IEC 60112 specifies the method for determining the proof and comparative tracking indices of solid insulating materials. The CTI is the maximum voltage at which a material can withstand 50 drops of ammonium chloride solution without tracking failure.
The test places two electrodes on the flat surface of a specimen and drops dilute ammonium chloride electrolyte solution between the electrodes while applying a specified voltage. If carbonized tracks form between the electrodes causing current flow, the material has failed.
For the elevated temperature variant, the test is performed with the specimen heated to a specified temperature above ambient, simulating conditions where insulating materials operate at elevated working temperatures near heat sources or in high-current applications.