By moisture or impurities between two points on the, in principle insulating, PCB base material, a measurable current flow is measurable if a correspondingly high voltage is applied. This current flow is called leakage current / tracking. The resistance of the material against leakage current is given as the CTI (Comparative Tracking Index) value and refers to the highest voltage which does not produce a failure through leaking current.
The measuring method is described by Underwriters Laboratories: A a few drops electrolyte solution on etched base material and increasing testing voltage on two test electrodes. A leakage current of 0.5 A is considered as failure.
Standard FR4 material has a CTI of > 175V, extra leakage resistant variants even have a CTI of > 400V up to >600V.
The measured values for the base material are grouped in Performance Level Categories (PLC).
| CTI range | PLC group |
|---|---|
| >600V | 0 |
| 400V - 599V | 1 |
| 250V - 399V | 2 |
| 175V - 249V | 3 |
| 100V - 174V | 4 |
| <100V | 5 |
When designing products for medical equipment, the CTI is differently classified. Here, material groups according to EN 60601-1 are used:
| CTI range | Material group |
|---|---|
| >= 600V | I |
| 400V - 599V | II |
| 175V - 399V | IIIa |
| 100V - 174V | IIIb |
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