Hi I've just had the same shower fitted and it did the same when the electrician tested it.
He realised what the problem was, and solved it.
Switch off EVERY trip switch in the consumer unit (fuse box)
then ONE BY ONE, switch them ALL on again. Don't ask me why, but nothing tripped out after that.
It works perfectly now.
Hope it sorts your problem too.
Let me know.
Sandy.
Sandy Brown, September 2006