What have I done wrong?

The short answer is that you should have used a qualified electrician! Apologies if this is in anyway insulting, but if you don't understand how a loop in lighting circuit works, its better to leave well alone. In the UK we commonly use a wiring system for lighting circuits know as the "loop in system" also refered to as "3 plate". The No1 DIY'er mistake it to take down the old light without noting the order of the connections. When putting the new light up, they then make an assumption and put all like coloured cables together and connect them up. For example they put all the reds or browns in one terminal and all the blacks or blues in the other without realising that one of the blacks or blues is in fact the live return from the light switch. The result of this is that the light will be on when the light switch is in the off position and then as soon as the light switch is operated the fuse or circuit breaker will blow / trip out the circuit, often with a bang and a flash. Whilst a very good DIYer maybe able to sort it out, once you have mixed up the three cables it can take a while to identify which one is the switch, as the grouped cables need to be separated and the return from the switch identified. So the best solution is to call a qualified electrician to sort it out. Normally a ten minute job! Hope this helps, if it does please click on the "like" button.

it sounds like you have done this "red to black, blue to bits"

It could be that you have connected the switch live to the neutral or something similar. An Electrician could solve this very quickly. Good Luck

Have you got all the blue cables and all the brown cable to together? You have mixed the wiring somehow.

you did it yourself thats where you went wrong ,get a qualified sparky ,your lucky you only blew a fuse next time IT COULD BE YOU

Sounds like you have connected the wires up wrong, get a Spark to sort it out for you.

Well it sounds as if you have wired it up wrong.

A tricky one to answer without seeing it but I think the most likely thing you have done wrong is to have mixed up the switch return with one of the neutrals. This is just one of many things you could have done wrong if you dont know what you are doing so, what I need to tell you is get someone who is qualified to sort it out for you before you hurt yourself or someone else. I'm not kidding. Kevin.

What you've done wrong is not get a qualified electrician in to do the job right and give you advice! your safety,and not burning your house down is important! if you and a problem with DIY electrics and have the old style consumer unit NON RCD PROTECTION its will not blow the fuse and can overheat! I THINK ALL SPARKS ABOVE ARE NODDING THEIR HEADS! THIS IS NOT A DIY FORUM FOR GAS NOR ELECTRICS Kieran.

common school boy error no offence

You have connected them up wrong, for example the blue or black wire to the lay man looks like a neutral, where in fact it could be a switched live. A qualified electrician will know this and be able to fix it easily for you. Tom