The 1 MV bushing of the ITER NB injector is probably one of the most challenging components of the whole accelerator, since such a high voltage (HV) in a contaminated and radiated environment requires the development of non-standard technologies. This paper intends to provide a guideline for the design of 1 MV bushing for the ITER neutral beam injector test bed. Although mainly focused on the SINGAP alternative concept of the injector, the reported general criteria can be useful for the design of other HV components, such as post insulators of the acceleration grids. For every different kind of required insulation, basic physical models describing breakdown phenomena are reported together with the main technological implications and the practical recommendations necessary to improve insulation performance. The results of the finite element analyses of 1 MV bushing, operating at CEA—Cadarache laboratories, are used to validate general criteria in a typical geometry. The effects of radiation, such as neutrons and gamma rays, on solid insulators and gases have also been taken into account, describing the most important phenomena and the related implications in terms of voltage hold-off capabilities. Finally the design of the bushing in the SINGAP option and the solutions adopted are described.