a method and device for monitoring operation of a high voltage generator which converts an input voltage into an output voltage which is higher than the input voltage , by measuring at least one first physical quantity which is representative of the input voltage ; deriving at least one parameter from the at least one measured first physical quantity ; comparing the at least one parameter with a second measured physical quantity which is representative of the magnitude of the output voltage ; and producing a response when a predetermined minimum differential exists between the at least one parameter and the second measured physical quantity .