On 07/11/2012 18:50, John Williamson wrote:

It depends on the cross sectional area of the cable cores. As is
correctly pointed out in the thread, the fuse is to protect against
cable faults, not internal faults in the box. If the cores are too
small, a 13 A fuse may not blow or carry enough current to rapidly trip
the MCB, leading to a cable that at the best is going to melt the
insulation, and at worst may start a fire.


Just anecdotally, I had a decorator in my house using a steam stripper.
He asked to borrow an extension lead, and I conscientiously checked the
extension lead was man enough for the job, then left him to it. Half an
hour later, the power went off.

Investigating, I found that he hadn't bothered to unreel more of the
extension cable than he needed to reach the job - despite clear
instructions on the reel to that effect. The entire lot inside the reel
had melted, and the power went off when the cores touched. This was a
man who had been doing this sort of work for over 10 years, mind.