Advantages of HTS cables are higher current carrying capability, very low thermal interaction with underground infrastructures, and very low external magnetic field designs. These advantages make them interesting for implementation in dense urban areas. There are several experiences worldwide with HTS cables at various voltage levels, but the long-term experience is still limited. As a result, various aspects of HTS performance, such as long-term reliability, are unclear and further study is required. This paper provides an overview of insulation materials, background, and uncertainties of recommended electrical insulation testing. The application of power law and the length effect will be discussed. This discussion may help to give direction and assure the quality of HTS cable for ongoing studies.