Abstract: There is provided an apparatus and associated method for the testing of a laminated core (100) for an electrical machine. The apparatus supplies an excitation signal to the winding (102) of the electrical machine which is of a frequency higher than the rated operating frequency of the machine. There is also provided a phase sensitive detector (110) which receives, from the laminated core (100), a signal in response to the excitation signal. The phase sensitive detector (110) resolves the received signal into a number of components. These components are resolved relative to a reference signal. The size of the components determines the severity of any fault that exists within the laminated core (100.