Comparative Tracking Index (CTI) is the extent to which the insulating material of the PCB can resist the unwanted current to flow between traces on the board. It is a measure of the ability of the PCB substrate to withstand any breakdown between two tracks on the PCB surface. It is used to assess the proportionate resistance of the PCB  in PCB manufacturer China base material and helps calculate the high-voltage isolation values between the tracks. The CTI / PTI value indicates how resistant the PCB material is against environmental influences such as dirt and moisture. The higher the value, the more resistant the material is. The default CTI value for FR4 is 175 and goes up to 600 on special materials. The CTI / PTI value is determined using standard test methods.

Tracking resistance is the resistance offered by the PCB  in PCB manufacturer China trace against the current flow. The higher the number of components placed on the PCB board the higher is its overall packing density, this leads to smaller trace spacings. Therefore, the CTI / PTI value in the PCB base material is of prime importance. These values help to deduce the overall PCB tracking resistance.

These values give an estimation for leakage or short-circuit as trace spacing becomes small due to the increased number of components. As per the IEC standard, the smaller CTI grade substrate offers good resistance to the electrical breakdown. CTI is a critical number while selecting a PCB material. Normal FR-4 ranges from 175 to 225V. IEC has prescribed three of the following classes for PCB CTI calculation: