Resistance of the human Body

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Dennis Alwon

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I was listening to a video by mike holt and I starting thinking. That is usually a problem for me...

Anyway I found that the National Institute for Occupational Safety and Health did studies that states the human body can have a resistance of up to 100,000 ohms but about 95% of the people tested had a resistance of 3,200 ohms.

Okay lets look at a scenario where someone is wet and there resistance is 50,000 ohms. Simple math I = V/R = 120V (assume this)/50,000 = 2.4 ma.

We know that a gfci will trip between 4-6 ma so if we come in contact with a grounded object and 120V on the load side of a GFCI will that gfci trip? I say know but I am curious if I am missing something. I am assuming anything over 4- 6 ma threshold is a sure trip also
 

Tony S

Senior Member
I?ll ask a couple of simple questions.

What voltage?
Where on the body?

It makes a big difference to the end results. Dead or just swearing at yourself for being a complete idiot.

I?ve survived 10,000V to ground and it was my own stupid fault. I didn?t follow protocol. I?m a stickler for it now.
 

Dennis Alwon

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I?ll ask a couple of simple questions.

What voltage?
Where on the body?

It makes a big difference to the end results. Dead or just swearing at yourself for being a complete idiot.

I?ve survived 10,000V to ground and it was my own stupid fault. I didn?t follow protocol. I?m a stickler for it now.

In the formula I stated 120V.

Lets say from arm to arm as I believe that was what the study used.
 

don_resqcapt19

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Dennis Alwon;1652077[SIZE=3 said:
]...[/SIZE]
We know that a gfci will trip between 4-6 ma so if we come in contact with a grounded object and 120V on the load side of a GFCI will that gfci trip? I say know but I am curious if I am missing something. I am assuming anything over 4- 6 ma threshold is a sure trip also
The standard says the GFCI must not trip if the current is less than 4mA, it is permitted to trip if the current is 4mA to 6mA, and it must trip if the current is 6mA and above.
 

Dennis Alwon

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The standard says the GFCI must not trip if the current is less than 4mA, it is permitted to trip if the current is 4mA to 6mA, and it must trip if the current is 6mA and above.

Okay so you agree it will not trip in the situation above but you should not get killed by it

Now a circuit with no gfci protect and let's say you are in the 95% group and have the resistance of 3,200 ohms = 37ma. According to the chart below why do we not get hung up on a simple 120v shock just thru our bodies based on the images below. The second one is really based on mass so there is variations


ry%3D400



ry%3D400
 

Tony S

Senior Member
Any shock across the chest cavity is liable to cause heart defibrillation.

50mA is supposed to be the limit, for defibrillation, that is a very loose figure. Your body mass and general health have a big part to play. Blood sodium levels cause a difference in body resistance.


How do you test a GFCI?
Our standard method is a ?ramp test? where the current is raised across the RCD (GFCI). There are strict limits on the time/current characteristics the device should operate at.



BTW
I was very lucky my right arm copped the lot. I didn?t escape unscathed, I was found unconscious and badly bruised after I hit the wall. 30 years on and the scars are still on my arm.
 

don_resqcapt19

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...
How do you test a GFCI?
Our standard method is a ?ramp test? where the current is raised across the RCD (GFCI). There are strict limits on the time/current characteristics the device should operate at. ...

The only approved test method is the test button that is part of the GFCI. If you push the button and the device trips, the device is working correctly.
 

junkhound

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Many of the charts do not tell the entire story.

For instance, fibrillation will only start at the low levels shown IF the shock occurs during the T-wave portion of the heartbeat, so actually the fibrillation part should have a time duration of at least the duration between a person's pulses, less time and statistics come into play (luck?).

The fatality curves first published in the 50's were based on lethalitiy of 1/2 of 1%. I've gotten 3 kV shocks from > 1A capable circuits twice in 70 years, lucky with the timing I guess.
Dozens of shocks at 10 kV or more for <10 mA current limited sources (e.g TV and CRT monitor HV circuits, oil burner and neon sign xfmrs, etc).

Painful is subjective and frequency related. About 20 mA actually feels therapeutic at 60 Hz, but starts being painful at 50 Hz, very unpleasant at 25 Hz, and acute 'displeasure' at under 6 Hz.

There is one recorded fatality I have read about of fatal electric shock at 18 Vac, 60 Hz. Understand that was in a hospital setting were person had catheters, etc. Probably one of the precursors for hospital grounding rules in NEC.


Sweaty hands with unbroken skin are as low as 400 ohms hand to hand, so even 120 Vac risky then. If you have a splinter thru the skin impedance can be much lower.

Very extensive subject.
 

templdl

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I would start with the conductivity being based upon the connection to the devices that are made. Depending upon whatyou or how the current passes tyhrouHow hard is the surface being touched or grabbed by your hand(s), is your hand(s) damp or wet etc.
 

GoldDigger

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The greater part of the total resistance measured at low voltage is the resistance of dry skin itself. Once you have a high enough voltage to punch through that the resistance of the salty wet tissue inside the body is much lower.
I would not use the 100,000 ohm or even 30,000 ohm values to calculate current from a line voltage exposure, especially if the contact point is sharp!
 

Dennis Alwon

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I would not use the 100,000 ohm or even 30,000 ohm values to calculate current from a line voltage exposure, especially if the contact point is sharp!


Sure you can't predict every case nor can we assign a single value of ohms in all instances. But the fact is it is possible to be up at the 3,200 ohms as stated earlier. So my question is can we get hung on that-- I say yes if there is no gfci. Interestingly enough when I was a kid I worked with my dad and was standing in water in a hallway. I touched a wire and it grabbed me but I was able to shake it loose somehow.
 

Rick Christopherson

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I only skimmed the thread, but I believe one thing you are overlooking is the parallel paths through the human body. For example, if you have 1-amp entering a fingertip, the amperage through a 1-inch square in the center of the body will be a small fraction of that 1-amp. That 1 amp is concentrated at the narrow finger, but as it passes into larger areas of the body it is spread out across the entire cross sectional area.

It is much like the conductivity of the earth. Dirt is not a low resistance conductor, but when you take into account the infinite parallel paths, the total resistance becomes very low.
 

Dennis Alwon

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So how do these charts make sense up above. They did studies that show that at certain levels of ma the body reacts to it. At 37ma it is way above the let go level
 

GrayHair

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I'd never given any consideration to anything like this:
'Another interesting point to consider is that in addition to acting like a resistor, the epidermis acts like a capacitor if placed in contact with a piece of metal (the underlying tissue is like one plate of a capacitor and the metal surface is like the other plate - the dry epidermis is the less-conductive material or "dielectric" in between) . In cases of electrocution by a DC voltage source, this capacitive property has little importance. But if the electrocution is by an AC source, the epidermis's natural resistance is "shorted out", allowing the current to bypass that part of the body's resistance and making the body's total resistance much lower.'​
https://van.physics.illinois.edu/qa/listing.php?id=6793

I feel the charts are "best estimates" since we are all infinitely variable. While Xma may bring a muscular reaction in person B but not in person C, would the same current bring the exact same reactions in the same persons next week?

My 2 cents.
 

junkhound

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would the same current bring the exact same reactions in the same persons next week


Have actually done that test with both feet in salt water and double isolated variac with current limit.

Consistent reaction for both self and wife over a span of years. DW threshold for everything is about 1/3 of my levels (e.g I cannot even feel 2 mA, DW hand to hand is already at startle shock level at 2 mA. DW has never allowed me to test her above 7 mA, too painful for her.

Safe way to verify this for yourself: small 9 V battery, put your tongue on the terminals, daily or weekly after risnsing mouth with the same pH water each time and see if there is any change in perception for you.
 

junkhound

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capacitance:

I have very large hands, and one hand around a pipe is about 12 nF (0.012 uF). That is about 210k ohms at 60 Hz, so the capacitive effect is minimal, e.g 210 k reactive in parallel with say a 20 k ohm skin resistance and 400 ohm inner body resisitance only raises the total current from just under 6 mA to 6.03mA.

Thus at 60 Hz, capacitance is negligible - the folks in the link may know electrical theory, but never bothered to quantify any comments.

If frequency higher, then skin effect (current flow on outer surfaces of conductor) starts to have large effect.

Those of you who are old enough to have serviced old tube televisions sets may recall that if one came in contact with the high voltage plate terminal of the horizontal oscillator tube (e.g 6BQ5), you would smell and feel your skin burning but never feel any electrical shock. The 15 kHz oscillator frequency current all flowed on the outside surface of your skin.
 

Fulthrotl

~Autocorrect is My Worst Enema.~
So how do these charts make sense up above. They did studies that show that at certain levels of ma the body reacts to it. At 37ma it is way above the let go level

well, those charts were based on research data that may be inaccurate.
if you think about it, how long ago was the original data produced, and
how many people have used it without question? once established as
fact, it's not likely to be questioned.
 

mivey

Senior Member
I was listening to a video by mike holt and I starting thinking. That is usually a problem for me...

Anyway I found that the National Institute for Occupational Safety and Health did studies that states the human body can have a resistance of up to 100,000 ohms but about 95% of the people tested had a resistance of 3,200 ohms.

Okay lets look at a scenario where someone is wet and there resistance is 50,000 ohms. Simple math I = V/R = 120V (assume this)/50,000 = 2.4 ma.

We know that a gfci will trip between 4-6 ma so if we come in contact with a grounded object and 120V on the load side of a GFCI will that gfci trip? I say know but I am curious if I am missing something. I am assuming anything over 4- 6 ma threshold is a sure trip also
Not with that scenario.

Okay so you agree it will not trip in the situation above but you should not get killed by it
correct

Now a circuit with no gfci protect and let's say you are in the 95% group and have the resistance of 3,200 ohms = 37ma. According to the chart below why do we not get hung up on a simple 120v shock just thru our bodies based on the images below.
Who says you won't? Most likely, the resistance is going to be higher than the 3,200 ohms unless we make very good contact and the skin is not dried out. Older skin is probably drier. Desk jockeys probably have moister skin. I remember back in the day they used to test voltage by touching the wire. Also, much like a ground rod, the earth is a good conductor but it is the connection to earth that is difficult. Body resistance is one thing but contact resistance is also part of the circuit.

Sure you can't predict every case nor can we assign a single value of ohms in all instances. But the fact is it is possible to be up at the 3,200 ohms as stated earlier. So my question is can we get hung on that-- I say yes if there is no gfci.
correct.

So how do these charts make sense up above. They did studies that show that at certain levels of ma the body reacts to it. At 37ma it is way above the let go level
Again, the resistance tends to be higher. Now kneel in some moist ground or lay up against some duct work and you will lower the circuit resistance because you lower the contact resistance. Brushing or casually touching a conductor is not the same as making high-pressure contact.
 
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