The 20Hz injection kit consists of two components a) 20Hz generator and b) 20Hz Band-pass filter, also called coupling filter. The 20Hz generator generates bi-polar square wave of 20Hz of +/- 30Volts. This signals is passed through a series LCR tuned band-pass filter to generate sinusoidal 20Hz voltage, which is injected / coupled to the neutral ground transformer (NGT). The band-pass filter has very high attenuation at 50Hz; thus protects the generator from 50 Hz voltage across NGT.