I have never seen a wire wound fire element go short, in fact, I don't think it's even possible as the heater element goes open in a failer and being as there is no earthed casing to contact it just sits there not working.
An 18 mega ohm element will trip ?????
That's around 0.0000127777778 amps, or 0.00293888889 watts.
Nobody mentioned wattage.
When an element fails it will usually "open" the electrical circuit and no further heating will take place. When this happens, the element simply appears to have stopped working.In some rare instances the element will "short-out" against the sheath which is the outer visible part of the surface unit.