Hi Manning You wrote:
> I love this question! The periodicity of ice ages is about 50,000 years. > The effects from glaciers is only periodic and not continuous. In fact, > I am told that if you sit in one place, you can see Kudzu grow, but if your > period of observation is continuous, the growth of Kudzu is hazardous > to your well being. The effect of the TNV is defined as being safe, similar > to SELV, since the ring voltage is intermittent, but I still would not like > to > be shocked by the 120V ring voltage. In this case the ring time is about > one second in length. How long does the ring tone have to be to become > hazardous? The GFCI will interrupt in about a 100th of a second and > again, I would not want to get shocked by a voltage to have the GFCI > to kick in. I am confident that the energy in the GFCI circuit is very > high, > so is this better than the clause 1.2.8.7 on Haz Energy. If the GFCI > operated in one tenth of a second would this be hazardous? > > There is a popular book with the title some what like coming into the light, > > where the author was struck by a continuous bolt of lightning and lived to > tell > all. There is a veiled relationship between the hazard severity and the > length > of the time for exposure. How short is short for being intermittent and how > long > is long for being continuous. The threshold of acceptability for the pain > or > physical reaction, blister, etc, and the rate of delivery of the energy to > the > person, hand, skin, etc. determines what is short and just too long. > > A similar pet peeve of mine is the frequency of occurrence versus the > severity of the event or hazard. One pundit proposes that there is a > zone of acceptability between the graph of these two, but I believe that > hazard reduction is for safety and frequency reduction is the domain of > insurance statisticians. > > So can we develop a graphical representation of the length of time of > exposure versus the severity of the hazard and have a zone of safety? > The continuous level is not an asymptote but a parallel to one axis of > the coordinate system. So any point under the parallel must be safe. > > So I would ask Rich's question the other way around, when can we stop > the test for a continuous application of a hazard? One hour; 2 weeks, etc.? > > To answer Rich's question, any point of time under the parallel is part of > of the continuum, including ground zero, at the inception of the application > of the hazard. In respect of time in relation to the effects of electricity on the Human Body, I use a British Standard document PD6519 part 1:1995. This is titled 'Effects of current on human beings and livestock'. It has some good graphs which show current/time relationships, and also which parts of the cardiac cycle are vulnerable etc. A quick abstract from this tome shows (fig14) 1 amp 50Hz for 0-200mS, and 100mA 50Hz for anything above 200mS will cause ventricular fibrillation. (Another word for death). This doesn't include I2R effects from high voltage and high skin resistance, which causes other forms of death! This document is, so far as I can tell, identical to IEC479-1:1994 Another tuppence worth.. Chris Dupres EMC Specialist. VG Microtech. [email protected] tel +44 (0) 1825 761077 fax +44 (0) 1825 768343 'Opinions expressed are personal, not necessarily Corporate'
