This International Standard applies to insulator strings and sets comprising string insulator units of ceramic material, glass or composite material for use on a.c. overhead lines and traction lines with a nominal voltage above 1 000 V and a frequency between 15 Hz and 100 Hz. This standard also applies to insulator strings or sets of similar design used in substations. This standard establishes a standard test procedure for power arc tests on insulator sets. It also establishes a standard test procedure for power arc tests on short strings. This standard does not apply to insulator sets mounted on non-metallic poles or towers. This standard cannot be directly applied to line post insulators or sets, or to insulating structures such as braced line-posts, since their mounting arrangement cannot be reproduced by the standard arrangements as described herein. However, this standard can be used as a basis for agreement for tests on such insulators and arrangements.