A method and a device for diagnosing the status of an electrical apparatus having a sealed fluid based insulating system. The device includes at least one input for receiving measurements of a current running through the apparatus and measurements of an actual apparatus fluid pressure as well as a diagnosing unit. The diagnosing unit determines a corresponding theoretical apparatus fluid pressure based on the measured current, compares the actual apparatus fluid pressure with the theoretical apparatus fluid pressure and diagnoses the status of the apparatus based on the comparison.