This part of IEC 60383 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. lt 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 IEC 60168: Tests on indoor and outdoor post insulators of ceramic material or glass for systems with nominal voltages greater than 1 000 V. Tests on insulator strings and insulator sets (for example, wet switching impulse voltage) are dealt with in part 2 of IEC 60383. The object of this part is: to define the terms used to define insulator characteristics and to prescribe the conditions under which the specified values of these characteristics shall be verified to prescribe test methods to prescribe acceptance criteria. This part does not include requirements dealing with the choice of insulators for specific operating conditions. Specific requirements on the use of coatings on ceramic or glass insulators are described in the informative Annex C. NOTE A guide for the choice of insulators under polluted conditions has been published, see IEC 60815-1 and -2. Numerical values for insulator characteristics are specified in IEC 60305, IEC 60433 and IEC 60720.