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The "standard" dust-fog test methods (Table I), although complex and time consuming, give sufficient repeatability of results (see appendix) to pennit comparative evaluation of the track resistance of materials over a very wide quality range (0.4-3,000 hours). This is apparently a much greater quality range than can be measured with the best drop-test methods standardized elsewhere. 4,12 A revrised form of the dust-fog test provides a much greater rate of degradation of enhancing tracking tendencies; also, the rate of degradation is less time-dependent compared with the "standard" tests. Both tests give results that may be correlated with the chemical structure and filler content of polymers; also both tests give results that correlate with the results of ASTM D495 arc-resistance tests for many unfilled and hydrated mineral-filled polymers. The D495 test may be employed to detect some wet-track susceptible materials and to predict the optimum contents of hydrated fillers used to improve the wet-track resistance of many types of polymers that do not contain too high a proportion of phenyl rings, chlorine together with hydrogen, or amine and amide groups in their structures. A short-time screening test is still needed for materials for which the dust-fog life is greater than 10 hours (International Electrochemical Commission tracking indexes greater than 1,000 volts). The use of diagnostic techniques (pH indicators, etc.) is informative.