When does the power trip?

[Captain Beer]

I was asked to look at a friends computer. The thing was dead, so I put a new power supply in and as soon as I push the power button the cable to the power supply starts burning like the fuse on a tom thumb, I quickly pulled the plug from the wall. The cable now has a hole in it and the wires inside are melted, the PSU is working fine with a new cable though.

So i'm guessing it was a defective cable, but shouldn't the power have tripped on my board? It has an earth on the plug and the cable.
I almost sh@t myself Lucky for me there were a few beers in the fridge to help calm the nerves.

 

[Voicy]

Power will trip from one of 3 conditions...

1) More than 20mA of leakage current from Live to Ground (i.e. faulty insulation / direct touch etc.)
2) More than 15A is being drawn on the circuit (depends on CB size)
3) Direct short between Live and Neutral up to 5/6 kA (otherwise the CB will explode before it disconnects)

Most likely you'll find that there's no earth leak and it was frying components before you could switch it off. If it were to shock you the Earth Leakage would've kicked in to switch off the circuit.

 

[UnUnOctium]

Cable most likely had slightly stripped isolation so you still had some form of resistance but more than (230/sqrt(2))/15. So you exceeded the cable's rating but not the fuse's trip rating. That's the most likely, logical explanation I can think of. I think it would've tripped the fuse eventually. If this didn't happen in only one spot then your PSU must've done something funky.