Comparative Tracking Index (CTI) is used to measure the electrical breakdown properties of the insulating material used in PCBs. It is primarily used to assess electrical safety especially for human users of electrical devices. PCB material manufacturers will provide these details in their material specifications sheet. The better the insulation, the higher the CTI (positive relationship).
The CTI value is used for electrical safety assessment of electrical apparatus, as for instance carried out by Underwriters Laboratory in USA and many other laboratories in the world. Performance Level Categories (PLC) were introduced to avoid excessive implied precision and bias.
| Tracking Index (V) | PLC |
| 600 and Greater | 0 |
| 400 through 599 | 1 |
| 250 through 399 | 2 |
| 175 through 249 | 3 |
| 100 through 174 | 4 |
| < 100 | 5 |
In design of medical products, the CTI is treated differently. Material groups are classified as shown below, per IEC 60601-1:2005, International Standard published by the International Electrotechnical Commission (IEC):
| Comparative Tracking Index (CTI) | Material Group |
| 600 ≤ CTI | I |
| 400 ≤ CTI < 600 | II |
| 175 ≤ CTI < 400 | IIIa |
| 100 ≤ CTI < 175 | IIIb |