In general, ok. And this is what the OP should be able to discern. WV is deceptively simple. The sqrrt(2) factor should be relied upon to only to provide the equivalent DC test voltage, not in calculating WV. In my world of power conversion there are some very hi crest factors, and some equipment environments depend too much on the 'standard ' 2500V impulse (OV II, 240 mains) for determination of test level and spacing. And as a cable is external to equipment, there could be many other considerations for spacing and materials selection. Finally, the mfr has to be able to provide complete conditions of acceptability for the equipment that his 1120V cable is connected to.
BTW, did Mr. Perkins or Mr. Nute do some seminars for this year's ISPCE? They typically talk about this stuff. Brian From: Doug Powell [mailto:[email protected]] Sent: Monday, May 12, 2014 11:15 AM To: Brian Oconnell Cc: emc-pstc Subject: Re: [PSES] Creepage and RMS Something to keep in mind, Peak Voltages (spikes, impulses, flyback, laser ignition, etc.) are more a concern for clearances than for creepage. Committees build creepage tables in slightly different ways and they sometimes ignore the relationships of Vrms, Vpk and Vdc which are often related by 1.414 and so on. On the clearance tables this is a somewhat less common practice, especially in secondary circuits where interpolation is allowed. One of the main keys is if you make allowance for high peak voltage values in clearance, the rule "creepage cannot be less than clearance" applies. On occasion, creepage calculations will come in lower than clearance in which case the creepage number must be automatically bumped up to match the clearance. thanks, -doug Douglas E Powell http://www.linkedin.com/in/dougp01 On Mon, May 12, 2014 at 11:45 AM, Brian Oconnell <[email protected]> wrote: The issue with some 60664 tables is that it is (for 0 to 2km) based on Paschen's Law, which is for a simple homogenous e-field. So creepage via RMS is an incomplete analysis. As others have stated, spacing should be determined by both peak and RMS. And because too many do not measure WV correctly, I strongly suggest reading papers and articles by P. Perkins and R. Nute on this subject, and "The physical fundamentals of low-voltage insulation co-ordination" by Klaus Stimper. In a very nutty shell, you need to determine the following for the intended end-use environment: - OV category - CTI of materials - pollution degree - atmospheric chemistry - frequency-based di-electric heating - WV for the complete range of rated operating conditions Have rejected more than one component mfr's CB reports due to incorrect interpretations of IEC60664 and calculations of 'RMS' (am looking at you people in NRTLs/SCCs/NBs). Finally, the OP indicated that "Creepage is based on the RMS value of the voltage so is based on 1120 V", which indicates a reference a particular safety standard. What does that standard say about WV measurements for the determination of spacing and test voltages ? Brian ----- Original Message ----- From: ce-test, qualified testing bv - Gert Gremmen Sent: 05/12/14 03:20 PM To: [email protected] Subject: [PSES] Creepage and RMS I have a question on creepage distances. A product creates a saw tooth High voltage of 2000 Vpeak The VRMS measured on the scope on a full period equals 1120 V Creepage is based on the RMS value of the voltage so is based on 1120 V. Now the frequency of the sawtooth is 3 Hz. Should I still consider the RMS value, and at what frequency should I consider this as a DC value in regard to creepage.... Anyone aware of the IEC 60664 standard in respect to low frequency signals ?? Regards, Ing. Gert Gremmen, BSc - ---------------------------------------------------------------- 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: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
