In regard to Fred Townsend's comment: I am aware that diodes will normally fail shorted. However, even in this state, they provide some current limiting. The internal construction can only pass a given amount of current before the diode will open. I have, however, seen a few poorly designed circuits where there was no fuse on the input. The available fault current was high enough that after the diode shorted out from the overcurrent, it suffered from thermal damage and became an open circuit. Even when the diode fails shorted, there is some resistance as noted which will provide at least some current limiting.
In regards to Robert Johnson's comments: I work in an industry where we have numerous field wired ITE products. I have products that are on a 50 A branch circuit. The available fault current can be very high. The selection of fuse for the switch-mode power supply is critical. Even with a common mode choke providing some protection, I have seen fault currents over 200 A when there is a component short after the diode bridge. Some of the small, circuit board mounted fuses have an AIC rating of only 50 A and they do not fail gracefully under worse conditions. I agree that most ITE will not be exposed to high fault currents, but this is not always the case. Ted Eckert American Power Conversion/MGE http://www.apc.com/ The items contained in this e-mail reflect the personal opinions of the writer and are only provided for the assistance of the reader. The writer is not speaking in an official capacity for APC, MGE or Schneider Electric. The speaker does not represent APC's, MGE's or Schneider Electric's official position on any matter. Fred Townsend <fred@dctolight.n et> To Sent by: ted.eck...@apcc.com emc-p...@ieee.org cc emc-p...@ieee.org Subject 03/17/2007 01:55 Re: Electrolytic Capacitors in AM Primary Circuits See comments below. ted.eck...@apcc.com wrote: I have to agree with Mr. Woodgate that the components between the capacitor and the AC mains make a difference. The available fault current on the AC mains can be very high. It may be 1 kA, 10 kA or even more. A component failure due to a short circuit can be very dramatic. The rectifier alone will likely limit the fault current. If nothing else, the diodes will act as fuses. You could design a rectifier circuit that would allow a high fault current, but you would have to set out to do so and you would spend a lot of money in the process. Ted: I have seen literally thousands of diode failures, mostly from lightning damage. I have never seen a silicon power diode fuse (open). They always fail shorted at about two ohms. When they fail the surge limiting resistor becomes very important. That's where they sometimes use 'fusible resistors'. Shorted diodes often cause secondary failures instead of protecting anything. Fred Townsend DC to Light Besides, proper abnormal condition testing will involve simulating a short circuit on the electrolytic capacitor. The purpose of the test is to verify that the system fails gracefully when the capacitor shorts out. You can't easily do this test on a capacitor directly across the line. The results of shorting out the X-capacitor are heavily dependent on the supply circuit. Ted Eckert American Power Conversion/MGE http://www.apc.com/ The items contained in this e-mail reflect the personal opinions of the writer and are only provided for the assistance of the reader. The writer is not speaking in an official capacity for APC, MGE or Schneider Electric. The speaker does not represent APC's, MGE's or Schneider Electric's official position on any matter. John Woodgate <jmw@jmwa.demon.c o.uk> To Sent by: emc-p...@ieee.org emc-p...@ieee.org cc Subject 03/16/2007 03:23 Re: Electrolytic Capacitors in PM Primary Circuits In message <be3336be85968d49be01e66d6e365b1e01b5a...@sjc1amfpew01.am.sanm.corp>, dated Fri, 16 Mar 2007, "Tarver, Peter" <peter.tar...@sanmina-sci.com> writes: ยง1.5.6 looks at X and Y capacitors, and those connected between the primary circuit and earth. The cathode of the electrolytic capacitor might not connect to earth, unless the bottom end of the rectifier is earthed. If it doesn't connect to earth, no exception is needed. I thin a little clarification is necessary. Between the mains conductors and the filter capacitor are, typically, a fuse, a common-mode choke, a rectifier diode or a bridge rectifier and a resistor to limit inrush current. In no case that I can envisage is the filter capacitor connected to both mains conductors. In fact, that would probably cause it to explode. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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