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I had the very same issue the other day and found it to b a neutral to earth fault that is allowing the RCD to reset but would not allow an RCD test , what you have to do here is to test all the outgoing circuits for IR , to confirm RCD is o/k then test it with all outgoing circuits disconnected at the red itself and as I also found the RCD will test o/k , the reason is the RCD is pre stressed and will not allow the test when the circuits are connected , don't assume its the cooker thats at fault although this could well add to the existing fault thats already there , which is in this case is usually a N to E fault ..