This International Standard is applicable to string insulator units of ceramic material or glass for use on d.c. overhead power lines with a nominal voltage greater than 1 000 V. It is not applicable to composite insulators. The application of this standard to insulators for use in other situations under d.c. voltage will be left to the relevant technical committees. This standard specifies: - the characteristics of insulators; - the conditions under which the specified values of these characteristics shall be verified; - acceptance criteria. It specifies basic characteristic tests in a manner similar to that of IEC 383-1. Certain additional tests have been found to be necessary to ensure satisfactory long-term performance of d.c. insulators. At present, these additional tests concern only cap and pin insulators. In view of the lack of significant performance data regarding long rod insulators on d.c. lines, it is not possible at present to define or validate special d.c.-oriented tests for such insulators. This standard does not include requirements, or special tests, dealing with the choice of insulators for specific operating conditions (e.g. ageing arising from specific operating conditions). NOTES 1 This standard does not include radio interference tests, artificial pollution tests or tests on insulator sets. These subjects and some relevant test methods are dealt with in the following IEC Publications: IEC 383-2, IEC 437 and IEC 1245. 2 Many of the requirements and tests in this standard are relevant to insulators for use on d.c. traction lines, but other tests may be required. Reference can be made to IEC 383-1.