Hi Doug:
If a partial discharge (ionized path), then only microamperes of 60 Hz current would flow until the voltage is zero. Then, the process will start with the next cycle if partial discharge conditions exist. If an arc, then 60 Hz current will flow until the voltage is zero (zero crossing). Then, the process will start with the next cycle if the arc exists. The current in the partial discharge or the arc will depend on the capacitance across the transformer or isolation barrier and the capacitance to ground or the connection to ground. Best regards, and happy holidays, Richard -----Original Message----- From: Doug Smith [mailto:[email protected]] Sent: Sunday, December 14, 2014 2:56 PM To: [email protected] Cc: [email protected] Subject: Re: [PSES] dielectric breakdown Hi Richard, There are many thousands of initiating events. The EFT test runs at 5 kHz to 100 kHz for 15 ms bursts every 300 ms for a minute. In real life, I have measured the EFT pulses coming in at 10 MHz, so the initiating events will extend for many cycles of the power and at all relative phase angles. If coming from a motor commutator, the EFT pulses would be coming continuously for a long period of time. I can regularly cause breakdown of various supplies that are UL certified using 800 Volt ESD or EFT, even lower to 500 Volts, and that is the generator setting. Less would be impressed across the barrier except for resonance. Obviously there is a resonance happening that increases the peak voltage and explains why some of the breakdowns are in the opposite current direction from the applied stress. So my question really is, given all these miniature breakdowns, within a cycle of 60 Hz, would the ionized path allow more 60 Hz current to flow even though there is not enough 60 Hz voltage itself to cause a breakdown? If so, a number of 60 Hz cycles would be involved, not just one, and in the case of a motor commutator, continuously for a long period of time. Doug University of Oxford Tutor Department for Continuing Education Oxford, Oxfordshire, United Kingdom -------------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 60941 ========= Boulder City, NV 89006-0941 _ / \ / \ _ TEL/FAX: 702-570-6108/570-6013 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: [email protected] \ _ / ] \ _ / Web: http://www.dsmith.org -------------------------------------------------------------- On 12/6/14 1:07 PM, Richard Nute wrote: > Hi Doug: > > > I assume you are assuming the initiating event is an air breakdown as opposed > to a solid insulation breakdown. > > Note that, according to Paschen's Law, air does not break down below about > 323 volts peak (at sea level). > > The mains voltage does not exceed 170 volts peak (for 120 volts r.m.s.). So, > when the initiating event is over, and the mains voltage goes through zero, > the arc is extinguished and cannot start again. > > For two-wire equipment, the current depends on the two capacitances to > ground. The first capacitor (transformer primary-to-secondary) breaks down, > and the second capacitor (secondary circuit to ground) controls the current. > > Safety standards specify minimum clearances so that breakdown due to > transient overvoltages is not likely. > > Hope this helps. > > > Best regards and happy holidays, > Rich > > > -----Original Message----- > From: [email protected] [mailto:[email protected]] > Sent: Saturday, December 06, 2014 10:01 AM > To: [email protected] > Subject: [PSES] dielectric breakdown > > Hi Everyone, > > Is it possible that a fast breakdown of the dielectric barrier, by a fast > event, like ESD or EFT, in a two wire mains power supply, cause a larger > follow-on current from the AC mains and therefore a safety problem. How about > follow-on current from a simultaneous lightning surge that would not by > itself cause a breakdown? Anyone know of an incident of either type? I could > set up an experiment but don't want to set my lab bench on fire. -:) > > Doug > > - > ---------------------------------------------------------------- > 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]> > > - ---------------------------------------------------------------- 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]>
