Electric shower tripping fuse - piglet john?

[TheLittleMe]

We bought our house 3 years ago, we replaced the electric shower immediately with a new one as the water would cycle scalding hot then freezing. The shower then worked well for about 6 months before the cycling started happening again. One day there was a bang and there had been a small fire in the fuse box! we had an electrician add a fuse switch (technical term- not!) in the fuse box. Now, instead of cycling hot and cold, the electric shower works 75% of the time. It then trips the switch in the fuse box maybe twice a week, if we have been in the shower too long. Does anyone know what might be going on here, and how I can resolve it?

[PigletJohn]

What is printed on the "fuse" that trips, and how wide is it?

Photo may help.

Also, do you know the kW power of your shower.