This part of IEC 383 applies to insulators of ceramic material or glass for use on a.c. overhead power lines and overhead traction lines with a nominal voltage greater than 1 000 V and a frequency not greater than 100 Hz. It also applies to insulators for use on d.c. overhead electric traction lines. This part applies to string insulator units, rigid overhead line insulators and to insulators of similar design when used in substations. It does not apply to insulators forming pa rts of electrical apparatus or to pa rts used in their construction or to post insulators which are covered by IEC 168: Tests on indoor and outdoor post insulators of ceramic material or glass for systems with nominal voltages greater than 1 000 V. It may be regarded as a provisional standard for insulators for use on d.c. overhead power lines. IEC 438: Tests and dimensions for high-voltage d.c. insulators, gives general guidance for those insulators. NOTE - An international standard dealing with tests on insulators for d.c. overhead lines is in preparation and is intended to replace the relevant clauses of IEC 438. Tests on insulator strings and insulator sets (for example, wet switching impulse voltage) are dealt with in part 2 of IEC 383. NOTES 1 This part does not include artificial pollution tests, radio-interference tests or residual strength tests. These subjects and relevant test methods are dealt with in the following IEC repo rts: IEC 437: Radio interference test on high-voltage insulators IEC 507: Artificial pollution tests on high-voltage insulators to be used on a.c. systems IEC 797: Residual strength of string insulator units of glass or ceramic material for overhead lines after mechanical damage of the dielectric. 2 The term "ceramic material" is used in this part to refer to porcelain materials and, contrary to North American practice, does not include glasses.