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BT18 is a range of elements rather than a specific model, some of them come with a built in cut off (which has a reset button you have to press) and others do not. As part of building regulations a immersion needs to have a safety cut off fitted, this may be separate sensor on the tank.
The fact your previous element had a integrated cut off suggests the previous engineer has installed the wrong type of element, and its overheated and burnt out as there wasnt a safety device fitted, this can be very dangerous, and would suggest calling the engineer who fitted the element to confirm they didnt fit a separate cut off on the tank, if they admit to that, then I would get them to come back and replace the element with the correct type free of charge.
Please let me know if I can be of any further assistance.
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