Hello Joe: Note 3 of table 6 refers to IEC 112 for the method of testing CTI to determine material groups for this table. CTI refers to "Comparative Tracking Index" which is a measure of how well a material resists tracking. What happens, is, given a high enough potential applied for enough time across a surface of a plastic, the material breaks down on the surface creating a conductive carbon path. This is called tracking, and is the reason for the creapage distance requirements, and why the required creepage distance varies with the working voltage. The longer the distance, of course, the lower the strength of the electric field for a given voltage. If one material resists tracking better than another, this creepage distance can be lowered for the better (material group 1 is better than material group three) material.
Pollution (grime, ionic contamination, etc.) also aids in breaking down a material, which is why "Pollution degree" is addressed in Table 18 also. UL has tested materials for CTI using a method very similar to the IEC method (UL has 5 categories of material group instead of three), and in the vast majority of cases, end up with the same rating as the IEC method for the same material. Look in the recognized component directory. Your board supplier, I would imagine, may be able to give you some guidance. Hope this helps, (the above is to the best of my knowledge, and not necessarily my employers opinion) Mel Pedersen Midcom, Inc. Homologations Engineer Phone: (605) 882-8535 [email protected] Fax: (605) 886-6752 ---------- From: Joe Finlayson[SMTP:[email protected]] Sent: Thursday, May 08, 1997 2:01 PM To: [email protected] Subject: IEC 950 / UL 1950 creepage and clearance requirements To any and all safety gurus, I am in the midst of trying to minimize the creepage and clearance requirements between TNV and SELV called out in IEC 950. My interpretation of tables 5 and 6 in sections 2.9.2. and 2.9.3 are
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