For proper operation of superconducting devices, a careful design of the dielectric insulation is essential. Techniques commonly used in traditional devices may not be effective in superconducting devices, since superconducting tapes need to be cooled to cryogenic temperatures. One of the first things to consider is the material choice: insulating materials that work well at room temperature may not be at all suitable for a cryogenic environment. Secondly, the cooling can take place in different ways, one of the most common of which consists in immersing the device windings in a bath of cryogenic liquid (typically liquid nitrogen). The cryogenic liquid itself, therefore, together with the solid cryogenic insulation of the superconducting tape, constitutes a solid-liquid composite insulation system with its own characteristics that must be studied in different operating conditions: in normal conditions, when the few thermal inputs cause the production of a limited number of bubbles in the bath; under quench conditions, when the transition of the superconductor involves a thermal input into the bath such as to produce a large amount of bubbles, which can transiently significantly lower the dielectric strength of the insulation. Don’t forget that the nitrogen bath produces nitrogen vapors, in which important active components such as current leads could be immersed. Part of the project also consists in verifying the dielectric strength of the gas phase and possibly modifying its composition or pressure to meet the project requirements. Based on the experience gained in the CIGRE Working Group D1.64 ‘Electrical insulation systems at cryogenic temperatures’, this report seeks to identify and collect literature data on some of the critical issues of cryogenic dielectric insulation, with a particular focus on SFCL (Superconducting Fault Current Limiters) devices. These devices, notably in the resistive configuration cooled in liquid nitrogen, present several critical issues which also emerge with other devices: in fact, the gas phase is present above the nitrogen bath, there is naturally liquid nitrogen with a possible production of bubbles and finally the solid insulation of superconducting tapes immersed in cryogenic liquid.