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Comparative Tracking Index

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The Comparative Tracking Index or CTI is used to measure the electrical breakdown (tracking) properties of an insulating material. To measure the tracking, 52 drops of 0.1% ammonium chloride solution are dropped on the material, and the voltage measured for a 3mm 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.

Performance Level Categories (PLC) were introduced to avoid excessive implied precision and bias.

Tracking Index (V) PLC
600 and Greater 0
400 through 599 1
250 through 399 2
175 through 249 3
100 through 174 4
< 100 5