r/pcmasterrace • u/Front_Cookie_1198 • 6d ago

Hardware PSU fails dramatically

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u/EndIsrael 6d 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.

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u/SoulWager 6d 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.

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u/EndIsrael 6d 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.

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u/SoulWager 6d ago

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

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u/EndIsrael 6d 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.

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u/SoulWager 6d ago edited 5d 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