The Comparative Tracking Index or CTI is used to measure the electrical breakdown (tracking) properties of an insulating material. To measure the tracking, 50 drops of 0.1% ammonium chloride solution are dropped on the material, and the voltage measured for a 3 mm thickness is considered representative of the material performance. Also term PTI (Proof Tracking Index) is used: it means voltage at which during testing on five samples the samples pass the test with no failures.
Tracking is an electrical breakdown on the surface of an insulating material. A large voltage difference gradually creates a conductive leakage path across the surface of the material by forming a carbonized track. Testing method is specified in IEC standard 60112.
Performance Level Categories (PLC) were introduced to avoid excessive implied precision and bias.
The CTI value is used for electrical safety assessment of electrical apparatus, as for instance carried out by Underwriters Laboratory in USA and many other laboratories in the world. The minimum required creepage distances over an insulating material between electrically conducting parts in apparatus, especially between parts with a high voltage and parts that can be touched by human users, is dependent on the insulators CTI value. Also for internal distances in an apparatus by maintaining CTI based distances, the risk of fire is reduced.
In design of medical products, the CTI is treated differently. Material groups are classified as shown below, per IEC 60601-1:2005, International Standard published by the International Electrotechnical Commission: