If it has not been used for a while, damp can creep into the elements, causing a slight earth leakage, sufficient to trip an RCD. Is it the main RCD that trips or the individual circuit breaker that serves the cooker? If it is caused by damp you may be able to cure it by heating the elements with a hot air gun. I have a few holiday cottages on my books and they tend to suffer from such dampness problems in the cooker elements as they are usually only let between April and October