I took some test data. That may be a bit of an exaggeration, but I made a few measurements.
This was at one of the electrical outlets where I got a one-handed shock. It is also where the electrolyte content of the rug is unknown but possibly not zero, on account of the four-legged members of this family are not as well-trained as I might hope. I measured the line potential phase-to-neutral and phase-to-ground and it was 118-120 Vac. I then configured the Triplett 9005 DMM as an ammeter, and connected from phase to floor. I measured to rug, then poked hard and ran the probe point into the sub-floor. I measured zero on scales 2 ma and 200 uA full-scale. I then went back to voltmeter mode and measured phase-to-floor potential. About 20 Volts. I removed the ground lead, so that the voltmeter had a single connection to phase, and just capacitance to ground. Reading dropped to around 6 Vac. I grabbed the voltmeter and the reading rose to about 10 Vac. I squeezed hard and the reading went to 18 Volts. I placed a fat pillow on the floor and stood on it and the reading went down a little. I am wearing socks but no shoes at this point. Then I decided to quit messing around. I connected the ground lead back to the voltmeter and grabbed the probe end while standing on the floor. The reading rose to 60 Vac. I got back on the pillow and the reading dropped a little. This is looking an awful lot like capacitance, folks. I think I can quantify the amount of capacitance. Standing on the floor in socks, the meter reads 60 Volts, or half the line potential when I grab the negative lead. That means that roughly half the line potential drops across the meter, and half drops across me and my path to ground. I don't have a manual for this meter, and specs on the internet don't show input impedance, but I think most DMMs are in the neighborhood of 10 MOhms. If that is a good number, then I and my path to ground were also 10 MOhms. At 60 Hz, that means a capacitance of 265 pF, which seems reasonable. In my shoes, the measured potential was 50 Volts, which means that the capacitance dropped to 220 pF. The implication is that about 6 uA of current was passing through me, and of course I couldn't feel it. But there is still a problem. If I left the voltmeter out of the circuit and grabbed the phase potential directly I would get the one-handed shock. But according to these numbers, the voltage would of course double to 120 Volts, and the current would double to 12 uA. I don't think that would be enough to tickle me, let alone be sensed as shock. I have a feeling it has something to do with the difference in source impedance, but I can't put my finger on it. I think the line potential could cause electrical ionization in my body that was not occurring with 60 Volts across me from a 10 MOhm source impedance. But if my body has a capacitance of 200 pF to ground, then it shouldn't really matter if my body looks like a few kOhms at low potential, or a dead short at high potential - the 200 pF is the limiting factor, not the body resistance. Comments? This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Scott Douglas [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
