The difference is in the pulse output voltage: INS-S220 has a maximum of 2 kV, and INS-S420 has a maximum of 4 kV. However, the EUT power supply capacity is the same for both: single-phase AC240V /16A DC125V / 16A MAX. For INS-S220, the configurable ranges for pulse width and repetition cycle differ depending on the pulse output voltage, so please check our catalog or HP for detailed specifications.