Encapsulation of electric and electronic components is necessary wherever protection against environmental influences, or high heat conductivity is needed. In case of coil windings potting resins help to reduce electrical losses due to heat transportation. In many applications, potting materials with so called UL certification are required. Most frequently this is the UL94 homologation. Further, also UL746 is increasingly important. While epoxy resins without any further test have an intrinsic RTI value of 90°C according to UL746, polyurethanes that were not subjected to a specific RTI test are only regarded as resistant up to 50°C. However, since shortly also polyurethanes with high RTI values are available and in use for encapsulation of transformers, stators and other electric applications. This paper gives an overview of the different UL norms that relevant for encapsulants and the related tests. It helps to interpret values on the so called “Yellow Card”. Further it shows, which new ways there are to reach such properties as flame retardancy and a high relative thermal index. It further informs the user about the typic conflicts of objectives in case of self extinguishing pottants.