Leakage current resistance
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|
|400V - 599V||1|
|250V - 399V||2|
|175V - 249V||3|
|100V - 174V||4|
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|
|400V - 599V||II|
|175V - 399V||IIIa|
|100V - 174V||IIIb|