GB 775 specifies the methods by which insulators are tested in China. An insulator has to satisfy requirements in three different areas, and the standard addresses each of them: the electrical tests, which establish the withstand of the insulation under power frequency voltage in dry and wet conditions and under lightning and switching impulses, the mechanical tests, which determine the failing load of the string or post and the residual strength that remains after a mechanical damage test, and the environmental tests, which characterise the behaviour of the shed and the surface under pollution or under artificial rain. The document defines the test circuit, the arrangement of the specimen and the rate of rise of the applied voltage for each test, the criteria by which a puncture, a flashover and a mechanical failure are distinguished, and the number of specimens that must be tested to obtain a valid result. Correction factors for atmospheric conditions and instructions for recording and interpreting the results are included.