Nonconducting material that provides electric isolation of two parts at different voltages.
It must have an electrical resistivity and a dielectric strength sufficiently high for the given application.
The secondary requirements relate to thermal and mechanical properties.
CTI is max voltage which material can withstands 50 water drops without without formation of conductive path.
Rated Impulse Voltage (Uimp) for Overvoltage category — I, II, III, IV at various nominal Voltage (Ue) is suggested in IEC 60664.
Thermal Classification of Electrical Insulating Materials, basis Permissible Temperature Rise above Ambient Temperatuer and Hotspot.