing. No failure is allowed per IEC 60384-14.The X and Y ratings really only point to the impulse voltage they are rated for. Y-capacitors have higher impulse ratings than X-capacitors.Y1: 8KVY2: 5KVX1: 4KVX2: 2.5KVSo, you can see that a Y1 or Y2 cap easily meets the requirements of X1 or X2. That is, Y caps are more robust than X caps.Product level safety standards (e.g. IEC60950-1 or IEC62368-1) require that capacitors used in certain circuit positions be of a particularrobustness.For capacitors used in line-to-earth or primary-to-secondary application, robustness is extremely important due to the risk of shock. Therefore, safety standards dictate that only Y1 or Y2 capacitors are used in this application. For capacitors used line-to-line application, robustness is of course important, but the risk of human injury is generally lower. Therefore, safety standards allow X1 or X2 capacitors to be used in this application. That is, as a practical tradeoff, these capacitors can be of less robust construction (lower impulse voltage), to allow for higher capacitance to volume ratios.If a capacitor manufacturer wants to pursue a dual rating, this is mainly a marketing/business decision.

author: coppercone2
author_rank: Super Contributor
timestamp: June 24, 2023, 04:08:53 pm
content: Well what would be a incentive to make a capacitor fail short, it seems its always better for it to fail open.A fail short would only be useful if the EMI filter is critical to some function where its better if it blows the fuse rather then functions without filtering. That is going to be a very specific design.You would think failing open would be better for not starting a fire then failing short. If they fail short, then there is a short circuit current going through them. If they fail open there is no current going through them. That never made much sense about fire protection with short circuit fail.The more open a capacitor becomes (its already a open!), the less energy it will dissipate!The only posisbility is that its meant to act as a protector for the MOV, so if the MOV gets hot, it melts the cap, and it shorts out the circuit at a lower temperature. So that would require good thermal coupling between the MOV and the capacitor. I never saw that before, it would be a bizarre gamble for a design. I don't think its good design to put the fuse inside of a capacitor designed to fail short to prevent the MOV from exploding. If you really wanted that, there should be a reverse spring loaded anti-fuse coupled to the MOV to short out and blow the inline fuse. I would call that like a thermal trigger fuse tripper. Because why not just put a regular thermal fuse, that opens the circuit, rather one that shorts the circuit out.SO if there is a mythical capacitor designed to fail short, it could act as a circuit breaker tripper that makes sure the MOV does not get above a certain temperature, but leaves the MOV connected in circuit during the whole ordeal. But fail-short reverse fuses that are really capacitors are a BIZZARE concept!UNless it means, that it fails short sharply, and does not settle at a low impedance. That would make sense.Are you sure the capacitors might not be designed to basically fail abruptly rather then drifting to a 'heater' value?

author: jjd
author_rank: Contributor
timestamp: June 27, 2023, 11:32:28 pm
content: Quote from: coppercone2 on June 24, 2023, 04:08:53 pmThis is how I understood this design is to behave in a failure. This behaviour causes the fuse to blow and that gets the job done. Is blowing the fuse is better than continuting to operate without the filter... I don't know and it may have been a bigger issue 30 years ago than it is today(?). The original design was also not servicable, housing was riveted together, repair path was to install a new supply.

author: jjd
author_rank: Contributor
timestamp: June 28, 2023, 12:03:52 am
content: Quote from: TimNJ on June 24, 2023, 03:45:50 pmIt's interesting when I was searching for Y caps, I pickedup some dual rated Panasonic caps from Digikey. When they arrived I noticed digikey had incorrectly listed the part as Y2/X1. The dual rating on that series was only for values .0068uF or less, and greater value only rated for X1 applications. A good reminder to always check the datasheet! Digikey was notified and they updated the ratings on capacitance values greater than .0068uF.https://industrial.panasonic.com/cdbs/www-data/pdf/RDI0000/ast-ind-127757.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: June 28, 2023, 12:43:48 am
content: Quote from: jjd on June 27, 2023, 11:32:28 pmmaybe it has to do with failing short quickly not sticking in some low but not shorted value. like collapse to dead short