Who can say why with any accuracy Adrian.
An RCD will trip when the current in the Live wire does not equal the current in the Neutral conductor - the assumption being that the current difference might be flowing to earth through a person - and electrocuting them - it's a good safety measure to have an RCD.
What might be causing it in your dishwasher? Well the time it happens in the washing cycle might give you a clue - on my Miele dishwasher for instance the heating element (a very probable cause in my opinion) does not get used to heat the water until about 20 minutes or so into the programme.
So if your RCD trips at the 20 minute mark then it may well be down to a defective heater.
Other things might be the culprit - the water inlet solenoid - the detergent solenoid - the circulating pump.
Pay attention to the timing of the RCD trip in relation to the machine's progress and you may learn something significant.
I do not get out much and I do so like the sound of my own prose.
Good Luck..., April 2016
