Electric shower problem

Can anyone suggest what it is I have done wrong?
I have recently installed a new 8.5kw electric shower in my new home which has been working fine.....untill my girlfriend got her hands on it!!
I had a sparky round to check out the wiring and after taking his advice that the exisying 10mm cable should be ok for this shower went ahead and fitted it, the problem is it works fine for about 20-25mins (which is long enough for any normal person to get clean!!) then the 30a switch in the CU trips out, if the switch is toggled on again it will again work ok?
Can any suggestions be made? or is more information required?
Many thanks in anticipation!!

Maybe because the breaker is overloaded. 8500 watts/230volts = 36.956 amps!

Or even at 8500 watts/240v= 35.41 amps!

your breaker needs upgrading.

Hopper said:

Can anyone suggest what it is I have done wrong?

Click to expand...

Yes - you've attempted a job which is beyond you, and as a result have screwed up. Please don't try to fit the new MCB yourself.

Also - did you think to tell your local council's Building Control dept about this shower installation?

Thanks Screwi for the advice!
Ban-all-sheds, I was looking for help not a lecture, I know I screwed up or I wouldn't have posted.

Fine - given your deep understanding of things electrical, fit the new MCB youself.