The Common specifications for high-voltage switchgear and controlgear standards ( IEC 60694:2002 ) tables 1 and 2 provide the rated insulation levels of switchgears and controlgears. In these tables, the withstand voltage applies at the standardized reference atmosphere (temperature, pressure and humidity) specified in IEC 60071-1.
The rated withstand voltage values for lightning impulse voltage (Up), switching impulse voltage (Us) (when applicable), and power-frequency voltage (Ud) shall be selected without crossing the horizontal marked lines. The rated insulation level is specified by the rated lightning impulse withstand voltage phase to earth.
For most of the rated voltages, several rated insulation levels exist to allow for application of different performance criteria or overvoltage patterns. The choice should be made considering the degree of exposure to fast-front and slow-front overvoltages, the type of neutral earthing of the system and the type of overvoltage limiting devices.
The "common values" as used in tables 1a and 1b apply to phase-to-earth, between phases and across the open switching device, if not otherwise specified in this standard. The withstand voltage values "across the isolating distance" are valid only for the switching devices where the clearance between open contacts is designed to meet the safety requirements specified for disconnectors.