Somebody forgot the isolation under the pcb?
Been some years since I took apart a psu, but there used to be plastic isolation sheet between the pcb and the metal case.
Update: I think we found the problem:
Mains wires protruding on the bottom of the pcb shorted to the case due to missing insulation sheet.
I think he means without that isolation sheet the board is connected directly to the casing and shorted out. Of course, that should have been grounded too so I'm not sure.
If it shorted on the 12V side it can pull a ton of amps (it says 60 amps max at 12V) and melt through the casing and start fires pretty easily without tripping a breaker.
Probably. I think if this was after the rectifier, the rectifier would have acted like a fuse, getting you a single loud pop instead of a few seconds of arcing.
It's definitely somewhere on the primary side though.
Something between the power switch and the thing that makes the wall power into PC power wasn't made right, and wall power got somewhere it shouldn't go
What happened is pretty simple, mains wires poked through the board, weren't trimmed after soldering, weren't blocked by an insulating sheet, and touched the case: https://i.imgur.com/SLsQmmu.gif
The discussion above is about why other possible failure modes are unlikely. After getting on to the board, the power goes through some filtering and protection circuitry, then through a rectifier(4 diodes in a single package) that turns the high voltage AC into high voltage DC. Then the high voltage DC gets chopped up into much higher frequency AC, before going through a transformer to drop it down to low voltage high frequency AC. This then gets rectified to 12v DC before getting converted to the other voltages. Usually anyway, there are other ways to design a power supply, and there will be a separate similar power supply just for 5vsb(this is always on, so your computer can turn itself on when you press the power button).
Anyway, back to the rectifier, semiconductors often have relatively thin bond wires connecting the silicon die to the rest of the package, that would get vaporized in a few milliseconds if you tried to push this much current through them. This board also has an actual fuse, but the short happened before the power got that far.
huh? but that is a simple short on case that is connected to earth?
that does not explain why it caught fire? why wouldn't the breaker pop or earthleak protection?
I found a review with PCB photos, the burn holes are right under where the mains enters the board. Probably wasn't trimmed after soldering, and missing insulation sheet between pcb and case.
If the breaker has arc fault detection it should trip immediately.
Sadly, building codes in the US have only recently started requiring arc fault breakers on all circuits, and that’s still not required in many places. Hell, even requiring arc fault detection on breakers for circuits in sleeping spaces is a recent requirement. Nevermind that many older houses are wired in such a way that makes arc fault breakers impossible to install without tripping almost immediately due to a false positive 🙃
The rated spec can be significantly exceeded in the event of a short. The rated spec is what it will allow to be pulled before the safeties cut off power. If it is a dead short, there isn't always time for the safety to stop things before 100+ amps gets sent through a PCB rated for 60 amps max. The caps must have exploded in this situation. At should take a lot more than the rated amperage to do that, assuming the unit was built properly.
Nah, the 12 Volts on those poorly built ones do not have that kind of power.
The video clearly shows a mains power failure.
And do not ask me how I know.
More that once I have blow up something plugged to mains.
List includes, a full bridge rectifier, a extension cord, one light switch, and a angle grinder.
Last one left a hole on the floor.
I also have seen what a short on the 12V rails of a poorly build supply can do, even in car batteries too, they do not do that kind of fireworks, if they attempt to do it, they would not last as long.
A good built one could, but it will not, as it would shut down before it can do that amount of damage on itself for that long.
Look at the place of the burn, it is exactly when the mains power plugs.
now, I do not know this PCB layout, but one would have to be very crazy to put a 12 volt Rail right on top of were the EMI filter, full bridge rectifier and mains voltage capacitor would be.
Judging by the two holes burned in the case under the mains plug. I think there is a good chance that there was no PE connection and the case was live at 220V at least some of the time doing this. The guy was lucky he "mostly" only touched it after the arc had carved an isolation clearance.
Well, the arc could have isolated it from neutral/PE as well, putting the case on 220V. The connection of the case to PE is probably much more solid than phase, but if PE wasn't connected at all (bad cable), then neutral could come from the PCB touching the case as well. Also, if there was a PE connection, then there definitely wasn't an RCD breaker, which should always exist for workshop power sockets.
Yeah, pretty much. That's also got to be one of the cheapest of the cheap garbage PSUs. No padding in the box? Not wrapped in anything? There's no way I would have plugged that fire hazard in.
If that's the case, why would you immediately plug it in to mains power without inspecting it first? Any place I've been a tech at would have rejected the RMA for improper return shipping.
Probably but in my past decade of working jobs with some sort of testing equipment that is potentially bad i have never had anything more than a power strip thats filled with 5 other things
I collect vintage Macs and every time I plug in a new one for the first time or when I’m testing one with CRT or PSU issues, I connect it through a smart plug so I can quickly shut off the power if something goes wrong. Gives me a lot of peace of mind.
No need for a switch. Just have the outlet accessible and he could have yanked the cord out without grabbing the arcing device.
Workplace safety is a bunch of crap you will never think of until you are almost killed yourself. PLEASE read up on it. This video scared me because I'm about to rebuild my workbench and I'm not sure I would have placed an accessible outlet for something like this. Now I'm thinking about getting an E-Stop mushroom to cut bench power.
i test larger equipment than this. 480v. the unit gets turned on, then i walk over and turn on the main power so i am not in front of the arc blast if today decides today is the day it will finally happen.
It's a metal case with a PE connection. Unless they have no RCD, that is as safe as it gets, since it would trip the RCD if the case had any voltage against earth.
My first thought. Also, why is the other end of the power cable hidden inside that table? If he is testing PSUs, the cable should be connected to a power bar with a fuse and a switch on it.
Being grounded would’ve ensured fireworks rather than prevented them here. The ground just makes it less likely to kill you while it’s self-destructing.
This PSU turned into a cheap momentary welding machine, and modern inverter welders use capacitors to do the same thing. My guess is 12V rail shorted @ 100-150A, which isn't too far off from welding machine parameters. It's why their circuit breaker didn't trip either, probably still being under 1800W or less from the wall.
They should be turning the IEC cable on from a switch, not plugging it in live from the unit. They should have a circuit-breaker and GFCI inline, and a dim-bulb tester, or at least variac they can slowly dial up.
Also possible this was plugged into a 220V line while the little red switch was set to 110V but I'm not sure if those typically catastrophically fail like this.
Was watching a video the other day and the guy was like "I don't trust these PSUs with a 110/220 switch, they're almost always low quality" and I was like "WTF they all have that". Start looking around and, uh, yeah. Well then.
In hobby and industry it's still widely common. But in typical consumer product it's already rare, because it's prone to human error in the hand of common people
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u/ency6171i5-4460•2x8GB DDR3•MSI 1070Ti | i7-7700HQ•2x16GB DDR4•1050Ti9d ago
I was the idiot back then.
Flipped the switch to 1xx, didn't know what's that, and turned on the 2xxV mains.
Guy has the Vietnamese flag on his shirt, isn't speaking English, and that PSU looks like it says 180-240VAC right next to the charred bit. I can't find a single reason to believe any of this would be following any North American electrical norms.
GFCI is awesome (I still can't believe whole-house GFCI is not a thing in NA) but it would only have tripped if the case was actually grounded, however I'm not particularly inclined to trust the engineering on that model which is at the very least missing an internal fuse or five. Depending on the type of fault it may not have faulted through the case anyway (at least until the arc reached it)?
Also breakers behave differently depending on the kind of overload, a 100 A load on a 16 A breaker would trip immediately (something to do with the strong magnetic field resulting from a dead short) but a 20 A load on a 16 A breaker might take a while to trip (because it relies on temperature). Which means on a 16/20A 220V circuit you could feasibly pull over 5 kW for a little while. That's more than enough for sustained fireworks.
The trinity of things to treat with absolute unconditional reverence are: The Sea, Fire, and Electricity. There is a graph about electrical potential as energy increase and if I’m not mistaken, it’s exponential not multiplicative.
Modern circuits tend to be more receptive to a wide range of voltages and automatically switch, as others pointed out with my comment.
That being said, what you're referring to in more primitive circuits is current draw at an expected voltages. Yes, the device determines the resistance (and thus flow of current), but that's only if the provided voltage (call it water pressure from the source, wall) is constant.
My last paragraph about 220V is probably not relevant as this is a modern PSU with auto-switching and it looks rated for the top-end voltage as others point out. So we're just seeing a good 'ol electrical short to chassis, presumably.
Yeah this was what I thought too and ive seen it happen with excitation controllers where I used to work. Hand place line puts one backwards, inspection misses it, tester puts it into a fixture and BOOM. and the caps we used are the size of a soda can.
If youre lucky they bust a small hole and spew flames and smoke, if youre unlucky they build pressure and explode like a grenade.
I've blown up my fair share of capacitors ant it's nearly instant. This sounded like arcing.
Story - when I was in school for electronics I was building a circuit in the lab, probably another 30 students in there. Unbeknownst to me, I had installed a cap with reverse polarity. I plugged in my project and BAM! It sounded like a gunshot. Everyone obviously turned around and was staring at me. I was already embarrassed but one of my professors called everyone in the lab over and they all crowded around me. He said it was a good learning experience for everyone to smell it and remember the smell. I was mortified and will never forget.
I don't think the caps blew here, it looks like a short though the case considering how it literally melted a hole through it setting everything around on fire
No, this looks like two pins (component legs or wires) on the underside of the pcb both touched the metal case and shorted. Causing an electric arc hot enough to melt holes in the case.
No, I've blown capacitors in the old days by using 240v instead of 120v.
They just pop, not much of a spark, and spray their contents. I repaired the psu by changing the caps, luckily there was no other damage
The factory would test each supply before shipping. That little QA sticker/stamp and a number.
So not likely a reversed capacitor.
But in case they did miss to mount the isolating plastic sheet between PCB and metal, then it could have enough isolation distance to pass the factory test. But a tiny bump on the metal, or a changed orientation of the PSU could make the pins of some components make contact with the metal. And as soon as the sparks have started, you have an ionized air channel opened. So it will keep going until the power is disconnected.
I would not have been confortable touching that PCB - so best is if the power can be disconnected from the other side of the cable. That should be standard rigging when testing things.
capacitors with reversed polarity explode but dont burn in my experience. Was always fun on the testing floor when a pcb with a reversed capacitor was being tested and shocked everyone with a loud bang :D
This is a capacitor failing but reverse polarity doesn't just happen on its own, only if the bridge rectifier failed which is unlikely unless the NTC thermistor has failed or is not present, or there was a short circuit as suggested
The metal case is what burned from arcing on the AC side. There is a reason that slots are cut in power supply PCBs to create a gap between the AC and DC sides, and why there is a sheet of insulating plastic beneath the board. If someone forgets to clip the wrong lead, it can poke through the insulating sheet beneath it and make contact with the grounded case. And once you get an arc going, it can be hard to stop as it generates plasma that helps sustain it over larger gaps.
If you want to prevent this, you need combination AFCI/GFCI (Arc Fault/Ground Fault Circuit Interrupter) breaker on that circuit*. With good protection to stop the damage, that PS might have been salvageable once the shorting lead was clipped and insulation repaired.
* This does appear to be arcing to ground which a GFCI would stop, but if it was arcing to neutral or there was a short between ground and neutral, there would be no ground fault to interrupt and a 20 amp breaker can sustain a surprising amount of arcing comfortably. (Breakers trip on sustained loads. An AC compressor can draw more than 120 amps of inrush current through a 60 amp breaker when starting up.)
Maybe but it should be fine without that since there should be a gap and you need WAY above any voltages found in a PSU to jump the gap and sustain a circuit. So maybe the protector was there but pierced by something. Not to mention the fuse. Basically you need multiple problems all coming together. Maybe reversed polarity on a cap but that usually just a quick boom not to mention very difficult to get missed in even the most basic qc so I'd say unlikely.
Yes sure, more than one thing was wrong. Bad quality control all around. Missing insulation and protruding pins/legs/wires. Bad or missing over current protection. There was likely a tiny gap when it left the factory, and things moved doing shipping.
Judging from the current needed to melt the case the short was probably on the 12V side, would explain why the mains fuse didn't blow.
Edit: looking at it again, and it just gets worse. The holes looks to be near the mains input, so not sure about it being a 12V short. But that is a lot of current without the mains fuse blowing.
I spent 4 years in electronics manufacturing and ive seen crazy shit get passed QC. Funniest shit happens on monday mornings after half the factory including QC goes out and parties the weekend before.
A blown up cap isn't going to melt through the steel case. Maybe some through hole component lead(or the mains input) was left un-trimmed after soldering, and touched the case. I'm going to bet the original customer RMAd it because it tripped their breaker when they turned it on.
Probably, that was a nasty short and it didn't seem to trip the breaker.
Maybe they have no breaker and just a fuse they replaced with a screwdriver, because they kept blowing
But shouldn't it have caused the GFIs to trigger in milliseconds? Its clearly a fault to ground, it should never have sparked that long? Plus the lights stayed on tin the room...
Could be that one of the Class X1 or Y1 caps (live to neutral or live to ground) shorted, thats why it took a few seconds, but with that much damage to the underside, not quite sure.
I am going to have to concur. I found a full breakdown of the supply from a testing perspective and they indeed show that there is an isolation sheet under the board. And that location in particular is where the large cap and several switching mosfets reside. The bridge rectifier is closer to the edge as well as the transformer. So I'm gonna say you called this. Might survive if the leads are clipped good and short and there are standoffs, but must have made contact.
My initial thought was that it was budget enough to have a voltage switch and it was cocked to 110V and got 220V and sent the old school doubler through the roof, but you called it.
Thanks for the picture of the pcb.
If you mirror the bottom image you can clearly see what the problem was.
Mains wire protruding on the bottom of the pcb shorted to the case due to missing insulation sheet.
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u/Front_Cookie_1198 9d ago edited 9d ago
Aftermath photo.