A diagnostic system for protecting a tap-changer comprising: a sensor, positioned to detect temperature changes of the tap-changer, wherein the diagnostics system is arranged to: obtain an initial measured temperature, indicating an initial temperature measured by said sensor prior to initiating a tap change; calculate a calculated temperature value, according to the particular tap change to take place; obtain a second measured temperature indicating an actual temperature measured by said sensor after a tap change has been initiated; characterised in that the diagnostic system is further arranged to: set a first threshold value determined based on said initial measured temperature and said calculated temperature value, at which an alarm is generated; generate said alarm when said second measured temperature exceeds said first threshold value; wherein the diagnostic system is further arranged to repeat the obtaining a second measurement temperature and generating said alarm during a selected time period, when the second measured temperature does not exceed a second threshold value, wherein the second threshold value is based on a reference temperature value, and said second threshold value is higher than said first threshold value; and send a trip signal, when the second measured temperature exceeds the second threshold value. obtain an initial measured temperature, indicating an initial temperature measured by said sensor prior to initiating a tap change; calculate a calculated temperature value, according to the particular tap change to take place; obtain a second measured temperature indicating an actual temperature measured by said sensor after a tap change has been initiated; characterised in that the diagnostic system is further arranged to: set a first threshold value determined based on said initial measured temperature and said calculated temperature value, at which an alarm is generated; generate said alarm when said second measured temperature exceeds said first threshold value; wherein the diagnostic system is further arranged to repeat the obtaining a second measurement temperature and generating said alarm during a selected time period, when the second measured temperature does not exceed a second threshold value, wherein the second threshold value is based on a reference temperature value, and said second threshold value is higher than said first threshold value; and send a trip signal, when the second measured temperature exceeds the second threshold value. set a first threshold value determined based on said initial measured temperature and said calculated temperature value, at which an alarm is generated; generate said alarm when said second measured temperature exceeds said first threshold value; wherein the diagnostic system is further arranged to repeat the obtaining a second measurement temperature and generating said alarm during a selected time period, when the second measured temperature does not exceed a second threshold value, wherein the second threshold value is based on a reference temperature value, and said second threshold value is higher than said first threshold value; and send a trip signal, when the second measured temperature exceeds the second threshold value.