This recommended practice applies to transmission system at 1000 kV ac and above. It defines standard insulation levels and specifies procedures for selecting insulation levels of ac transmission lines and substations. It also specifies reliability criteria under switching and lightning overvoltages. Suggestions on insulation coordination design are described. Overvoltage mitigation measures are recommended according to the characteristics of such ac systems. Some examples of insulation coordination are also presented. Purpose Ultra-high voltage (UHV) ac power transmission system is an effective approach to achieve efficient allocation of energy resources utilizing the advantages of large capacity with long distance@ and low loss. Some countries have already carried out research on UHV technologies@ and several UHV ac projects have been constructed and put into operation. This recommended practice is formulated specifically for the design and application of insulation coordination for transmission systems at 1000 kV ac and above.