This paper provides a detailed overview of the dielectric design and insulation coordination applied to a medium voltage modular multilevel converter prototype. The complete system has ratings of 0.5 MVA and is designed for connection to a 10 kVdc supply with a system voltage of 6.6 kVac. The choice of air as insulating and cooling medium requires careful considerations regarding the clearance and creepage distances with respect to the selected materials. The design considerations from the submodule to the cabinet level are presented in the paper, considering safety standards and their requirements for a selected over-voltage category and pollution degree. The final dielectric design is verified experimentally with dielectric ac withstand test and partial discharge measurements.