One of the items that comes up regularly when reviewing the performance of materials is the resistance to arc tracking. This may be one of the properties of polymers and plastics that have the largest number of tests methods for evaluation.
No different in this regard is the ASTM D3638 test method for comparative tracking index.
While not frequently called out for on aerospace materials, this test nonetheless is another method for examining this property.
What makes the D3638 test method different is than other methods (such as the ASTM D495) is that this is performed at a relatively low voltage (up to 600V) with the addition of a contaminate.
In this test, a contaminate liquid is slowly dripped between two electrodes on the surface of the material. By adding this contamination, electrical conduction between the two electrodes is started and carbonization of the polymer slowly occurs. Once the carbonization occurs and the electrical current exceeds the threshold set on the test, the test is halted, and the voltage is decreased. This continues until sufficient amount of data is gathered to interpolate the number of contaminate drops necessary to achieve the electrical current threshold.
When the test is performed, the test is typically started at the maximum of test voltage range and, after each failure, the voltage is decreased and a test is performed on a new sample. The best fit curve is developed and the voltage necessary to cause a failure after 50 drops is calculated. This is called the comparative tracking index. An example of the CTI is shown in the chart below.
The results from the tests provide a means to gather comparative data on two or more material types. Those with a higher CTI can be considered as more arc track resistant, and are less likely to arc track than those materials with lower CTI values.