My Defy 621 stove causes the earth leakage to trip after a few minutes when the oven is swithed on. The thermostat was resently replaced. Could it be one of the elements, although it seems they are working.

It is possible, old plate have larger capacity i.e. wattage and new plates have lower wattage. Did you replace same value plates ?

replace its shorting out as it gets hot

To turn on the oven on your Defy Thermo Fan gas stove, follow these steps:

1. Make sure that the oven is properly connected to a gas supply and that the gas valve is turned on.
2. Turn the oven control knob to the desired temperature setting.
3. Press the oven ignition button to ignite the gas. You may need to hold the button down for a few seconds until the gas ignites.
4. If the gas does not ignite, check the ignition system and make sure that it is functioning properly. You may need to consult the stove's manual or contact a professional for further assistance.
5. Once the oven is ignited, it should begin heating up to the desired temperature.

Keep in mind that it may take a few minutes for the oven to reach the desired temperature, depending on the setting you have chosen. You can use the oven control knob to adjust the temperature as needed.
If you are having trouble turning on the oven on your Defy Thermo Fan gas stove or if you have any other questions, you may want to consult the stove's manual or contact Defy customer support for further assistance.

by replacing switch

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Contact DEFY and fill in a service request there to assist your qualified electrician to do it safely.

https://www.defy.co.za/manuals/?ma=6&cat=9

These links should help:

http://www.newappliances.co.za/uploads/documents/DOWNLOAD%20MANUAL%20DDS491.pdf
http://www.manualsonline.com/support/defy/stove/kitchenaire-621-5556179
DEFY Kitchenaire Stove 621 User Manual
Electric Oven Repair

The elements in ovens consist of an outer earthed metal tube and an inner resistive element that is insulated from the outer tube by what is effectively a compressed ceramic insulating powder. If the element is not used for a while, enough atmospheric moisture, (or from spills, etc), can penetrate the insulation to cause enough earth leakage to trip the RCD. If the RCD is tripping at the correct level, (They can get too sensitive - see this article here:
Diagnosing RCD earth leakage breaker nuisance trips), then the only long term solution is to replace the element, although they can sometimes be dried out by heating them.

ISOLATE THE POWER TO THE APPLIANCE BEFORE OPENING
There are several electrical conponents that can cause an earth leakage, but usually there are temp control swithes shorting to earth, the internal of the componet brakes down and it is screwed to the metal chassis of the oven. When the temp gets too high this is ment to break connection, if it has went fauly it can pass current to earth causing your RCB to trip. Have a look to find the location it is usually a semi oval case with two terminals, you should have continuity through the two terminals as it is normally closed, check for impedance to the case with the component disconnected, is there is any conductivity, replace.
There are as I said before several things that can cause this, a faulty heating element is another, check them while you have the appliance open. Black Grey spots not carbon or food it should feel rough, or if not fully blown a small bulge on the metal. In order to do these test you will require a mulimeter, and some basic knowledge of electrics.
Hope this has been helpful
john

Yes it can, especially if it is not dried out immediately, It gets into the plates and insulation and will cause your earth leakage to trip out. With the stove unplugged or switched off, use a hair drier or heat gun to dry out all the water,especially the plates as water seeps into the internal insulation and causes a short. If there are signs of rust, clear this off with a fine wire brush (remember to keep the power switched off). If you can, after drying out the stove, check for current floe between earth and neutral and earth and live to make sure that there is no chance for a leakage there - use a multimeter set on the ohm scale or a Megger meter. There should be no continuity.