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[FAQ] What is Comparative Tracking Index (CTI)?

What is Comparative Tracking Index (CTI)?

  • Please note that all definitions and references are based on the latest standard IEC 60747-17


    CTI – Index used to define the resistance of a material to tracking.

    From which certification/standard is this specification derived:

    IEC 60747-17 which refers to IEC 60112 to define CTI (IEC 60112 is the Method for the determination of the proof and the comparative tracking indices of solid insulating materials).

    How it is specified in TI datasheets:

    How it is validated:

    The CTI of an isolator, or any other device/material, is determined by applying a test voltage to the material, while adding 50 drops of 0.1% ammonium chloride solution to the surface. If tracking is observed, the test voltage is marked as a fail and the test will be repeated with a lower test voltage until a passing voltage is found.

    Why does this specification matter and how is it relevant to isolated systems?

    Tracking is the process that produces a partially conducting path of localized deterioration on or through the surface of an insulating material as a result of the action of electric discharges on or close to an insulation surface.

    As high voltages are applied to an isolator, electric discharges close to the surface of the package can cause tracking of the mold compound. This deterioration creates a conductive path across the isolator and is often referred to as tracking. How susceptible an isolator is to tracking is dependent on the chosen material. If a large amount of tracking occurs on the surface of an isolator, the isolator can fail, potentially allowing high voltages to pass through the system. It is important that the CTI is considered when designing a system with isolation.


    Does CTI influence isolation?

    CTI does not directly influence the isolation rating of an isolator. However, the CTI is used to determine the isolator’s material group, which is used to define the minimum required creepage to achieve basic or reinforced isolation for a given working voltage.

    Refer to the appropriate standards for your application to determine the required creepage for your chosen isolator’s material group.

    Other notes:

    Due to the relationship between CTI and creepage, an isolator with a higher CTI could support a given working voltage with a smaller package than another isolator with a lower CTI.