keeps tripping electric?

My baumatic double electric oven,one large fan one small normal with 5 gas rings keeps tripping the electric off when using the large oven,the small one is fine.A year ago it did this and I looked at the round element around the fan and it had blisterd so I replaced it and it was ok,about a month ago it did it again so I looked at the round element but it was ok ,I replaced it anyway,it was ok for 2 days but now trips out again in this large oven we only use the round element not the side elements or the grill so it seems like its the round element doing it or something to do with it,help please ,mick