Existing measurements of an input component (such as voltage or current) in a three phase power supply system are decomposed into a negative sequence component. The negative sequence component, which is significantly higher when a rectifier fuse is blown, is compared to a threshold and a determination made that a rectifier fuse is blown when the negative component exceeds the threshold. In an aspect, an adaptive algorithm is used to make the detection work better in the range of the nominal frequency of the input voltage. In an aspect, the negative sequence is determined indirectly from the existing measurements.