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HFAC safety

Posted by dave slagle (M) on January 7, 2017 at 08:49:46

This is just a silly thought but here goes.

I just found out that the impedance of a body is not linear with frequency. I always found that I could pinch the leads of a transformer and have some interesting effects on the shape of a 10K square wave. Knowing the body has something like 50K-200K resistance when grabbing a DVM and since simply placing that value resistance across the leads on the bench did very little I came to the conclusion that we must be more complex than just a simple R so I played with adding some C and still was unable to get results that matched what my body did to load a square wave.

The other day I looked back at the subject when I was measuring something with a friend and this time I tried to measure the simple Z and saw two things that I hadn't considered. First the Z went down as frequency went up which tells me there must be some capacitance involved and the phase of the impedance is moving - even at 100hz. (-90º is capacitive, 0º is resistive and +90º is inductive). Here is a simple impedance plot of my body from hand to hand (across the chest cavity) and the pattern is consistent with what my meters say.

We have all been taught that it takes 100ma across the chest cavity to kill us and that typically refers to the 100V at 50-60hz line voltage. Applying ohms law with the 100K impedance of my body I only come up with 1ma, so I tried grabbing my line voltage and that didn't go so well. As it turns out once the voltage is high enough to "establish a conductive path" the resistance (impedance?) breaks down to a lower value causing a runaway situation and since we know that line voltage can kill you it seems clear that that 100K initial value @ 50-60hz must break down to 1K if you hang on long enough.

I realize this was a long story to get to my question but if my graph is representative and the bodies impedance at 60khz is 1/10th of what it is at 60hz, do we need to consider that the voltages in use for filament supplies might be dangerous too?

I'm guessing they aren't since these supplies are readily available without warnings but that makes me wonder what piece of the puzzle I am missing / overlooking in this.

dave

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