How long is this "continuous" going to go. Its a waste of everyones time!
>>> Tony Fredriksson <[email protected]> 11/15/96 10:32am >>> >>>Every reasonable instance, that I can imagine, of injury or damage resulting >>>from an energy hazard, will happen very fast and be over and done with in less >>>than one second (human reaction time) or even faster (20 J @ 240 VA = 83 ms). >>>So what possible significance can there be in relating this to a test that >>>runs for one minute or even longer? All, I believe that an energy hazard does not need to be an instantaneous event as discussed above. I recall working with a computer once for which, during assembly, it was possible to pinch one of the 5Vdc wires. As this insulation broke down, a limited short circuit developed that was not enough to trip a fuse or to open the circuit, but was enough to heat up the enclosure plastic such that it ignited and smoldered, ultimately opening a sizeable hole in the plastic. The thing sat there for hours and the problem got worse and worse. Had the system been on a desk with papers in contact with the area, a buliding fire could have easily been triggered. Limited short circuit conditions are often insidious and can go unnoticed for long periods of time. Building fires from shorted electrical wiring are another example of this phenomenon. Egon is right that a PROPERLY chosen fuse can limit such hazards. In such cases, limited shorts under the fuse hazard would not draw enough current to cause and overheating problem with resultant fire hazard. My 2 cents worth... Regards, [email protected] ---------- From: Egon H. Varju To: IEEE Subject: RE: How long for continuous? List-Post: [email protected] Date: Thursday, November 14, 1996 6:58PM This thread is getting very interesting. Trust Rich Nute to keep us on our toes and help keep our minds from atrophying from disuse. Many times when when I hear this question, the intent seems to be to salvage a poor design and find out "how much can we get away with." Wouldn't it make more sense to find out "how much is safe?" Most responses so far seem to indicate that safety agencies consider one minute or longer to be a reasonable number. Does this mean that we are assuming that normal people are stupid enough to stand there for one full minute, or longer, and watch their finger melt off? Seems to me that in the unlikely event that any such people should exist, they probably don't belong to the human race and, therefore, shouldn't be protected by our safety standards or our laws. Every reasonable instance, that I can imagine, of injury or damage resulting from an energy hazard, will happen very fast and be over and done with in less than one second (human reaction time) or even faster (20 J @ 240 VA = 83 ms). So what possible significance can there be in relating this to a test that runs for one minute or even longer? Well, I think we all know the answer to that: we're trying to use fuses to limit enegry hazard. But fuses aren't intended to operate in fractions of a second, except at extremely high overcurrent levels. In fact CSA/UL fuses aren't even calibrated for operation in less than 2 minutes (@200%). Perhaps Rich's question should be "can a fuse be used to limit energy hazard?" Just some food for thought ... Egon Disclaimer: Definitely personal opinions!
