Relative to your comments in the attached Email on testing MOVs 3. In my opinion, use of these parts must include a disconnect means to cover the fault case.
About fusing MOVs I have an application designed for the US market and now being considered for the European market. It can be powered from 48 to 250 VDC or 120 to 240VAC. Presently there are MOVs line to line and each line to ground. For EMC emission reasons I need to recycle the design. I have witnessed the same events you describe and have read the various postings concerning surge suppression. MOVs obviously require some care in applying both from an engineering and standards point of view. I have considered limiting impedances but I believe I couldn't afford more than 100 ohms and I believe I am just shifting the problem out a few standard deviations from occurring. I have thought about fusing. It appears that I would need two fuses since I could be sourced from US 240 with two hot circuits (a floating neutral). I could restrict the application to systems with neutrals referenced to earth. My application does not like fuses (a protective relay) so I also have considered using Positive Temperature Coefficient "PTC" ceramic fusing devices from Murata. They are sold as protection devices for transformer overloads and have voltage ratings up to 265 Volts. On over current they open like a fuse and therefore would limit the joule exposure to the MOV, after the event they would recover and I would be powered again. Of course I would need to coordinate the reaction time to the joule ratting of the MOV. > Does anyone have experience in this area? > I have also considered scrapping MOVs altogether! Using passive inductors, Caps resistors to pass surge. I am still foggy on the restrictions imposed by the LVD versus local safety standards. Any feed back would be appreciated. Does any one know of good sources for inductors used in this application? I have calls into Siemens USA to get catalog on their VDE 565-2 recognized inductors. Regards Chris Wells Sr Des. Eng. Cutler-Hammer well...@ch.etn.com ------------- Original Text From: C=US/A=INTERNET/DDA=ID/bobdb(a)VNET.IBM.COM, on 5/15/97 10:36 PM: from--bo...@vnet.ibm.com Several years ago I tested MOV devices from several manufacturers. All had a nominal rating of 70 volts and 10 joules. My testing forced breakover with a 10-ohm limiting resistor. Source voltage was 120 mains and simple fill-wave rectifier. The results were quite interesting and (my opinion) point out that the circuit designer must consider all aspects of a part when used in a safety-sensitive application. 1. All parts failed rather violently, with a large plume of dense smoke and some flaming, as the overcoat materials decomposed and burned away. 2. Following the flaming stage was a 'cooking' stage. These parts collapsed to approximately 7 volts breakover zener characteristic and soaked up the high energy available. Judging by the white-hot color the surface temperature was approximately 1200 C. I let several of these tests run for 10 minutes--- these are very rugged parts, basically a ceramic sprayed with metallic coating for lead connection. Some of the parts lost connectivity and ended the tests, but this is not reliable. 3. In my opinion, use of these parts must include a disconnect means to cover the fault case.
