Hi Ron:
You ask some good questions. I should use these for my next "Technically Speaking" column in the PSEN. Insulation resistance and dielectric strength are two different parameters of insulation. Insulation resistance is just that: the resistance, in ohms (or teraohms) of the installed insulation. Dielectric strength is the maximum voltage the insulation will withstand without breakdown, usually in kV (for the installed insulation) or kV/mm (for the raw insulation). Both parameters are directly proportional to the distance through the insulation. The greater the insulation thickness, the greater the insulation resistance and the greater the dielectric strength. Neither parameter can be substituted for the other parameter. As for history of insulation resistance testing as a measure of safety, I can only guess. Fifty years ago or more, paper was the principle insulation material in transformers, motors, etc. My guess is that insulation resistance of paper insulations was a valid measure of the quality and reliability of the installed insulation. Those insulations measured in the range of 100s of megohms to 100s of gigaohms. And, they tended to be hygroscopic, which could be measured by the change in insulation resistance between low humidity and high humidity. Today's insulations measure in the range of 100s of gigaohms and up. These materials are such very good insulators that insulation resistance need not be measured. And, they are not hygroscopic. For safety purposes, a dielectric strength test is not better than an insulation resistance test, and vice-versa. They are largely independent parameters of insulation. For example, an insulation may have a very high insulation resistance, but a very low dielectric strength. Air is an example of such an insulation. And, an insulation may have a very low insulation resistance and a high dielectric strength. When I worked for the oscilloscope company, the crt cathode operated at -1000 V dc in a very high- impedance circuit. We had difficulty finding insulating materials with sufficiently high insulation resistance for this application. Another example of extremely high insulation resistance (but not necessarily high electric strength) is that used in the construction of ionization chambers. A primary circuit dielectric withstand test is to show that the safety insulations have adequate electric strength to withstand the normally- occurring transient voltages that appear on the mains. (Safety standards fail to state this as a requirement.) So, the insulation resistance test and the dielectric strength test are independent tests and one cannot be used in lieu of the other. And, I can only guess at the history of why insulation resistance was included in safety standards. Here's a quiz for you: What does DCC mean when referring to wire? Best regards, Rich > -----Original Message----- > From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf > Of Ronald R. Wellman > Sent: Tuesday, July 10, 2007 4:49 PM > To: ri...@ieee.org; emc-p...@ieee.org > Subject: DC Insulation Resistance Testing > > > Technically speaking, since the all wise Rich Nute has > provided us with a history of the "on" and "off" mains power > switch markings, I was wondering if anyone knows the history > behind DC Insulation Resistance Testing and why it continues > to be performed in Europe and Asia when a Dielectric > Withstand test is a better test to indicate insulation failure. > > Best regards, > Ron Wellman > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
