Comparative Tracking Index

The tracking index indicates the insulation resistance of the surface ( Creepage ) of insulating materials, particularly under the influence of moisture and contaminants. It defines the maximum leakage current under standard test conditions ( predetermined voltage Leitschichtmaterial ) must adjust in a defined test set (distance of electrodes, electrode shape ) itself.

General

A high tracking resistance means that a measurable current produced on the surface of the specimen only when a correspondingly high voltage.

In the printed circuit board manufacturing and designing them, where the distances between the structural elements can be only a few 10 microns, is a high tracking resistance of the substrate is of great importance.

Insulating materials for insulation and protection for high voltage must have high tracking resistance, the creepage distances (distance between the live parts along the surface of the insulating material ) must not fall below certain values ​​. The dielectric strength of a creepage distance is often less than that of an equally long distance in air, even if the insulating material itself well insulated.

Creep resistance of an insulating material is influenced by its water absorption capacity and its behavior under thermal and ionizing ( predischarges ) stress.

Determination of Resistance to tracking

Creep is the CTI value (English Comparative Tracking Index ) is determined and checked with the PTI value (English Proof tracking index). The CTI value indicates up to what voltage the base material will not be tracked (base material becomes conductive under voltage ) when 50 drops of standardized electrolyte solutions (A or B, accordingly KA or KB value ) are dripped. Is measured on the etched-away surface, every 30 seconds a drop falls between two platinum electrodes. Failure criterion is a leakage of> 0.5 A. Details of the measurement method of the CTI value are included in IEC 60112 regulated.

Typical CTI values ​​of some materials:

  • Phenolic resin: 125
  • Polyimide Kapton: 150
  • FR4 ( PCB base material, glass fiber- reinforced epoxy resin ): ≥ 175/250
  • FR4 type KF: 400
  • LDPE, HDPE (polyethylene ): 600
  • Polyester resin: 600
  • PTFE ( polytetrafluoroethylene ): 600

The CTI value is normalized only for voltages up to 600V. For higher voltages, there is also the possibility of testing a Hochspannungskriechstromfestigkeit (so-called IPT value of closely. Inclined Plane Tracking).

Related to material groups

The standard EN50124 establishes a connection between material groups and the CTI value here:

  • Insulating material group I CTI> 600
  • Material group II 400 < CTI < 600
  • Insulating material group IIIa 175 < CTI < 400 ( FR4)
  • Insulating material group IIIb 100 < CTI < 175
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