Your MCB is an overcurrent protection device. It is tripping because it is
undersised.
For a 9.5 kW shower fit a 45 Amp MCB.

"smb" wrote in message
...
Hi all.

My brother is having a problem with his electric shower. You know the
sort, the instantaneous ones which heat the mains cold water as it passes
through. It keeps tripping the MCB. There is nothing else on the
circuit, which is also RCD protected. It hasn't always happened, but it
is now occuring very frequently. The MCB is rated at 40amps, and the
shower is a 9.5 kilowatt unit (at 240volts).

Any ideas on what could cause the MCB to trip before the RCD, given that
the shower shouldn't be pulling more than 40amps?

I guess the simple answer is that somehow the shower *is* pulling more
than 40amps, or the MCB is naff and trips too easily.

Cheers
Simon