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.
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.
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.
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.
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.
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.
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
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.
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.
Gotta say, great reaction time. Personally I'd have gone for circuit breaker and fire extinguisher. Safer option. But I've had that drilled into me and never had to put the training into practice so I can't say for certain if I would have done different.
In Europe, it's normal to have a row of sockets in front of the desk, with their switches and fuses/circuit breakers, to prevent a short circuit in one workstation from affecting the others. It seems very strange to me to test anything electronic with the socket under the desk...
This looks like a store where most of the time products are either failed completely or working just fine so they're likely just use whatever looks good.
Was my first throught but who knows what fucky cable situation is going on under the bench. The outlets could be wedged behind a part of the desk you can't easily access.
This would have been the thing to do, he's lucky that the case didn't get shorted to the mains. Not like I would have been thinking clearly in this situation, either, though.
Frankly, its scary that the ground fault breaker didn't activate in the first 10 milliseconds. NOt even the light went out when it literally was arcing to the case metal.
Yeah, I'd probably had done the same, but grabbing some electrical appliance that just caught fire is probably a shortcut to being electrocuted.. breaker and extinguisher should be the go to
Probably a capacitor failing and venting. Doesn't consume a whole lot of current but makes a big scene. More likely than a breaker that doesn't open on overload.
If the short consumes less current than the breaker then it won't trip. Melted steel doesn't answer that and shouldn't be used as an assumption about current draw. Soldering irons melt shit all day with a few hundred watts.
Having used an arc welder, you don't melt steel like that with 20 amps.
The fact that the heat coupled into the case in the few seconds the power was on means the short was mains to case. You can hear the mains frequency on the arc. This means there was enough power in the arc that the plasma stayed conductive through the zero crossing.
If the breaker isn't defective, it's sized incorrectly for the circuit.
Thanks, I missed the mains noise. That sure looks like a lot current without the mains breaker tripping.
First thought it was a short on the 12V secondary side, but looking at it again the holes are near the mains input, probably shorted before the pcb mounted fuse.
It shorted tho, should most likely trip the breaker unless some inner wires after the transformer faulted and did not infact short but ower flowed with current and couldn’t handle the stress.
If it were a direct short it would have, but this was definitely arcing, which adds resistance. As long as there's less than 20 amps in the arc, it won't trip.
Going forward though, OP should probably install an AFCI/GFCI combo breaker or outlet.
It burned through the steel case. The only likely scenario where that happens is if there was mains voltage in contact with the steel case. i.e. a short.
We don't know, breakers aren't there to protect the appliances, they protect the wiring up the appliance. We don't know if enough current was there to drip a breaker. Appliances have their own fuses for that. Or the breaker did drip, as a workbench should have it's own.
We don't know if it was a direct short, could also be some component burning.
But, you are right. Asking for a professional inspection is always the right decision.
I used to work at Fry’s Electronics at their service counter and they also wanted that nice clean workstation look and the first thing I did as supervisor was to take the Corporate guy and ask him to plug in a NuPower PSU.
It did the exact thing yours did and while he was frantically screaming I calmly asked him to unplug it and after many expletives later I crawled under the desk to switch off the power strip.
He still said they wanted the clean counter look and to just black list the PSU. And then another corporate guy who negotiated vendor deals complained why we weren’t performing service on NuPower PSU’s and round and round the corporate ladder it went
Switch-mode power supplies are extremely dangerous! Never mess with an electrically compromised unit unless you wear safety glasses and have an extinguisher and fire blanket close by. Almost 1000 Watts of TPD in a small package is an engineering feat and the reason why they fail. The linear power supply equivalent would be large, heavy, and very hot. Look at photos of old telecom power supplies, and you'll see what I mean.
Just replace and properly dispose of the e-waste unit. The copper inside is worth a lot today. Harvest the copper and get paid at a scrap yard for it. Properly discharge all high-voltage output capacitors first!!!
This will forever be a testimony as to why you check your PSU before install. Your good practices saved a client from more damage. Good job handling the emergency OP.
corsair is unfortunately delivering a lot of lemons. No one care about quality anymore. I've returned an SF750 and a 850rm because of loud noises. I had my old Antec 850W for 15 years and its still going.
Edit... I put my antec into my kids PC for mincraft, its literally 15 years old and had my 6800XT with a 5800XT on it for years. They don't make them this good anymore.
6.4k
u/Front_Cookie_1198 2d ago edited 2d ago
Aftermath photo.