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The high voltage (HV) levels in standards start at ratings above 52 kV in both IEC and IEEE. Below these voltage levels, the equipment is classified as medium voltage (MV). The typical GIS high‐voltage ratings can be grouped into four design classes, even if the split may vary somewhat. The lower high‐voltage ratings are in the range from 52 to 72.5 kV as the first level range. The second level range includes 100, 123, 145 kV, and in some cases the 170 kV rating. The third level range covers 245 and 300 kV. The fourth level range includes the voltages 362 and 420 kV. The 345 kV voltage level is considered as 362 kV and is no longer recommended by standards today. The third and fourth level have been historically developed in North America and Europe, typically as 245 and 420 kV in Europe and 300 and 362 kV in North America. The reason behind this is the availability of technical solutions like insulators at the time when the new voltage levels were established. An overview of the voltage ratings is given in Table 1.5.

Only two voltage ratings left in the IEEE and IEC standards, which have different options for power frequency switching and lightning impulse values, are 245 and 362 kV. 550 kV rated voltage offers two insulation levels for the rated power frequency withstand voltage.