Tracking resistance
The creepage resistance characterizes the insulation strength of the surface (creepage distance) of insulating materials , especially when exposed to moisture and contamination. It defines the maximum leakage current that can be set under standardized test conditions (specified voltage, conductive layer material) in a defined test arrangement (electrode spacing, electrode shape).
General
A high creepage resistance means that measurable currents only arise on the surface of the test body when a correspondingly high voltage is applied.
In the manufacture of printed circuit boards and in their design , where the distances between the structural elements can only be a few 10 µm, a high tracking resistance of the substrate is of great importance.
Insulating materials for protective insulation and in the case of high voltage must have a high creepage 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 lower than that of an air distance of the same length, even if the insulating material itself insulates well.
The creepage resistance of an insulating material is influenced by its water absorption capacity and its behavior under thermal and ionizing ( pre-discharges ) stress.
Determination of the tracking resistance
The tracking resistance is determined with the CTI value ( English Comparative Tracking Index ) and checked with the PTI value ( English Proof Tracking Index ). The CTI value indicates the voltage up to which, measured in volts, the base material shows no tracking (base material becomes conductive under voltage) if 50 drops of standardized electrolyte solutions (A or B, corresponding to KA or KB value) are applied. Measurements are made on the etched surface, with a drop falling between two platinum electrodes every 30 seconds. The failure criterion is a leakage current of> 0.5 A. Details on the measurement method for the CTI value are regulated in IEC 60112.
Typical CTI values for some materials:
- Phenolic resin : 125
- Polyimide , Kapton: 150
- FR4 ( printed circuit board base material, glass fiber reinforced epoxy resin ): ≥ 175/250
- FR4 type KF: 400
- PE-LD, PE-HD ( polyethylene ): 600
- Polyester resin : 600
- PTFE ( polytetrafluoroethylene ): 600
- PBT ( polybutylene terephthalate ): 500
The CTI value is only standardized for voltages up to 600 V. For higher voltages, there is also the option of testing high-voltage leakage current resistance (so-called IPT value from Inclined Plane Tracking ).
Norms
- DIN EN 60112: 2010; VDE 0303-11: 2010: Procedure for determining the check number and the comparative number of the creepage path formation of solid, insulating materials
- DIN EN 60587: 2008; VDE 0303-10: 2008: Electrical insulation materials that are used under difficult conditions - test method for determining the resistance to tracking and erosion
- DIN EN 61302: 1996; VDE 0303-12: 1996: Electrical insulating materials - Test method for assessing the resistance to tracking and erosion - Cyclical testing
Relation to insulation groups
The standards EN 50124 / DIN EN 60664-1 (VDE 0110-1) establish a relationship between groups of insulation materials and the CTI value:
- Insulation group I: 600 ≤ CTI
- Insulation group II: 400 ≤ CTI <600
- Insulation group IIIa: 175 ≤ CTI <400 (FR4)
- Insulation group IIIb: 100 ≤ CTI <175
See also
Literature / individual references
- High-voltage technology , script from the Institute for High-Voltage Technology and System Management; TU Graz, 2005, 125 pages