An RCD will trip when there's a difference between Live and Neutral of 30mA - the assumption being that the "missing" current is running to earth possibly through a person - it's for safety. RCDs of other ratings can be had but 30mA is typical.
Your oven has a leak to earth type fault - the heating element will be a very likely - probable cause.
An appliance engineer will be able to properly diagnose and repair it for you.
Good luck...
Peccavi, November 2009
