Rick, In past years I spent significant time researching this question. I also found that most materials manufacturers do not understand CTI. Even though the concept has been known for more than 50 years.
As far as standards go I put this table together simply to help correlate the International, European and UL standards: VOLTAGE UL 746 IEC 112 VDE 303 600 or greater 0 I I 400 - 599 1 II II 250 - 399 2 IIIa IIIa 175 - 249 3 IIIa IIIa 100 - 174 4 IIIb III b 0 - 99 5 n/a n/a (If the columns do not line up, use tabs to delimit) If you have the UL yellow Recognized Component directory, you should be able to find the UL CTI info at the beginning of the plastics section. It is on page 1893 in my 1994 version. The 205 figure that you mention does not look like a CTI number, rather a thermal performance number. FR-4 "typically" is greater than 180C in thermal performance and the flame rating is UL 94V-0. CTI is a function of the organic content of the material, material that is able to be carbonized. For example anorganic materials such as glass and ceramics always have the highest possible CTI ratings and essentially do not track. If you will note, when using FR-4 for PCB's most standards including IEC950 and EN 60950 make a special case and provide a separate spacing table. The creepage numbers generally correlate directly to equivalent CTI of 600V. If you are using the FR-4 for something other than PCB's you will need to get exact information from the laminate manufacturer. Try to get past the sales people and you may still need to educate technical types as to what CTI is before you can get meaningful information. Chances are most all FR-4 laminates are CTI 600V, depending on glass content.. Doug Powell Advanced Energy Industries, Inc. Fort Collins, Colorado USA ---------- From: Rick Busche To: [email protected] Subject: Comparative Tracking Index List-Post: [email protected] Date: Thursday, February 19, 1998 8:26PM IEC 950 defines CTI (comparative tracking index) in terms of material groups for the determination of creapage distance. Specifically groups I, II, IIIa and IIIb. are mentioned. In paragraph 2.9.3 (note 3) the actual index (CTI) is referred to as <600 or <400. My questions are as follows: 1. Is this CTI (as assigned by UL in the yellow book) actually a voltage level? 2. The FR4 laminate we use has been rated at 205. As such the maximum creapage distance is mandated. Is it common to find better material? Is this 205 level typical of common FR4 materials? Thanks Rick Busche Evans & Sutherland [email protected]
