Defines terms and insulator characteristics and prescribes conditions under which specified values of these characteristics shall be verified. Also prescribes test methods and acceptance criteria. Applies to string insulator units, insulator strings, insulator sets and rigid overhead line insulators and to insulators of similar design when used in substations. Also applies to insulators for use on d.c. overhead electric traction lines. May be regarded as a provisional standard for insulators for use on d.c. overhead power lines. Publication 438 gives general guidance for those insulators. It does not apply to insulators forming parts of electrical apparatus or to parts used in their construction or to post insulators which are covered by Publication 168.