13A power socket - blackened earth connector on - how / why?

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So, ( because people hate that ). I've been going around the 'new' house and replacing the existing power sockets ( volex and GET )with MK Logic Plus. So far all good, apart from one, in which the earth socket on one side seems to have suffered a burn at some point. I also noticed on the socket face the plastic on the edge of earth hole has a tiny nick taken out of it. You have to really look at the socket to see it. I don't think I've used the socket.

I also noticed both of the exposed sections on the earth wire ( all the wires are green / yellow sleeved ) were black too.

I'm really curious to understand what could have caused this?

Ta.

[boxedin]

Thanks for the replies.

1. Yes its on a RCD. No, its never tripped.
2. I've replaced the socket.
3. I can only assume that something was 'jammed' into it previously.