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 For inquiries, bookings, and testing updates please send us an email on [email protected] <mailto:[email protected]> or call 01274 911747. Our lead times for testing and consultancy are typically 4-5 weeks. From: Richard Nute <[email protected]> Sent: 29 September 2026 20:59 To: [email protected] Subject: Re: [PSES] Spacings “Clearance” is an unfortunate IEC term (description) of air insulation. As with solid insulation, air insulation electric strength is a function of distance through the air – the greater the distance, the greater the electric strength. The peak breakdown voltages of a 50 Hz sinewave, both inhomogeneous and homogeneous fields, as a function of distance, were measured by Hermstein and were published in ETZ-A in 1969. Pfieffer did the same for 1.2/50 impulse, published in ETZ-B in 1976. (The references are not available on the web.) This data was picked up by IEC SC28A and published in the Appendix to IEC 664, First Edition, published in 1980. Air breakdown is a time function of the applied voltage. The first step before a breakdown occurs is “ionization.” When a sufficient number of air molecules are ionized, breakdown occurs. Hence, there are two breakdown voltages for the same distance, one for the 50 Hz peak, the other for the 1.2/50 impulse. (The 1.2/50 impulse is the man-made version of a lightning impulse.) Paschen has shown that air does not break down, regardless of distance, below about 327 volts dc at sea level. Nevertheless, the various standards require air insulation (clearances) distances down to 30 volts. Where conductors emerge from solid insulation (e.g., an appliance coupler) or lie on solid insulation (e.g., printed wiring) or equivalent, the conductors at the surface of solid insulation are insulated from each other by both air insulation and solid insulation in parallel. Since air has lower electric strength than solid insulation, the distance between conductors along the surface of the solid insulation must be at least the air (clearance) dimension. See IEC 60664-1, 5.2.2.6. However, the surface of solid insulation can be contaminated by deposition of unknown materials, which comprise a third material in parallel with the air and solid insulations. These materials may compromise the electric strength of the surface of solid insulation. If the contamination has a lower electric strength than the air insulation, the contamination is likely to break down in micro-arcs. The micro-arcs heat the solid insulation and may cause pyrolysis of the solid insulation surface and thus, over time, reduce the electric strength of the solid insulation surface between the conductors. This deterioration is a long-term phenomenon. As such, the functional voltage, r.m.s. or DC, between conductors is the major cause of the surface-damaging micro-arcs. While transient voltages can also cause micro-arcs, transient voltages are very infrequent and therefore do little damage to the surface. See IEC60664-1, 5.2.2.2. See Stimper, “The Physical Fundamentals of Low-Voltage Insulation Co-ordination,” VDE 57. Solid insulation surface distance (creepage distance) requirements only apply to solid insulation surfaces that are subject to pollution. According to IEC 60664-1, 4.6.1: “Means may be provided to reduce pollution at the insulation under consideration by effective use of enclosures, encapsulation or hermetic sealing.” Solid insulation surfaces that are not subject to pollution are not subject to surface distance (creepage) requirements. Others (and some standards and some certification organizations) may say that all solid insulation surfaces must comply with surface (creepage) distance requirements. Richard Nute Bend, Oregon, USA Note: The IEEE is doing away with my [email protected] <mailto:[email protected]> e-mail forwarding address. My fall-back e-mail address is [email protected] <mailto:[email protected]> . (My thanks to those pointing out the typo in my fall-back e-mail address!) _____ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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