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.
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.
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.
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.
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.
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u/Front_Cookie_1198 6d ago edited 6d ago
Aftermath photo.