The photos taken with the socket after i turned it Off at the board but it was showing correct polarity, its possible the manager was getting confused and it was the OLD melted plug that was tripping the MCB on the Non rcd board, before he fitted the new plug, however he said he fitted the new plug then tried in 2 seperate sockets, which i tested and both were right polarity and earth loop tested ok. but he said the unit never worked. so he left it unplugged.
Its quite possible the thing was butted up to the earthed oven causing a short, It was unplugged when i arrived and it was only by chance i checked the plug as I was unsure who fitted it.
He said the previous melted plug was fitted by the oven engineer and he did copy that as confirmed by the picture he showed me of the old plug.
I am glad i did not plug it in after i had pulled the unit out, as it was free standing on a wet kitchen floor.
The 13 amp internal plug fuse was blown, not sure its suitable to plug in anyway and the rating labels worn away.
As you say its a mystery IF it did work before.
