The main harmonic of an electrical emission (60 Hz in the USA and 50 Hz elsewhere) will be most prevalent in corona. As the condition becomes more severe, there will be fewer and fewer 60 Hz harmonics observed. As an example, arcing has very few 60-cycle components. Mechanical looseness will be “rich” in 60 Hz Harmonics and will have little frequency content between the 60 Hz peaks and will also demonstrate harmonics other than 60 Hz.