There are only two reasons I can think of that would cause this problem. Firstly, there could be a fault within the oven, such as a short on the fan circuit. However it might also be that the oven has been connected to an electrical supply which isn't up to the job.
So the first thing to do is to check that the RCD isn't simply tripping out because the oven is drawing too much current, even though it's working properly. (If it is, you need an electrician to sort out your wiring. If it isn't, you need a service engineer to repair your oven).
So what's the model number of your oven, please, and also what's the rating of the RCD?