The paper touches upon the phenomenon of absorption of moisture by insulating materials and systems and their degradation. The paper examines in detail the phenomenon of absorption current underlying standard recommendations for measurement of insulation resistance and related to assess the condition of rotating electrical machines. It is brought out that during the process of humidification, fast decaying components appear in the absorption current. As a result, the rate of decay of this current increases. Hence to assess the state of humidification and to eliminate likely ambiguities due to extraneous conditions experienced in standard practices, these measurements are proposed during the discharge of a charged system at a short time interval. Further, coefficient of state of insulation is proposed. Data for this coefficient on large rotating electrical machines is presented. It is felt that the coefficient can serve as a better index of inherent characteristics of insulation and state of humidification.
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