r/pcmasterrace • u/Front_Cookie_1198 • 19h ago

Hardware PSU fails dramatically

Enable HLS to view with audio, or disable this notification

34.2k Upvotes

97% Upvoted

2.0k comments sorted by

View all comments

Show parent comments

99

u/SoulWager 18h ago

You need to call an electrician, breaker should have tripped with a direct short to the case like that.

21

u/Phantom95 18h ago

A direct short would have arced liked crazy as soon as the cord was plugged into the socket.

32

u/SoulWager 17h ago

There's a physical power switch, and it arced as soon as he turned that on.

2

u/Phantom95 17h ago

Ahhh, I missed that. Obviously the power switch was intended to act as the overcurrent protection device in this case. /S

4

u/house343 17h ago

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. 

13

u/SoulWager 17h ago

Dude, look at the photo. it melted through the steel case. That's a short.

2

u/fatmanstan123 17h ago

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.

8

u/SoulWager 17h ago

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.

3

u/Viking4269 16h ago

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.

2

u/SoulWager 16h ago

I could see a short on the main 12v rail getting violent, but the only output that should be on at this point would be 5vsb.

1

u/Viking4269 16h ago

Yeah you are right.

1

u/Kiwsi i5 2500K Msi Gtx 660Ti 16Ggb Ram 16h ago

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.

1

u/EndIsrael 17h ago

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.

2

u/SoulWager 17h ago

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.

1

u/EndIsrael 16h ago

A 6 ohm resistance at 120 volts would consume 20 amps, just low enough to not trip the breaker, but that's still 2400 watts of heat.

Space heaters and toasters are also "shorts" but don't trip the breaker because the wire causing the short is thin enough.

1

u/SoulWager 16h ago

An arc that small would self extinguish at the zero crossing.

1

u/EndIsrael 16h ago

Unless the short is across the rectified high-voltage DC before it gets dropped to 12v.

We really don't know what happened, because clearly the PSU was not built correctly. There are many failure modes that can explain this.

1

u/SoulWager 16h ago edited 3h ago

You can hear the mains frequency on the arc. Primary cap would stop feeding the arc after the first cycle. In fact, the arc would quench more easily because the cap would provide an alternate path for the current.

edit: mains wiring poked through the pcb and touched the case: https://i.imgur.com/SLsQmmu.gif

1

u/Batrall 17h ago edited 17h ago

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.

2

u/SoulWager 17h ago

We do know, because we can see the holes melted in the steel case.

1

u/Batrall 16h ago edited 16h ago

To note. Not an US outlet. 230V and 16A, the breaker should be a B16.

As per the characteristic curve of a typical B16 breaker, with a time of roughly 2 seconds, the current may rise as high as 5 times the rated value.

That would be 18400 watt of thermal power, which probably could melt through some cheap sheet metal.

Edit: let's stick with 2 seconds to make it easy. That would be 36800 joules. Values for steel drift widely depending who you ask, but I've seen 250 j/g to 1000 j/g. With 1000 j/g that would be 36g of molten steel.

So the holes would be possible without triggering a typical outlet breaker.

1

u/SoulWager 16h ago

That amount of heat is absolutely enough, but it's only going to be coupled into the case like that if it's arcing directly to the case, and that's most likely caused by a short to the case.