3.4　Engineering Meteorological Investigation and Surveying

3.4.1　The basic data referenced in meteorological analysis and calculation shall be reviewed in terms of reliability，consistency，and representativeness.

3.4.2　Multiple methods should be adopted for calculation and determination of the design wind speed and ice thickness for areas where no sufficient data is available，and the calculation results derived from these methods shall be analyzed comprehensively so as to select reasonable results.

3.4.3　Where lines run through remote mountainous areas and it is impossible to borrow data from the meteorological stations in adjacent areas，special meteorological observation stations shall be established according to engineering requirements to observe icing，wind and other meteorological elements.

3.4.4　The design reoccurrence interval for protection against strong wind and icing shall be 30 years for 330kV overhead transmission lines and 50 years for 500kV-750kV lines.Authorities having jurisdiction may also determine the criteria of design reoccurrence interval based on the importance of lines.However，such criteria shall not be lower than those specified in this code.

3.4.5　Icing areas of overhead transmission lines should be divided into the following three categories:Light icing area（the design ice thickness≤10mm），medium icing area（10mm＜the design ice thickness＜20mm），and heavy icing area（the design ice thickness≥20mm）.

3.4.6　Where the lines run through heavy icing areas，special study on icing shall be carried out.

3.4.7　For micro-terrain or microclimate heavy icing areas with complicated terrain and harsh climate，the calculated values shall be increased by 10% as safety correction.

3.4.8　Where an emergency event such as strong wind or icing occurs，field investigation shall be carried out in time to make further study and analysis on the design meteorological conditions，make correction as necessary，and propose proper engineering measures.