Electrics, fuses & wiring!

Hi, i need a very temporary power outlet in a cupboard in my home. There's an unused outlet that was originally intented for an imersion heater that i decided i could wire into. So i basically used some decent 3 way cable and put a twin socket box on the end after wiring into this imersion heater outlet.
The imersion wiring is wired into the fuse box seperatly with an 16A fuse.
The first time i switched it all on everything was fine. But now it trips the whole fuse box, the circuit breaker thingy. Then when i switch the fuse box back on everything is fine and power is ok through the twin socket.
So, at first it trips but then once the circuit breaker is reset everything is fine until its switched off again.
This happens whether there is anything plugged into the socket or not.
Thanks for any help.
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