IEC Technical Specification 62271-304 Revised
IEC TS 62271-304:2008/COR1:2010
Corrigendum 1 - High-voltage switchgear and controlgear - Part 304: Design classes for indoor enclosed switchgear and controlgear for rated voltages above 1 kV up to and including 52 kV to be used in severe climatic conditions
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A more recent version of this publication exists:
IEC TS 62271-304:2019