The tripping of the RCD would suggest an earth leak fault, When you install the element as all terminals clear from the metal case, as it sounds to me the element may be making contact with the metal case and tripping that way.If all wires (other than earth) are not likely to come in contact with the outer case or element, it could be the power regulator which is running at two high a voltage, this is easily checking use a multi meter.The last option is the element is the wrong part number, I would have expected an element to last 5 years easily for residential appliances.
Hello James You are spot on suggesting an earth leak fault. All the elements fitted were genuine replacements supplied by AEG with the correct part number. Each element failed after approx 18 months with visible signs at the point of failure. Your mention of the power regulator is certainly an item of interest, I was not aware that some parts operated at less than 240v mains. I have looked on the AEG website, entered the model number to bring up a list of parts specific for this model and it lists a transformer but then informs that this part will not fit this appliance. I do own a multimeter and will continue to diagnose the fault.