Pete, I have always 'lived' with increased end-of-life VDR leakage; and the MTBF calculation reflects this. But more recently, what has drawn my attention and raised my concern is that some VDR vendors have re-issued their reliability data, which may be indicative of increased vigilance for the requirements in the 2d edition of IEC60950-1 -> speculation on my part.
As for gas discharge tubes, at least for the few that I have tested, they are not as mechanically robust as the disk VDRs. I have never had a disk-type VDR fail during HALT. I have had gas discharge tubes fail during HALT. In general, my employer will design in a gas discharge device only for certain custom models. Brian > -----Original Message----- > From: [email protected] [mailto:[email protected]]On Behalf Of Pete > Perkins > Sent: Thursday, June 10, 2010 11:52 AM > To: 'Brian O'Connell'; [email protected] > Subject: RE: [PSES] VDR recommendations > > Brian, > > Historically it is well known that solid material based > VDRs exhibit > a leakage current that grows with time; the end point is a > large current > that destroys the device and (hopefully) blows the fuse > protecting the > circuit. > > Because of this increasing leakage current phenomena, > it has been > common practice to put a Gas Discharge Tube in series with > the VDT. The > GDT, of curse, does not conduct under normal operation thus > stopping the > normal VDR leakage current which eliminates this long-term > failure of the > VDR. When the GDT fires because of an impulse the VDR limits > the current > appropriately, as it is designed to do - else the GDT acts > as an ongoing > short circuit & blows the fuse. > > The GDT needs to be sized to not fire under the test > conditions to > be applied; the VDR needs to be sized to handle the impuse > fault current > then return the circuit to normal operation. > > There has been long-term controvosy over this belt& > suspenders (belt > & braces to our English friend) approach but it seems to > work quite well. > Users are happier when the equipment continues to run > normally rather than > have the fuse blow or the device die under, seemingly, > unseen & unknown > conditions. > > br, Pete > > Peter E Perkins, PE > Principal Product Safety & Regulatory Consultant > Tigard, ORe 97281-3427 > > 503/452-1201 fone/fax > [email protected] - 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.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
