I have no idea as to what exactly the "Protection Present" LED indicates. I believe that the 50dB, 75 dB and 100dB indicates the level of noise rejection in each bank. For example, when your AC line is hit with a 10kV surge, it is reduced down to ~32V, ~1.8V and ~0.1V in the 50dB, 75 dB and 100dB banks, respectively.