If it's an intermittent fault it could well take more than an hour. One would hope that the circuit the MCB is tripping needs breaking down and investigating and a Live to Earth fault would certainly cause an RCD to trip.
If the RCD trips when a load is switched on many people ( including some "electricians" ) assume the fault has to be on the circuit supplying that load. The fault most often Neutral to Earth can be anywhere and can be on a circuit whose MCB has been switched OFF
This may help you and your "electrician" to understand and thus locate the fault that is causing your RCD to trip.
.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local