For instrument transformer, there are various standards in different countries and areas. Mainly including GB 20840 (China), IEC 61869 (Europe) and ANSI 57.13 (USA). The standard value of the highest voltage for equipment regulated by different standards is different, resulting in possible differences when selecting transformers. While most standards say that, for equipment to be installed under normal environmental conditions relevant to insulation U_m shall be at least equal to U_sys. Further comparison of Um value as followings:

It can be found that the IEC standard corresponding to the U_m = 36kV transformer, in the national standard GB does not have the corresponding value U_m, should choose U_m = 40.5 for installation. Meanwhile, we can observe that, in this case IEC requires testing with a rated lightning impulse withstand voltage (peak) of 170kV, and the test value required by the Chinese national standard GB is usually 200kV. Therefore, we say that the testing requirements of the national standard are more strictly than those of other standards.