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Chief Flunky
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What do you mean by this?

Again the question is about the amount of voltage needed to move a certain amount of current over an already specified resistance. I don't need to soak my hands or puncture my skin. That would give me a different resistance and is not the question. I don't want to know how different resistances or amounts of current affect me. The question is how different voltages affect the amount of current I would receive with my specified resistance. Actually, I already know how they should affect me the question was about the mathematical assumptions made.

It seems the biggest factor I overlooked is how the capacitance of the body blocks to an extent DC current.
DC cannot cause fibrillation. AC interferes with the heart sinus rhythm which is what makes it so deadly at very low currents.

DC can cause a great deal of pain but doesn’t kill you unless it cooks you (over 10 A). But it causes involuntary contraction of muscles at milliamperes of current. The contraction is the pain source and when it stops the reaction from the muscles can be so violent it can break bones.

Back in the 1950s we really didn’t know anything about the effects of electricity. Charles Dalzeil was the first to study it. There is a set of 4 technical papers on the subject that he published that are freely available. Since that time some refinements have been made but the basic concepts established by Dalzeil still hold true.

Among those concepts Dalzeil established that clinical effects are related to body weight and current, not voltage. Voltage is strongly affected by skin resistance. Internal resistance is affected mostly by body weight and is almost universal even among different mammals. So for instance the thresholds tend to be lower in human females due to smaller body weight. This is fortunate as it allowed him to do lethal testing on rabbits instead of humans.

Dalzeil did some experiments on skin and found resistance varied widely but that a lower threshold was about 1 K ohms but dry skin could be quite a bit higher, Voltage matters too but you may not be interested in “Meggering” yourself. I know I really don’t like doing it accidentally. IEC published details on the effect of voltage on skin resistance. So the results you get at 9 V with a multimeter will be significantly higher than at 100+ V where it matters.

If you are getting to voltage thresholds it is generally held that 50 VAC is safe. I have read somewhere that an extreme situation of a man with a pace maker pinned so he could not escape was killed by under 50 VAC in an MSHA area (mining) but even they admitted this was an outlier. Roberts (Canadian) did some questionable math with the IEC standard and came up with 28 VAC as a threshold and justified it by some essentially unverifiable reports of incidents in China. Based on preponderance of evidence I’m not buying any of it. For DC no similar equivalent exists. 70E did use AC=DC for a while and was widely criticized for it. The proposals mostly recommended around 150 VDC as a minimum safe limit.

I have personally experienced shocks at 1400 VAC and had coworkers that have taken up to a 4160 V (7200 V L-L) shock and lived to tell about it and I’ve known of cases even higher. It is fatal depending on the path it travels through the body (hand to hand being the worst) and as you’ve found out skin resistance. The fact that the sensation and pain thresholds are a lot lower than fatal shocks and given that many people have experienced much higher voltages and lived through it tells you that fatal shocks are not that common. But when it does happen you have minutes to address the heart afibrillation that occurs. An AED can in most cases recover normal rhythm. In my AED training it’s around 80% success. Fortunately I have not had an “opportunity” to test this on either myself or others. But on a couple incidents involving “Megger” testing where residual polarization has accidentally hit me pretty hard I have experienced DC involuntary muscle contractions first hand. I do a lot of MV work where those tests are 1000-10,000 V.

So not sure what you are after but don’t expect safety-thresholds meant for worst case to correspond to average situations and don’t expect fatal conditions just because of a one time incident with say 120 V. It only takes once but no lethal shocks are a lot more common than most electricians admit to.
 

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Discussion Starter · #22 ·
DC cannot cause fibrillation.

DC can cause a great deal of pain but doesn’t kill you unless it cooks you (over 10 A).

the reaction from the muscles can be so violent it can break bones.
This is all very interesting but I find these statements very hard to believe. Not arguing it as I’m interested but do you have any sources for these claims?

It’s my current understanding that a shock of a certain amount of mA can begin to mess with the heart rhythms. Maybe this is 100mA maybe less. Either way it could be AC or DC but AC moves current into our bodies easier meaning more current at lower voltages. This appears to be due the capacitance mentioned earlier. In fact it’s not really that AC moves more current but that DC current gets blocked due to the bodies capacitance.

So for instance let’s say you have very low resistance of 1200 ohms and 120v AC. This would get you to 100mA. It seems due to capacitance however it could take 2-5 times the DC voltage to provide the same 100mA of current. It seems a persons capacitance varies like a persons resistance. Still a 100mA current from either could mess with the heat rhythms.

It sounds very dangerous to suggest that DC current under 10 amps won’t kill you. I have seen nothing that suggests as much other than what you stated. Am I misunderstanding you?
 

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This is all very interesting but I find these statements very hard to believe. Not arguing it as I’m interested but do you have any sources for these claims?

It’s my current understanding that a shock of a certain amount of mA can begin to mess with the heart rhythms. Maybe this is 100mA maybe less. Either way it could be AC or DC but AC moves current into our bodies easier meaning more current at lower voltages. This appears to be due the capacitance mentioned earlier. In fact it’s not really that AC moves more current but that DC current gets blocked due to the bodies capacitance.

So for instance let’s say you have very low resistance of 1200 ohms and 120v AC. This would get you to 100mA. It seems due to capacitance however it could take 2-5 times the DC voltage to provide the same 100mA of current. It seems a persons capacitance varies like a persons resistance. Still a 100mA current from either could mess with the heat rhythms.

It sounds very dangerous to suggest that DC current under 10 amps won’t kill you. I have seen nothing that suggests as much other than what you stated. Am I misunderstanding you?

SA node (sinoatrial node) – known as the heart's natural pacemaker. The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract

Ac electrical shock causes multiply cells to fire out of rhythm. Biology teach gave the example of a good heart beat as a rock throw in water causing a out going ripple affect. fibrillation after a shock looks like a hand full of gravel thrown in a pond where all the ripple collide.
Im not sure what dc does to the heart but as its smooth it probably doesn't cause fibrillation. Also dc is used to fix a heart that is in fibrillation.
 

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Chief Flunky
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This is all very interesting but I find these statements very hard to believe. Not arguing it as I’m interested but do you have any sources for these claims?

It’s my current understanding that a shock of a certain amount of mA can begin to mess with the heart rhythms. Maybe this is 100mA maybe less. Either way it could be AC or DC but AC moves current into our bodies easier meaning more current at lower voltages. This appears to be due the capacitance mentioned earlier. In fact it’s not really that AC moves more current but that DC current gets blocked due to the bodies capacitance.

So for instance let’s say you have very low resistance of 1200 ohms and 120v AC. This would get you to 100mA. It seems due to capacitance however it could take 2-5 times the DC voltage to provide the same 100mA of current. It seems a persons capacitance varies like a persons resistance. Still a 100mA current from either could mess with the heat rhythms.

It sounds very dangerous to suggest that DC current under 10 amps won’t kill you. I have seen nothing that suggests as much other than what you stated. Am I misunderstanding you?




There is a lot more but it’s behind the IEEE pay wall. IEEE standard 80 and the yellow book in particular have whole chapters dedicated to this topic. There are tables and charts showing all of it. All of Dalzeil papers plus Robert’s and many references to Dalzeil papers as they have been refined go into great detail. If you have an IEEE XPlore subscription or access to a local university library that has one or willing to pay tons of money I can just give you the IEEE references.

Capacitance has nothing to do with DC behavior. Look through the above papers and capacitance is not mentioned even once. Dalzeil picked numbers for DC shock hazards not because of fibrillation but the pain threshold.
 

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Discussion Starter · #26 ·
Capacitance has nothing to do with DC behavior. Look through the above papers and capacitance is not mentioned even once.
I'm going to read through your links now thank you, but capacitance has everything to do with DC behavior. Not sure where that statement comes from other than the fact that it's not mentioned in your reference material. Not being mentioned however is far from not having an effect. Capacitance blocks DC current. I've never seen this debated before. This article mentions the effect and has references at the bottom of the page.

 

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My first Jman's basic warning was, "Boy, it depends on how you are grounded, so get some rubber soled shoes, and avoid shocks across your body." i.e. you left hand is holding onto conduit, and you grab 120volts with your right hand. I remember when Square D came out with the first GFI breakers and their literature said that they trip at 6ma because if you are grounded properly (Like sitting in a bath tub), 22ma can kill you. My Jman's advice has served me well down through the years... plus insulated needle nose pliers are you best friend.
 

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Chief Flunky
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I'm going to read through your links now thank you, but capacitance has everything to do with DC behavior. Not sure where that statement comes from other than the fact that it's not mentioned in your reference material. Not being mentioned however is far from not having an effect. Capacitance blocks DC current. I've never seen this debated before. This article mentions the effect and has references at the bottom of the page.

Yes, that article gives somewhat of an explanation but it is fundamentally flawed. See figure 6 on this paper:


For perception (Figure 2), it shows the wrong pattern. More current is required as frequency increases. This indicates an RC filter...the capacitor is in parallel with the resistance, not series.

Also check out Let-go (Figure 6). Here we see the trend that you are claiming...let go thresholds first decrease from DC down to around 20 Hz, which would seem to indicate a series capacitance. But then they increase above about 500 Hz, just as they do with perception, again indicating parallel capacitance, not series.

Now there are major differences between testing skin resistance or more correctly at this point impedance, and testing the effects on the human body such as perception, let-go, and fibrillation. If we want consistent experimental results then we need somewhat consistent skin resistance or to insert electrodes through the skin and test that way, although that would lead to more questionable results. So the solution is to thoroughly wet the skin first. So your capacitance claims simply don't exist in the above test results.

OK now the big question...can you find anything talking about the fibrillation threshold of DC? Dalziel focused entirely on the let go threshold and used this as the basis for his maximum "safe" DC threshold. All others have followed suit since then. The DC let-go threshold is about 3 times higher than the AC one. Why did he do this? If you read carefully let-go with DC is excruciatingly painful and when it releases it does it so violently that it can actually break bones. Dalziel was doing safety studies so he was looking for two concerns: at what point are electrical shocks first perceived (perception limit) and at what point does it cause permanent harm. Whether it breaks bones or causes fibrillation is all the same: permanent damage.

See this article talking about the design basis behind IEEE standard 80. IEEE standard 80 only touches the surface of Dalziel's work but nicely summarizes the thresholds with calculations. IEEE Yellow Book (commercial systems maintenance) standard 902 goes much more in depth concerning Dalziel's work but you can get a much better understanding from the horse's mouth.


Also see this one where Dalziel is working on the DC let-go threshold.


So getting back to the issue of capacitance. This only comes up with dry skin. Some skin resistance testing was done in the 1950's and is summarized in the IEEE Yellow Book but it isn't really a focus when it comes to safety. Aside from testing purposes, most of the time I do my best to keep my hands clean and dry. Often I have to work with various lubricants though which are mostly oil or wax based, and I may be working outdoors in wet or hot and humid conditions so I might be wet or soaked with sweat. So rather than a skin resistance of 1 megaohm I might actually be closer to 1 Kohms or less at times. So from a safety perspective we can safely ignore skin capacitance. If it is a factor it will show up in one of two ways. First it can be in parallel in which case it might have some influence on the high frequency response to shocks. This might be a useful explanation but that's about it. In terms of influence on DC, a parallel capacitor might cause the DC voltage to linger but with so little capacitance it won't last very long. If it is in parallel then it would indeed block DC but if that's the case then your own experiment showing skin resistance of around 1 megaohm (pretty typical for the "grab the leads" just about proves that capacitance doesn't matter for practical conditions. Either the RC time constant is so high that it takes minutes to hours for the charge to build up or the capacitance simply doesn't exist, or that again its in parallel with a resistance that nullifies the effect.

What electronicsforu missed is that TIME is a major factor in considering health effects of electricity within the body, and this was something that Dalziel studied quite a bit. Above 5 seconds if you haven't caused fibrillation or any of the other nasty effects, it isn't going to happen. Below 8 milliseconds unless you are at flesh burning current levels, again nothing is going to happen. The explanation is quite simple...if the pulse is fast enough, the muscles don't have time to react to it anyway. Similarly we see the let-go threshold increase dramatically above 500 Hz and this is for the same reason...again the muscle tissue doesn't have time to react to what is happening. Sure we can consider skin capacitance as a component of skin impedance but Dalziel pretty much nullified it by making sure that skin was wet at the time of the experiments. So I see capacitance here as a moot point, and that the electronicsforu article might be trying to use that to explain DC vs AC effects but the problem is that it doesn't really do that. The effects and data that they are referring to (Dalziel's work and that of others) already nullified the effect of skin capacitance.
 

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