Wiring Problem?

Last night I was wiring up a new socket in the kitchen, the wiring was correct because I had the new wires connected in a chocolate block while the plaster around the switch box set and everything worked fine.

Once the plaster had set I connected the new switch up and turned on the electricity. Everything was fine until I plugged in the kettle into an existing socket and I heard a fizzing noise. I went to turn the electricity off and noticed the fizzing was coming from behind the main fuse box and I could also see sparks. I re-checked the wiring in the new switch and everything seemed fine but i gave a bit more slack in the wire and screwed it back up, now everything seems fine.

What I would like to know is:

a) Why was my fuse box fizzing
b) Shouldnt the fuse wire in the fuse box have blown? It had 30amp fuse wire in it, is this the correct size?