Physiological Effect of Current

[TAFEDout]

If your finger was to come into contact with a ‘live’ 240 volt 50 Hz conductor in a wiring installation with earthed neutral (assume you have dry unbroken skin, rubber-soled boots standing on a wooden floor) – it would hurt!

It would seem from my calculations that taking into account the resistance provided by rubber-soled boots and the body’s capacitance and resistance (at said voltage and frequency) the impedance would be too high to produce an electron current flow of significant magnitude to earth. Even assuming that the body is a ‘dead-short’, I could not get a current of anywhere near 30 mA through the boots to trip an RCD.

I assume the physiological effect would therefore be largely due to a movement of electrons as in static electricity.

In several reference texts I have viewed, they show a standing person touching a live conductor. However, unless they are bare-foot on an earthed metal floor, the point they are trying to make of a dangerous current flowing to earth, seems unlikely.

I would be very grateful if anyone could shed some light on this.

 

[DRWeig]

There is an old, old Lancet article on this, I read it ages ago but I can't seem to find it. What I remember of it is that it takes only 15 mA through the torso on average to have really bad cardiac consequences, but if it's not through the torso it can be somewhat higher. But even with only microamps flowing, the potential difference between your body and the earth causes havoc in the nervous system. Pain, involuntary muscle action, messed-up vision, but not necessarily a burn if you're insulated from earth well. If the potential difference is high enough, you might not be able to let go of the hot conductor if it's in your hand.

If you are grounded well (wet bare feet), the initial current will be small like you noted. But the path that the current takes will rapidly ionize bodily fluids and reduce your resistance (resulting in high currents), causing burns and near-certain cardiac interruption.

Please don't take this as gospel, I think it was over 30 years ago that I was given this article to read because I asked the same question you just did. My memory isn't what it used to be. Wait for others to respond.

Best to you,

Goober Dave

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[VEBill]

Your likely false assumption is, given the circumstances you've describe, that "it would hurt". Depending on how many tens or hundreds of micro amps leakage current there is, you could perhaps feel it.

You obviously must not experiment with this. One false move, or a bad assumption about the dielectric strength of your footwear, and you could wake up dead.

What are you up to?

 

[IRstuff]

You could "shed some light on it" if you posted your model.

Nonetheless, people do get electrocuted, either by accident or on purpose. In the case of executions, the criminal is well wetted to ensure good electrical contacts. In the case of accidents, there is typically water involved, although, as an EE, you should be well-familiar with the traditional habit of EEs to keep one hand in one's pocket to minimize the risk of making a complete circuit.

On the flip side, it's a common thing in science museums to allow people to touch Van de Graff generators, but ONLY if they're on an isolated platform, and ONLY touching the Van de Graff. So, obviously, someone, brave or idiotic, experimented until they determined a safe amount of isolation and insulation that would protect someone when in contact with the generator.

Conversely, there are lots of old stereos and other equipment where you can feel a sticky, draggy, sensation when touching the chassis. That's from a non-grounded chassis where there's some level of AC bleed into the chassis and into you.

In any case, the RCD, or GFCI to those who don't keep up, current threshold is designed to handle primarily physical contact between live wires, a person, and a good conductor. The value of the threshold requires less than 4 kohm of resistance, which is, obviously, somewhat of a challenge to achieve. Nonetheless, people routinely get electrocuted, although, the good news is that the statistics have been historically declining, due to a combination of improved product design, and less exposure. That's particularly evident when one considers the near complete elimination of the "stereo" as the primary source of music. See:

http://www.cpsc.gov/library/shock95.pdf

for a breakdown of statistics. Also:

http://injuryprevention.bmj.com/content/8/4/306.full

http://en.wikipedia.org/wiki/Electric_shock#Body_resistance

http://www.elcosh.org/en/document/5...nstruction-workers-being-electrocuted%3F.html

TTFN
faq731-376

 

[zeusfaber]

I did the 240V, 50Hz, rubber soled shoes thing about thirty years ago while training a theatrical spotlight - I was a bit perturbed to see the light go out accompanied by arcing between the mounting bracket and the steel structure of the building (especially since I was holding the bracket at the time). A couple of minutes' investigation revealed that the neutral and earth cores had both pulled out of their terminals in the plug and had got wound together. The current path was up the live, through the lamp, down the neutral lead, back up the earth lead, then through the lamp body, building structure and earth to the transformer. No RCDs in those days. When I loosened the mounting nut and the light went out, the case, I, and the 30Ft steel tower I was perched on all went live to mains.

Didn't feel a thing. On the other hand, I could always make somebody flinch at about 240 microamps using a neon tester and a gentle touch to the earlobe. The skin is remarkably sensitive if you use a small contact area to increase the current density. It takes milliamps rather than microamps to kill, partly because the nerves that matter are buried deep in the body and the current densities tend to be lower than at the skin surface.

You can't guarantee that the victim is going to have dry skin, that there isn't a drawing pin sticking through the sole of their shoes and that they aren't clinging on to a steel roof truss with their other hand. All of these can turn a near miss into something that hurts (or worse) and becomes sufficient to get noticed by the RCD. This is why the "back of the knuckles" test is such a poor way of proving that conductors are safe to work on.

A.

 

[LiteYear]

It's a very interesting conundrum, and one that's hard to find impartial evidence on. I recommend reading IEC 60479 for some authoritative background on the relationship between touch potential and body current. You'll see that ever since live experiments on humans and animals became a bit of a no-no some decades ago, the literature has stagnated a bit. What some of the pioneers were able to subject themselves to before that to give us the empirical baselines we have is quite extraordinary.

To answer your question directly, yes, under ideal circumstances, rubber soled shoes will prevent a current from a 240V source reaching the threshold of ventricular fibrillation. But consider all the effects that worsen the situation:

* Cracks in the shoes may have accumulated wet soil.
* A knee might have bushed against a metal ladder or chair leg.
* At the instant of application, the dV/dt is enormous so all the capacitances move charge very quickly.
* The event that caused the contact may have also interrupted the normal circuit and therefore involve an inductive voltage kick.
* The muscle spasm may cause contact with other parts of the body.

And many more. If it just takes an unfortunately combination of any of these extra factors to kill, then you can see why it might be easy to trip an RCD even if in steady state it doesn't seem possible.

The physiological effect I don't believe needs to be related to static. The threshold of perception is much lower than 30mA (more like 0.5mA) so even with 20kOhm boots you can still get a decent kick.

Hope that helps shed a little light. It's a complex topic that ought to attract more discussion, IMHO.

 

[IRstuff]

Just bear in mind that if the body resistance is less than 2 kohm, a 240 V voltage application provides an instantaneous 120mA of displacement current into an uncharged capacitance/unterminated. The good news is that it would only last for less than 300 us.

TTFN
faq731-376

 

[IRstuff]

oops, read the value wrong, less 1 us

TTFN
faq731-376

 

[TAFEDout]

Thank you all for the valuable input you have provided. The reason for my question VE1BLL is that after a few decades as an Engineer, I have decided to try my hand at teaching. I soon discovered as LiteYear indicated, that the physiological effect of current was a complex topic that ought to attract more discussion.

 

[electricpete]

It sounds like you have gotten good answers covering almost all the angles including some I never thought of.

I hope you will reinforce to your students that safety procedures should never be modified based on such a casual analysis.


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