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. The object of this standard is – to define the terms used, – to prescribe a standard test procedure, – to prescribe criteria to evaluate the results of the tests. Power arc tests are not an obligatory element of line insulator specifications. The standard test procedures and the evaluation criteria described in this standard are intended to provide testing guidance when power arc tests are felt to be necessary. It is not the object of this standard to introduce a general obligation to execute these tests.