How does a moulded plug just fail?

Spark123 said:
The other thing that could have happened is a poor connection on one of the pins when plugged into the socket causing a heating effect of the plug. Maybe worth investigating the socket as well.
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If that were the cause, I'd expect to see at least some discolouration of the socket fascia around the holes and probably some arc pitting on the plug pins too. Nothing, both plug and socket are like new visually.

To everyone who's suggested it, yes, I am intending to conduct an autopsy at some point over the weekend and I will be posting the results!
 
The autopsy...

plugwash said:
Sometimes the joins between the different parts are not as good as they could/should be.
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I've seen that too but this one was very well moulded, it took a good 15 min. of intensive carving and prising to get into it, but in the interests of scientific discovery I persevered despite the not inconsiderable risk of slicing a finger...

deadshort said:
My bet is the cable has shorted at the entry to the plug.
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That would have been my bet too. We both would have lost money on that one.

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Internal insulation breakdown between live and earth crimp terminals, note the black marks on the clear plastic cover. There was a visible arc here with 500V from the insulation tester applied. What actually caused the breakdown is still not entirely clear, some sort of contamination during manufacture possibly? Certainly there was none of the residue you'd normally expect to see from liquid ingress.
 
DetlefSchmitz said:
Looks mirror imaged to me (L on the left).
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No, but the brass strip connected to brown does go to the LH end of the fuse, the RH end being connected to the live pin on the right. The far right terminal is earth, the strip goes across the centre of the fuseholder to the earth pin.
 
Whatever it was it must have provided workable insulation at first but then later failed.

I wonder if there was a stray strand of wire which either moved and created a short circuit or created a weak point in the insulation that an electrical spike could break through. The resulting short circuit (whether triggered electrically or mechanically) would then destroy the stray wire and leave the area badly contaminated.
 

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