Three-phase photovoltaic "on-grid" three level with neutral point clamped (3L NPC) have become very interesting for high-power photovoltaic systems from the aspect of output voltage quality and reduction of harmonics injected into the power grid. This type of inverter shows several advantages compared to classic three-phase inverters with two levels. One of the important aspects of the reliable and safe operation of this grid photovoltaic inverter is the types of insulation applied in it. When we talk about isolation, it means the isolation between the power circuits of the inverter and the grid power supply on the one hand, and the isolation between the control and communication electronic circuits and the power stage of the inverter. The paper presents an example of the analysis and coordination of the insulation of a "on-grid" 3L NPC photovoltaic inverter, based on the EN62109-1 standard. The input DC voltage of the analysed PV inverter is ≤1500 V, the mains voltage is 3x400 V, 50Hz, the protection system is TN-C, and the photovoltaic inverter is connected to the power grid via a three-phase power transformer with a transmission ratio of 2:1 (800 Vac/400 Vac).