roy@phri.UUCP (Roy Smith) (10/30/88)
henry@utzoo.uucp (Henry Spencer) writes: > an electrical fire is *hot*. This is getting a bit far afield, but what the heck? We've got a rack of MultiTech modems. Supposedly the cards are hot swappable (i.e. you can pull and insert cards with the power on) but if you get them misaligned in just the right way when you insert them, you short the +12V and -12V power rails in the card cage. The modular power supply has 2 1-ohm, 2(5?)-watt current limiting resistors. The first time we shorted a card like this one of the resistors blew (you would think the power rails would be fused, wouldn't you?). We decided to fix it outselves but couldn't find any suitable resistors around, and Radio Schlock doesn't carry that size and value either. However, we did come across some 1-ohm/foot nichrome precision resistance wire. We just measured out a foot of it, coiled it up into neat open-wound coil and soldered it in place. Worked like a charm until the next time we shorted a card. Care to guess how much power 1 foot of 20-or-so gauge nichrome wire can dissapate without melting? A lot! If I ever want to build a toaster to run off my car battery, I know what to do. It took 10-20 seconds or so for us to notice something was glowing inside the rack; by the time I had the door open, the coil was glowing bright cherry red, and the PC board was starting to burn (I didn't even know PC board material was flamable, but there it was, burning away even after we cut power). The amazing thing is that after I removed the short and wiped away most of the soot, the power supply still worked, charred PC board and all! In fact, right now, I'm dialed up via a modem in that rack, with that very power supply, with the nichrome resistor still in place. One of these days I gotta see about ordering a spare supply.... -- Roy Smith, System Administrator Public Health Research Institute {allegra,philabs,cmcl2,rutgers}!phri!roy -or- phri!roy@uunet.uu.net "The connector is the network"
tim@hoptoad.uucp (Tim Maroney) (10/30/88)
In article <3446@pt.cs.cmu.edu> ns@cat.cmu.edu (Nicholas Spies) writes: >Mercedes pulled out of auto racing in the 1950's after a magnesium-bodied >car crashed and sent burning debris into the crowd (at Le Mans?). Either >68 or 168 people died (sorry I don't have a reference handy). Definitely more >flame than your average power supply fire would yeild, however... The fatality number I recall is in the eighties; horrible accident. However, please note that a car crash tends to make lots of small metal fragments, which (in the case of magnesium) would be trivial to light from a burning gasoline tank, or just from sparks. Now, if the Next box happens to get involved in an explosion which shatters the case, it might be worse, but probably not otherwise. -- Tim Maroney, Consultant, Eclectic Software, sun!hoptoad!tim "Mere opinion without supporting argument is no more than the American Bandstand school of literary evaluation." -- Tom Maddox