Call it a surge protector, spike buster or whatever you choose. Most of these devices use the marvelous MOV (Metal Oxide Varistor) as I like to call them.
I suggest you give this a read. When a MOV breaks down they frequently make a ticking sound and each time a MOV breaks down it degrades. When they fail they normally break down into a short. They can also fail open in which case they simply no longer do anything. They can begin breaking down well below their rated breakdown voltage. In that case if we measure the MOV it will appear open, however, a 250 volt MOV may be breaking down at 100 volts. This is why there is a fuse or mini circuit breaker inline with mains hot power.
As to the outlet or socket. If a socket becomes worn it will not carry its rated current. The resistance between the plug blade and socket increases and results in excessive heat being generated. This condition can be caused by wear or a loose connection of mains power to the socket as was already covered in this thread. This is exactly why back in April Kiss stated:
Replace the socket IMMEDIATELY. The plug can probably be cleaned with emery paper if there is any sign of discoloration.
Apparently we all do not share the same meaning of
"Immediately"?
1. Just because the pin and socket seem to fit tight and proper does not mean there isn't a loose socket mains screw terminal.
2. The plug and socket based on your own explanation were destroyed as a result of excessive heat. The melted plastic is a good indicator.
#. I covered earlier, the MOVs have failed and breakdown below mains voltage.
That is my guess....
<EDIT> Beat by Nigel again.
Yes, and did you replace the wall socket? I assumed you did but...... </EDIT>
Ron