Comparative tracking index (CTI)
Bulk insulation and surface tracking are different paths
- Permittivity
The dielectric changes the relationship between electric field and stored charge. Permittivity is not a dielectric breakdown rating.
- Dielectric strength
Breakdown crosses the material under a defined electric stress. State thickness, electrodes, conditioning and test procedure.
- Insulation resistance
A defined voltage produces a measured leakage current. Temperature, moisture and time affect the measured resistance.
- Comparative tracking index (CTI)This term
Wet-contaminant testing establishes a comparative index. Surface tracking differs from breakdown through the insulating body.
- Proof tracking index (PTI)
The material is assessed at a specified proof condition. That result does not determine its complete comparative tracking index.
An electric field crosses an insulating body, while leakage and wet-contaminant tracking can follow its surface. Permittivity, dielectric strength and tracking indices describe different properties.
Conceptual electrical paths, not test circuits or voltage limits. Material condition, geometry and the applicable test method determine each result.