The Use of Copper Electrodes for the Comparative Tracking Index Test

If failure of electrical insulation occurs after a product has been in service for a considerable time, the mechanism involved is usually that of a carbonaceous track which develops between electrodes under conditions of intermittant moisture. A number associated with specific insulations and designed to rank insulations in order of their ability to avoid tracking is known as the comparative tracking index (CTI). There is little doubt as to the importance of the test that establishes the CTI value but there is much concern about its accuracy. This paper proposes modifications of the test equipment in an attempt to make the test more closely represent the conditions in the field and to make determination of the CTI value less subject to individual interpretation of the data. The proposed changes have been tested by laboratories in this country and Europe.