Circuit board glossary

Item: Leakage current resistance

Description:

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 rangePLC group
>600V0
400V - 599V   1
250V - 399V 2
175V - 249V 3
100V - 174V 4
<100V5

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
>= 600VI
400V - 599V   II
175V - 399V IIIa
100V - 174V IIIb