As menctioned the fault was caused on installation, most showers now have a label over the terminal block teling the installer to check that all terminals are made secure.

The mcb will not trip as there is no overcurrent and the rccb/rcd will not trip as there is no fault to ground, so from arcing the cable heats up nicely melting insulation. So all that can be done is replace burnt cable and terminals

sorry, wasn't disagreeing with you there in my badly worded reply, I was just saying that I had been to a clients house that had the same problem, for the same reason, and remarking on the coicidence of what appeared to be the same shower. What had shocked me was the fact that a plumber had previously been out to check the problem, and had walked away leaving the cables in that condition without warning them not to use the shower until they had someone else look at it at the very least.

I guess plumber would be a very loose term in this instance.

Should terminal blocks be used for 15amps?