Elementary confusion  - Page 1
Forum section: electronics / beginners

author: jeffjmr
author_rank: Regular Contributor
timestamp: December 29, 2021, 04:59:07 pm
content: I guess I’m having a senior moment.The mains circuit on my Heath IG-5218 is shown below.The two paralleled primaries of the transformer measure 50 ohms resistance. 50 ohms at 120v calculates to 2.4A. Yet this circuit is protected by a 3/16A fuse. What am I missing?Also, how can a capacitor be both X1 (fails shorted) and Y1 (fails open) at the same time?Thanks,Jeff

author: langwadt
author_rank: Super Contributor
timestamp: December 29, 2021, 05:04:50 pm
content: Quote from: jeffjmr on December 29, 2021, 04:59:07 pmit is a transformer not a resistor

author: bdunham7
author_rank: Super Contributor
timestamp: December 29, 2021, 05:15:04 pm
content: Quote from: jeffjmr on December 29, 2021, 04:59:07 pmInductive reactance. 120VDC would blow that fuse very quickly. The inductance of the primary coil gives it an impedance much higher than the DC resistance.http://www.learningaboutelectronics.com/Articles/Inductor-impedance-calculator.phpAnd, of course, for a pure inductor, the current will be 90 degrees out of phase (lagging) with the voltage. With a mix of DCR and inductance, it will be somewhere in between.QuoteThose are outdated and simplified notions of the requirements for X and Y capacitors. Specifically, the X capacitor is notrequiredto fail short.https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

author: jeffjmr
author_rank: Regular Contributor
timestamp: December 29, 2021, 10:38:35 pm
content: In a nutshell, I grossly underestimated the effects of impedance in transformers. Got it now.I had already read that article on X and Y caps as well as others, before. I’m still stumped.From that very article:“ 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.”“Class-Y safety capacitors are designed to fail open.”So let me be more specific: What happens when a cap rated as X1/Y1 fails?I aak because my IG-5218 SG has two 1nf X1/Y1 caps, one line to neutral and one line to ground. Said SG is plugged into a UPS. I’m thinking it might not be healthy for the UPS should either of these caps fail short. In addition, the EMI/RFI filter on the mains cable of my SG has another 470nf X1 cap between line and neutral, as do most of my test equipment that have either internal EMI/RFI filters or an external one or built into the power cable.Any help to understand what could happen here would be appreciated.Jeff

author: bdunham7
author_rank: Super Contributor
timestamp: December 29, 2021, 11:34:25 pm
content: Quote from: jeffjmr on December 29, 2021, 10:38:35 pmAs I understand it:An X-rated cap may fail short, but is not required to. Many similar non-safety capacitors will also typically fail short, it's just a general characteristic. A Y-rated capacitor should not fail short, only open. An X/Y capacitor thus should only fail open. Presumably they have some extra mechanism that causes this, sort of like a fuse. I don't know any details of how they accomplish this.

author: Capernicus
author_rank: Frequent Contributor
timestamp: December 30, 2021, 02:26:29 am
content: FEED HIM LIES!!!!!

author: Cerebus
author_rank: Super Contributor
timestamp: December 30, 2021, 02:34:51 am
content: Quote from: Capernicus on December 30, 2021, 02:26:29 amHave you thought about going and finding a nice free energy conspiracy forum to hang out on, and leave the good folks on here alone?Quote from: bdunham7 on December 29, 2021, 11:34:25 pmIt's a geometry thing. I too am ignorant on exactly how they arrange to get the geometry right, but I am clear on the basic mechanism. They arrange the capacitor geometry so that if any conductive part shorts it can only happen over a very small area, small enough that it will act as a fuse for any current that flows through it and thus blow the short apart and "self heal".

author: tooki
author_rank: Super Contributor
timestamp: December 30, 2021, 02:43:34 am
content: Quote from: Capernicus on December 30, 2021, 02:26:29 amAnd you wonder why people “hate” on you…

author: Capernicus
author_rank: Frequent Contributor
timestamp: December 30, 2021, 02:52:29 am
content: It was only a joke.

author: Cerebus
author_rank: Super Contributor
timestamp: December 30, 2021, 03:23:28 am
content: Quote from: Capernicus on December 30, 2021, 02:52:29 amOh gosh, and there was me thinking that a joke had to have some sort of humour involved; unless you mean joke as in "That man's opinions are a complete joke".All you ever seem to do is shitpost or announce your capacitor electronics 'discoveries' that are contra-physical, it's getting a bit wearing.

author: jeffjmr
author_rank: Regular Contributor
timestamp: December 30, 2021, 03:48:51 am
content: Back to the subject at hand.It’s not hard for me to imagine how and why a X-rated safety cap could fail closed and the advantages to certain circuits to have that feature.It’s not hard for me to imagine how and why a Y-rated safety cap could fail open and the advantages to certain circuits to have that feature.It is also not hard for me to imagine the problems associated with an X-rated cap installed where a Y-rated cap is called for, and vice versa.What I can not get my head around is how a cap can be both, and what kind of circuit could benefit from not knowing which way it's going to go.My immediate concern, (ignoring for a moment the obvious educational benefit from this discussion) is I can imagine that shorting line to neutral which is my understanding of what an X cap WILL do, and an X/Y cap MAY do, will be injurious to a UPS before a fuse or CB can trip. And that’s before we get to the possibility that an X/Y cap on line to ground could do what Y caps are specifically designed NOT to allow; line voltage to chassis!Hopefully someone is going to chime in to make sense of this madness!Thanks all for your responses, even the sarcastic ones. I have lost some of my senses but not the funny bone.Jeff

author: Cerebus
author_rank: Super Contributor
timestamp: December 30, 2021, 04:03:46 am
content: No, you're still stuck on the idea that an X class capacitor WILL fail short. It won't necessarily. It MAY fail short, it MAY fail open. What it won't do (in theory, yes RIFA I'm looking at you) is go up in flames when it fails.A Y class capacitor on the other hand MUST NOT fail short, and should be designed so that it does NOT. A Y class capacitor is always an acceptable replacement for an X class capacitor, but NOT the other way around.

author: IanB
author_rank: Super Contributor
timestamp: December 30, 2021, 04:30:59 am
content: I'm going to guess that a capacitor connected directly across the mains supply is exceedingly unlikely to fail short. Either the capacitor will will self-heal (in which case it will no longer present a short circuit), or it will explode (in which case it will no longer exist to cause any problems). Either way the short circuit will not remain.If connected across a UPS, then maybe things wouldn't be so dramatic, but there should at least be a fuse or something to prevent too many fireworks.Broadly speaking, failing short circuit is not what you want, so quality capacitors with the appropriate rating should be very unlikely to do this.

author: tooki
author_rank: Super Contributor
timestamp: December 30, 2021, 01:41:14 pm
content: Quote from: Cerebus on December 30, 2021, 04:03:46 amExactly. To be extra clear (since not everyone is aware of the defined meanings of "may", "shall", etc. in technical documentation), what's meant here is:- a class X capacitor is allowed, but not required, to fail short, and is not allowed to fail in any way that causes a fire.- a class Y capacitor is not allowed to fail short.Additionally, each class has subclasses with specific voltage/pulse/impulse ratings. A capacitor that's rated for, say, X1/Y1, must meet all the requirements for X caps in general and Y caps in general, as well as the specific requirements of both X1 and Y1 classes. With regards to failing open or short, the "must fail open" of the class Y trumps the "is allowed to fail open or short" of class X.Quote from: IanB on December 30, 2021, 04:30:59 amWell sure, even if it is unlikely (which, AFAIK, depends entirely on the construction of the cap), but the point of the Y safety class is that is mustneverfail short.

author: jeffjmr
author_rank: Regular Contributor
timestamp: December 30, 2021, 03:06:44 pm
content: Thanks, Tooki. That adds some clarity for me.But several articles refer to the practical uses of X caps by placing them line to ground specifically to trip a breaker or GFCI in the event of a failure. If they can’t be relied upon to fail short, then it seems their only real purpose in that situation is EMI/RFI reduction.Given that understanding, I can not envision what an X/Y cap would ever do that a Y cap wouldn’t do. Can you give me an example where an X/Y cap would be specified instead of just a Y cap?All the Shurter power inlet EMI/RFI filters I have, some fused some not, have X2 caps across the mains. This seems backwards to me. Seems to me they should be Y. Why would you want a cap to MAYBE fail shorted across mains, EVER?Jeff

author: tooki
author_rank: Super Contributor
timestamp: December 30, 2021, 03:35:55 pm
content: Quote from: jeffjmr on December 30, 2021, 03:06:44 pmI can only find one source making that express claim. My suspicion is that that article is written by a novice who misunderstood the causal chain.The idea is this:ifa class X cap fails short, then the fuse or breaker must trip (and thus, must be present, which is always the case since we have them in our electrical panels), to prevent fire.Quote from: jeffjmr on December 30, 2021, 03:06:44 pmThat isliterallythe caps' only purpose. They aren't circuit protection devices.Quote from: jeffjmr on December 30, 2021, 03:06:44 pmNo, I'm not knowledgeable enough in this area to have any examples, sorry! But if you look at what the various subclasses of X and Y require, you'll see that they don't really overlap that much. Some things are specified only for X caps, and different things are specified only for Y caps. So if your application requiresbothof those things, you have to get an X/Y cap.If I had to make nothing more than an educated guess as to why you'd want an X/Y cap, it'd be to simplify purchasing, so you can use the same part instead of stocking two different parts. Hopefully others can chime in who know more about it.Quote from: jeffjmr on December 30, 2021, 03:06:44 pmTolerating ≠ wanting.X caps come in larger values than Y caps, so I guess if you need the higher capacitance, but not necessarily the failing open, then you go with the class X, tolerating its limitations.(This is a similar issue to "why are electrolytic caps polarized? does this have any advantages?", to which the answer is "no, there's no situation where being polarized helps anything, and many situations where it's decidedly unhelpful. But we tolerate that limitation, when possible, due to the much higher values that are possible with electrolytics.")

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2021, 03:56:54 pm
content: Quote from: jeffjmr on December 30, 2021, 03:06:44 pmYou'd have to dig through the actual testing specs, but I believe that the surge and endurance testing procedures for X and Y (actually X2 and Y2, as there are also X(1-4) and Y(1-4) designations) are different and don't completely overlaps, so a Y2 capacitor has not necessarily passed the X2 testing. An X2/Y2 capacitor will have passed both tests. I'm not aware of any particular application that would require (by regulation) both X2 and Y2 ratings, but perhaps they exist. However, I use and would specify an X2/Y2 capacitor in a design just because they are readily available and nobody really needs the additional hazards of line-voltage capacitors shorting out.Across the mains is explicitly the 'X' application. Presumably any short triggers current overload protection somewhere. Line-to-ground may not trigger current overload protection if the ground circuit is open and thus failing short would create a severe hazard on any device with an exposed surface connected to the device ground.