I can't verify the part that after a wrench was dropped on the -48V bus bars, all the other equipment was blown, but I can throw a little light on this. The previous company I worked for made Telco rectifiers. Our largest system had an output of over half a megawatt (!) at -48V. The bus bars were not little 1" diameter rods, but laminated copper bars that were 6" x 4" in cross section. One customer was worried about what would happen when ol' "Bubba" dropped his wrench across the bars, and I had to prove that the bars would not tear themselves loose due to the repulsive force caused by peak short circuit current from the batteries being charged by the rectifier system. The current was not insubstantial: 100,000 Amps would flow for about 15mS before the fuse cleared. It was assumed that not only would Bubba's wrench vapourize, but so would Bubba.
Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Robert Macy [mailto:m...@california.com] Sent: February 25, 2002 10:34 PM To: Jim Bacher; ieee pstc list Subject: Re: Use of PCB Traces as Fuse and Voltage suppressor Jim, You touch on an important issue concerning a fuse - just how does it "blow"? Years ago I discovered by accident that fuses were designed with some remarkable properties, when we had to make our own transient generator to verify some telcom equipment's compliance to a BABT power supply transient spec. The BABT spec required that you simulate some very husky power transients. It was like a short occurs in adjacent electronics followed by the inductive kick. The -48 voltage would clamp to around 10 volts then "pop" up to over 300 volts capable of supplying 500A for something like more than 50mS. If you didn't design your protection properly you would have a lot of unintentional PCB trace fuses. [ Actually heard that the spec originated because a workman had dropped his wrench across the 1 inch diameter rods which supply the -48 to the telco building from the battery building. After the wrench evaporated, they found the whole room of equipment was blown, thus the spec. Somebody verify that? ] The simulator used 4 deep discharge current vehicle batteries supplying the telcom equipment through 50uH of inductance (that was 0000 cable on a spool). Parallel to that you used a starter solenoid to short out a fuse with a dead short. Amazingly the larger fuses never produced much kick back. They were designed to blow gently away. Tried all kinds. Most of the 8AG didn't do much, other types, nothing, even the 100 amp cartridge types, nothing, The absolute best was a 1A 8AG type. When that went, you'd get a flash of light, 300 volts trying to drive 500 amps into everything, and even the coil would "jump" up off the floor. Talk about PCB traces acting like fuses. Anyway, I learned a respect for people who design fuses to make them go away so gently when there is an incredible potential for some extremely high voltage transients. - Robert - Robert A. Macy, PE m...@california.com 408 286 3985 fx 408 297 9121 AJM International Electronics Consultants 619 North First St, San Jose, CA 95112 -----Original Message----- From: Jim Bacher <jim.bac...@paxar.com> To: 'Cortland Richmond' <72146....@compuserve.com>; Chris Maxwell <chris.maxw...@nettest.com>; ieee pstc list <emc-p...@ieee.org> List-Post: emc-pstc@listserv.ieee.org Date: Monday, February 25, 2002 3:04 PM Subject: RE: Use of PCB Traces as Fuse and Voltage suppressor > >Long time ago we found that the traces worked well as fuses when the >batteries were fully charged. However, when the batteries were mostly >discharged, the PC Board traces did not work well as fuses. At lower battery >charge levels, the traces became very hot and ignited the PC Board rather >than opening the traces up. I therefore would recommend against using PC >Board traces as fuses. > > >Jim > >Jim Bacher, Senior Engineer >Paxar Corp. >e-mail: jim.bac...@paxar.com or j.bac...@ieee.org >voice: 1-937-865-2020 >fax: 1-937-865-2048 > >-----Original Message----- >From: Cortland Richmond [mailto:72146....@compuserve.com] >Sent: Thursday, February 21, 2002 12:53 AM >To: Chris Maxwell; ieee pstc list >Subject: RE: Use of PCB Traces as Fuse and Voltage suppressor > > > >When do you need a fuse? Level II is the only time you are allowed to lose >functionality, and the requirement for THAT is, it can't catch fire or >explode. I've seen "trace fuses" tried. The problem comes after the trace >blows. You are at the mercy of your board shop, and if you use a number of >them, results might not be all that repeatable. AS i said earlier, I've >had a board catch fire in my hand (though not as a result of stress, but a >solder splash). It is instructive. > >Cortland > >------------------------------------------- >This message is from the IEEE EMC Society Product Safety >Technical Committee emc-pstc discussion list. > >Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > >To cancel your subscription, send mail to: > majord...@ieee.org >with the single line: > unsubscribe emc-pstc > >For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: davehe...@mediaone.net > >For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > >All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
