This International Standard describes a method of test for comparing the performance of insulating materials, when they are exposed intermittently to a liquid contaminant and then to an a.c. electric stress along their su rface. It should be realized that the test was developed to compare the resistance to tracking and erosion of materials belonging to one specific family, so that comparisons between results in different families may not be fully valid. The test apparatus uses a rotating wheel on which standard' ) rod- or tube-shaped specimens, fitted with electrodes at each end, are located around the periphery of the wheel. The wheel, mounted at a small angle to the horizontal, is rotated slowly so that the test specimens are repeatedly dipped in a specified salt-water solution (contaminant) and then withdrawn. After withdrawal, the solution is allowed to drain or drip from the surface of the test specimen for a short time before a specified voltage is applied. The voltage causes electrical discharges (tiny arcs, scintillations) across dry areas which develop on the su rface of the specimens. These discharges may cause surface degradation to progress; until flashover occurs, or the leakage current exceeds a specified value 2) . The time to flashover, or to exceed the specified leakage current, and observation of the nature and severity of degradation are the criteria used to compare materials. The test conditions are such that materials suitable for use out of doors normally fail within a few thousand hours. IEC 112 and IEC 587 (see annex B) describe tests of much shorter duration, on smaller samples, to assess the ability of materials to withstand electric stress when exposed to liquid contamination on one surface. These tests may rate materials differently from tests of longer duration. Salt-fog tests (not yet standardized by IEC) are also in use. Such tests permit comparison of materials in the form in which they will be used in service. The test results are then dependent both on the materials used and the design of the test specimens. Such tests may also evaluate materials differently.