I used to install power lines in rural areas. Take an aluminium line stretching for a mile from wooden pole to pole. Leave for isolated for the winter. Line picks up enough charge to zap you good and dead. AGSC ------------------------------------------------------------------------ ------------------------ On 2006-Jul 17, at 03:06hrs AM, Russell McMahon wrote: Reading area: Interest and experience Subject: Things that go bang. How not to die 101 What was the mechanism ... Had an interesting experience which I can't offer a certain explanation for. I have a "reverse dynamometer" where a 3 phase AC motor drives an alternator whose output is being tested. A single phase AC input / 3 phase output inverter is used to drive the 3 phase AC motor. Mains are 230 VAC and inverter DC rail is peak rectified mains ~= 400 VDC. Alternator is rated at under 1 kW. Turned on drive with setting at low speed. Auto ramp up (usually provides a soft start). Wurrah wurrah, system starts to rotate, ... poof band flash. Smoke drifts from inverter. Sadness. I need the dyno urgently at present. [[I've since managed to borrow another one for now]]. Can we fix it? Yes we can! . Well, maybe. Mains plug out. Check 4 + times. Cover off. Bit of a fight. Quite some minutes as mounting screws are masked by cover tower which is ... 5+ minutes after flash bang I have inverter open. Charring on plastic cover by 2 of 3 motor terminals :-(. Peer in at PCB. Misc charred surface mount pieces flake away :-(. Wonder if the power switches, which must be underneath somewhere, are OK. can't imagine. Several more minutes attempting to demount from heat sink. Was that the slightest of tickles I felt in a finger? Hmmm. Would be VERY wise to discharge the main caps. Meter. 390 V. Ah so! 890 uF x 400V x 2 of in parallel. 130 Joules or so! Discharge time. Reaches for SFR16's. Fist to hand is 10K. Smiles 400^2/10000 = 16 Watts. Not exactly useful in the circumstance, but, seeing its in my hands... :-) Wearing $2 shop $2 2 dioptre glasses. Also an X dioptre head magnifier tilted up. Tilts down head mag to provide 2 quality [tm] layers of eye protection. Grasps SFR16 [TM] in side cutters (insulated, discharging for the use of). Apply 10k across cap terminals. Smoke curls, as is expected. Resistor starts to glow, as is expected. Next to be expected is a small fiery burst as the resistor disintegrates. It didn't happen. My eyes/head are perhaps 8" from there sister. *BANG*. Better than a "double happy". Probably less than a thunder cracker. Deafening. Can't recall if there was a visible flash. NO charring or spark path or debris or any visible damage. Resistor measures 24 ohms. Cap now has 100 volts on it still. It just lost about 100 Joules in one go. That's about twice the energy in a 9mm pistol slug, about 7 times the energy in a 0.22 long rifle slug or 2/3 the energy of a 5.56 x 45mm rifle round. Down a bit on a 38 Magnum. Ears sing for perhaps 10 minutes afterwards. But, I don't know what caused the extremely loud bang. I've shorted out all sorts of capacitors at all sorts of voltages over the years (up to about 1200 VDC) and while I've seen lots of sparks and arcs and chewed up shorting implements, I've never had a genuine explosion before. Best guess so far is that the carbon film resistor layer vaporised and formed an arc which worked "just right" and released most of the energy available "rather fast". Lack of confinement makes the bang puzzling. 100 volts remaining is interesting - although the arc could break at any time. Arcs can be drawn at well below 100 V in appropriate circumstances. The amount of material available was small. The biggest lesson is, discharge capacitors proactively. In my enthusiasm to troubleshoot I could have died. 1600 uF at 400V is very very lethal. I know very well the dangers of charged capacitors. Lesson 2 is discharge caps with an appropriate wattage and resistance resistor. Anyone want to buy a slightly used inverter ? Russell -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist Gus S Calabrese Denver, CO 720 222 1309 303 908 7716 cell I allow everything with "spamcode2006" in the subject or text to pass my spam filters -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist