Deals with basic, supplementary and reinforced insulation subjected to high-frequency voltage stress within low-voltage equipment. The dimensioning values directly apply for basic insulation; for reinforced insulation additional requirements apply according to Part 1. It is applicable for the dimensioning of clearances, creepage distances and solid insulation stressed by any type of periodic voltages with a fundamental frequency above 30 kHz and up to 10 MHz. This part of IEC 60664 can only be used together with IEC 60664-1 or with IEC 60664-5 (in this standard called Part 1 or Part 5). By using Part 1 or Part 5 together with this part the frequency limit of Part 1 or Part 5 is extended to frequencies higher than 30 kHz. This part also applies to Part 3 for frequencies greater than 30 kHz and protection of type 1. For type 2 protection this question is under consideration. Applies to equipment for use up to 2 000 m above sea level having a rated voltage up to a.c. 1 000 V. Specifies the requirements for clearances, creepage distances and solid insulation for equipment based upon their performance criteria. lt includes methods of electric testing with respect to insulation coordination. The minimum clearances specified in this part do not apply where ionized gases occur. Special requirements for such situations may be specified at the discretion of the relevant technical committee. This part does not deal with distances - through liquid insulation, - through gases other than air, - through compressed air. The object of this standard is to guide technical committees responsible for different equipment in order to rationalise their requirements so that insulation coordination is achieved when specifying clearances in air, creepage distances and solid insulation for equipment. The major changes made during the revision of IEC 60664-4 were the following: - inclusion of more recent information about the withstand characteristics of insulation at high-frequency voltage stress (see Annexes A, B and C); - inclusion of requirements for the dimensioning of clearances at high-frequency voltage stress (see Clause 4); - inclusion of requirements for the dimensioning of creepage distances at high-frequency voltage stress (see Clause 5); - inclusion of requirements for the dimensioning of solid insulation at high-frequency voltage stress (see Clause 6); - inclusion of diagrams to provide guidance on dimensioning with respect to high-frequency voltage stress (see Annex F); specification of tests with respect to high-frequency voltage stress (see Clause 7). - inclusion of test circuits for high-frequency voltage withstand testing and partial discharge testing (see Annex D.1 and D.2.1); - inclusion of design criteria for partial discharge test circuits at high-frequency voltage (see Annex D.2.2); - inclusion of criteria for dealing with non sinusoidal voltage stress (see Clause 8 and Annex E). Has the status of a basic safety publication in accordance with IEC Guide 104.