The double set of Fischer FCC 5-50 LISN's are DC LISN's. The FCC 5-50 LISN's are predominantly used for Conducted Emissions testing, being utilised by placing it between the power supply and EUT to accurately measure the interference that the EUT is emitting at its supply input.

Where would this be used? The FCC 5-50 LISN's are used to supply the connected equipment with a known, reliable impedance and isolating the equipment from the power source, preventing external noise from interfering with measurements.