According to IEC60950, creepages distance shall be so determined that , for given RMS working voltage and pollution degree ,no flash over or breakdown of insulation (for example ,due to tracking ) will occur .
So . material groups depend on the comparative tracking index (CTI) and are classified as follows:
Material Group I: CTI ≥ 600
Material Group Ⅱ 400 ≤ CTI 〈 600
Material Group Illa 175 ≤ CTI 〈 400
Material Group Illb 100 ≤ CTI 〈 175
The material group is verified by evaluation of the test date for the material according to IEC60112 using 50 drops of solution A .
Zhilitong Electromechanical Co., Ltd manufactures tracking index tester ZLT-LDQ1 which is complying with IEC60950 standard.