Beginner Q: Is this an acceptable fair-quality PC PSU failure mode? - Page 1
Forum section: electronics / general

author: jewelie
author_rank: Regular Contributor
timestamp: April 17, 2016, 12:20:06 am
content: Hi folksWell I'm not a happy bunny. With very bad timing (I'm currently losing a battle with Crohns, blargh) our whole PC desk appeared to be dead......a couple of minutes later I worked out that it was the Corsair RM550 550W Gold PSU that's barely 18 month into it's 5 year warranty and still looks mint.It has not only died, but done so in a nasty way that I just would not expect of a "quality" component that connects to the mains!Firstly, I did a very basic bit of testing with a DMM (yes... whilst completely disconnected from everything, especially the mains) but it doesn't seem to be a simple short-circuit at DC or live to earth fault or anything that obvious. Next, I isolated it from other stuff and nervously connected it to mains alone, it immediately blew the 5A UK plug mains fuse. I tried another, the same, so this is repeatable. Then I wondered if it was a one-off requiring a higher than 5A transient switch-on current for some reason and tried a 13A fuse (very very very carefully with my better half at the fuseboard ready to send the house into darkness) and it knocked out not only the 13A plug fuse but the relevant 250V 32A circuit breaker on household fuseboard too (not the RCD, just the ring main 32A circuit breaker.)So when it starts, it attempts to draw 32A+ @ 230V?So there's no fuse or similar basic over current / short circuit protection inside it!?It's the one component that can bugger up everything else, including our home and lives, so we purposefully didn't get a budget PC PSU (it was £85 - approx US$120 at the time), hence why I'm very much less than pleased! But, before I start getting uppity and awkward and demanding a refund in full (because I'm not going to be happy with a like-for-like replacement) I thought I'd query you knowledgeable folk first....Question: Should such a failure mode, that relies purely on the household's electrical safety protection rather than blowing an internal fuse within the device itself, really ever be seen with a "decent" PC PSU?Warm regardsJuliePS On the upside, at least the HDD and SDD have survived - can't check the rest as I don't have a suitable alternative PSU (not to mention that I'm supposed to be resting anyway.... but rest and sleep are for the dead, lol, and I'm angry right now!)

author: FireFlower
author_rank: Regular Contributor
timestamp: April 17, 2016, 12:35:42 am
content: Short circuit, under voltage, over voltage protection means output voltages, not input.So if input side gets fried for reason or two or manufacturing error then it will blow fuse like any other device. I would be worried if you cannot trust your house hold electrical safety devices like fuses which prevent fires.

author: jewelie
author_rank: Regular Contributor
timestamp: April 17, 2016, 12:43:57 am
content: Quote from: FireFlower on April 17, 2016, 12:35:42 amThanks for that.I'm aware that the standard protections are for the output not the input, but I had assumed that a quality unit would at least have its own fuse on the input too somewhere.Oh well. I said I'm a newbie. :-/On the upside, after this experience I feel less paranoid about my being really careful and conservative on the fuse ratings I put in plugs etc - on that side of things, everything worked as it should.Warm regardsJuliex

author: wraper
author_rank: Unknown
timestamp: April 17, 2016, 12:47:18 am
content: It has a fuse, and most likely it blew on your last attempt. Don't see anything nasty or unusual in the way how it failed.

author: Brumby
author_rank: Unknown
timestamp: April 17, 2016, 01:02:52 am
content: I've not had a lot of experience with computer PSUs, but your experience would seem to be rather rare. I can't say I've ever heard of such a serious example - but as I said, my experience is not significant.It would seem there is a very solid dead short right at the input to the circuitry. You don't pop BOTH a 32A circuit breakerANDa 13A fuse in one go without some serious energy.I would beextremelyinterested to have a look inside the case as I believe the cause could be very obvious - but you won't want to try this unless you want to throw away your warranty.As for a replacement PSU, as far as I'm aware, Corsair aren't known for dodgy gear and this could be one of those instances of a freak manufacturing fault that made it into the wild ... and you were just 'lucky' to be the one who got it. If they offered me a replacement, I wouldn't be as reluctant as you are - but then, it's not my house and family.Quote from: jewelie on April 17, 2016, 12:20:06 amI can't say I've noticed many PSUs with fuses - but even if there was one, this degree of short would indicate it could be before the internal wiring even gets to the fuse (yes, as short as that would be).

author: nogood
author_rank: Contributor
timestamp: April 17, 2016, 01:07:38 am
content: If your RCD wasnt in working order it could have been a short in the switching transformer to earth, since the output is earth referenced.Of course only if the internal fuse of the PSU is on the "wrong" side, because you have an indexed plug in the UK, so Live->Broken Transformer->Earth there would be no FET in line because it is the "neutral" side with no fuse.Does your RCD work ? Maybe test it if it has a button for that.Otherwise I couldnt think of a scenario where the internal fuse wouldnt pop first (if there is one, which I would expect for a brand name PSU).With a working RCD the current should only be between live and neutral so if there was a fuse inside the PSU it should have been blown.PS: Every ATX-PSU I ever owned had a fuse, even the cheapo that blew up at 4 AM while in standby (fuse + FET and a ceramic cap all in one go)Edit: That said, *maybe* the internal fuse is now gone (after the 13A fuse). The characteristic of the 5A fuse in your plug probably was to similar to or even rated for less current than the internal one (inrush current, 120V etc. as already mentioned)

author: Ian.M
author_rank: Super Contributor
timestamp: April 17, 2016, 01:13:03 am
content: Here's the product page:http://www.corsair.com/en-gb/rm-series-rm550-80-plus-gold-certified-power-supplyIts rated for 550W output with a typical efficiency of 92%, so the input power must be fairly close to 600W. Its universal input, so may be expected to run from a 120V supply. If running from 120V, 600W is 5A, and it needs some margin for startup surges etc. so it wouldn't be surprising to find a 10A slow-blow fuse in it so blowing 5A plug fuses that would power it satisfactorily from 240V is not unexpected.The results with a 13A fuse take you into the knotty issue of over-current protection discrimination. You'd have to do the research to see what range of fault currents provide discrimination with your specific breaker and make of fuse. Its also possible that the BS1362 13A fuse was fake, or if the 32A breaker is old and has tripped a lot, that it has become over-sensitive.

author: Monkeh
author_rank: Super Contributor
timestamp: April 17, 2016, 01:25:24 am
content: Internal fuses are often quite slow. If it's a severe short, you WILL trip an MCB before a fuse blows, their magnetic trip is incredibly fast.The short is likely on the DC side of the rectifier. Primary drive transistor, probably.

author: jewelie
author_rank: Regular Contributor
timestamp: April 17, 2016, 01:28:53 am
content: Quote from: blueskull on April 17, 2016, 01:07:55 amOnce I'd identified the PSU as a culprit, the power cord was the first bit I looked at and swapped, of course.Quote from: nogood on April 17, 2016, 01:07:38 amRCD: Fairly modern build house (<20 years old), only one owner, lol. But good point - I just gave it a quick test.Fuse in PSU: See belowQuote from: wraper on April 17, 2016, 12:47:18 amA-ha. Excellent point. I just checked... and the PSU does appear to be open circuit now! So there probably was a fuse or something and, as you say, the 32A+ @ 230V finally blew it.Quote from: Brumby on April 17, 2016, 01:02:52 amThe fact that it does finally seem to have properly blown a fuse or similar on the last attempt does make me feel more comfortable. I suspect it has to have been a fuse too given the total lack of noise, smell, smoke or hints of components frying!Okay, feeling more comfortable about accepting a replacement now. But still, this is only the 2nd PC PSU I've ever had that's died (and I'm not exactly young.)THANK YOU everyone.Warm RegardsJuliex

author: jewelie
author_rank: Regular Contributor
timestamp: April 17, 2016, 01:40:19 am
content: Quote from: Ian.M on April 17, 2016, 01:13:03 amAll the bits on the computer desk have been going through a couple of 5A mains fuses for many months and never blowing, when under load, so I can't imagine that an unloaded and disconnected (except for mains) PSU should be suddenly blowing 5A fuses and then a 13A fuse and tripping a 32A breaker, repeatably, immediately, without it being a genuine fault?

author: jewelie
author_rank: Regular Contributor
timestamp: April 17, 2016, 01:51:54 am
content: Quote from: Brumby on April 17, 2016, 01:02:52 amThis is the downside to warranties, of course. I'm sure my doggied determination to not attempt to fix things until they were broken stunted my electronics development.All my best learning and most growing experiences come from when things are otherwise far outside of warranty and otherwise considered beyond economic repair - and in most instances I end up surprising myself by getting things working (and well) again! I really really really do not like dealing with mains stuff. I'm a scaredycat. In my head, mains and above is my Dad's domain (he was a sparky - hence why me and my sister didn't get away with growing up not knowing how to at least properly wire a plug.) 9V or under, that's my comfort zone, lol. I still need to learn to be a bit less _scared_ of higher voltages (whilst of course retaining adequate _respect_ and _seriousness_ for that area.)

author: Ian.M
author_rank: Super Contributor
timestamp: April 17, 2016, 01:57:15 am
content: Its certainly a genuine fault, but expecting the internal fuse to blow first is over-optimistic. It *CANNOT* be fused at less than 6A if it's to deliver its rated output at its minimum rated input voltage of 100V AC, and will probably have a slow-blow fuse to handle startup surges.Your 5A BS1362 fuse must have had a fast enough characteristic to blow before the internal fuse or the 32A circuit breaker at whatever the fault current was. As the internal fuse is now blown. you cant check the input resistance to estimate the fault current. I suspect that the input resistance was quite low, with either the bridge rectifier or the PFC chopper having blown SC. Another possibility is that it has a MOV before the bridge rectifier to protect it against transient overvoltage spikes that has failed shorted.Without breaking the warranty seal, one can only speculate . . . .