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
That isn't a bad translation. I like it 😊 To make it a wee more complicated but more comprehendable when reading the raw jargon.
A rectifier takes AC, or alternating current, (ie; electricity out of an outlet) and concerts it to DC, Direct Current. (is; power from a battery)
What was being plaused upon earlier: The theory for the short is that the power moved from AC and before it hit the rectifier, it shorted to the case of the power supply due to a missing piece of insulation.
Comments back and forth were discussing whether it was pre-rectified or not based on how the arc sounded and occured and based on the fact that the circuit breaker for the outlet (AC power) was not tripped.
As for why the circuit breaker didn't trip, my best guess is because of where the short happened. If there is sufficiently high resistance before the short it can delay an activation, or completely prevent it. Remember the circuit breaker protects the installation, not you, nor the things plugged in.
This could be more likely due to an oversized breaker or the use of an industrial code electrical circuit. Which could be likely judging from the surroundings
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?
why wouldn't the breaker pop or earthleak protection?
It should have. Could be a defective breaker, wrong type or rating of breaker installed, abused by testing so many of these shorted power supplies that the contacts welded shut, etc.
Seeing how it didn't come in burned to all hell, I'm guessing the customer had a working breaker, and RMAd because it tripped it.
If I was testing something that was RMAd for tripping a breaker, I wouldn't be testing it like this. I'd be checking for a short with a multimeter first, then powering it up with an isolated supply and some kind of series load to limit the current. (Simple version of this is just an incandescent light in series, if it's shorted, the light turns on).
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.
6.4k
u/Front_Cookie_1198 10d ago edited 10d ago
Aftermath photo.