Hi James:
CTI… from Stimper, VDE 67: “The International Electrical Commission defined the procedure IEC 112 [3] for determining the tracking resistance of low-voltage insulations. In this procedure the investigated materials are stressed with surface scintillation of increasing intensity and it is determined to which extent the material withstands without tracking. “The procedure uses two platinum electrodes in the distance of 4 mm on the insulation surface. Every thirty seconds a drop of NH4CI-solution is applied onto the inter-electrode surface of the insulation. As a measure for the tracking resistance the maximum voltage is determined at which the specimen withstands fifty drops during five subsequent tests. The definition of failure is that the surface current exceeds 0.5 A for more than two seconds or that the material burns. The fifty-drop-voltage is called 'comparative tracking index' and is recorded with the unit "CTI" and not "V".” As near as I can tell, CTI only applies to organic materials, and not to inorganic materials. Organic materials can dissociate and become resistive when tested for CTI, but inorganic materials do not. I do not have the research for “material groups” which are used in IEC 62368 in conjunction with the CTI rating of the base insulating material. I suggest you study IEC 60112 for more information about determining CTI. Also, I suggest you look at Google “Comparative Tracking Index” for a good summary of the subject. Best regards, Richard Nute [email protected] <mailto:[email protected]> From: James Pawson (U3C) <[email protected]> Sent: Wednesday, September 30, 2026 12:46 AM To: [email protected] Subject: Re: [PSES] Spacings Hello Richard, Thank you for the interesting background, it helps with my understanding of the “why” 😊 Where does the Comparative Tracking Index (CTI) of the material come into play? Is this a measure of the pyrolysis characteristics of the material i.e. how easily the non-insulating components of the insulating material can be separated and turned into conductive materials? Or is there another force at work? Thanks and all the best James ( <https://www.ggtemc.co.uk/esd/> shameless plug for my new book) James Pawson Managing Director & EMC Problem Solver Unit 3 Compliance Ltd EMC : Environmental & Vibration : Electrical Safety : CE & UKCA : Consultancy <http://www.unit3compliance.co.uk/> www.unit3compliance.co.uk | <mailto:[email protected]> [email protected] +44(0)1274 911747 | +44(0)7811 139957 2 Wellington Business Park, New Lane, Bradford, BD4 8AL Registered in England and Wales # 10574298 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected] All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://pses.ieee.org/wp-content/uploads/2024/12/EM-PSTC-List-Rules.pdf For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
