The high frequency current (call many names such as dirty power, electrical pollution, noise, harmonics, trash, hash, transients, etc) on the neutral wire in the electrical system is positioned to return directly to the power system via the feeder neutral to the substation, and possibly beyond. However, as is often the case, the utility feeder neutral is not adequately sized (as required by law). If this is the case, much of the return current will flow through the ground rather than the feeder neutral. Under these conditions, STETZERiZER® filters still provide benefits, but utility companies should be taking the necessary steps for public safety by updating power lines, providing larger, higher-strand neutral returns to handle the high frequencies.
At 60 Hz the filters act as capacitors and normally improve the power factor of the customer load, which are normally slightly inductive.