Resistance of the human Body

[LMAO]

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

I am a little late to this party and others might have already mentioned it but this is a really broad question. I think the proper term should be "resistivity" not "resistance". Resistivity is the resistance of "material" over a unit surface area and length (it is directly proportional to length and inversely proportional to surface area). Also, different parts of body have different resistance. I can image your bone is almost a perfect insulator while less rigid parts of your body are better conductors.

So to answer to your question: 120V of voltage and 2.4mA only result in about 0.24mW. You won't probably even feel it. In reality though, 120V will draw much more current through your body.

 

[zog]

The resistance of the human body can vary dramatically depending on several factors. Essentially it is the skin, along with such factors as area of contact, tightness of contact, dryness or wetness of the skin, and cuts, abrasions, or blisters that introduce the variables.

Excluding the skin, human body resistance is about 250 Ohms per arm or leg, and 100-500 Ohms for the torso. The more muscular the person, the lower the resistance. Unless the skin is punctured, the skin will provide additional resistance. The worst-case scenario is considered to be 500 Ohms hand-to-hand. Some typical values of skin resistance are:


(Dry /Wet)
Finger touch 40 k-1 M/ 4-15 k
Hand holding wire 15-50 k/ 3-6 k
Finger-thumb grasp 10-30 k /2-5 k
Hand holding pliers 5-10 k /1-3 k
Hand around pipe (or drill) 1-3 k /0.5-1.5 k
Palm touch 3-8 k/ 1-2 k
Two hands around pipe 0.5-1.5 k /250-750
Hand immersed -------- 200-500
Foot immersed -------- 100-300

Using these typical values, a person can estimate their approximate body resistance. Let?s say a person grabs a wire in a 480/277V panel that they assumed was deenergized while touching the panel door with the other hand. The worker would have about:

250 ohms for the arm +
250 Ohms for the other arm +
32,500 Ohms for the skin
A total resistance of about 33,000 Ohms
277V/33,000 Ohms = 8.4 mA (Mild shock)

Now lets look at the same scenario, but this time with wet or sweaty skin.
250 ohms for the arm +
250 Ohms for the other arm +
4500 Ohms for the skin
A total resistance of about 5000 Ohms
277V/5000 Ohms = 55.4 mA (Respiratory Paralysis, possible fibrillation, may be
fatal)

At voltage levels above 600V, the resistance of the skin ceases to exist; it is simply punctured by the high voltage. For higher voltages, only the internal body resistance impedes current flow. At levels about 2400V and higher, burning becomes the major effect, lower voltages fibrillation and asphyxiation.

How well you are grounded is also a factor. Wearing rubber soled shoes or gloves along with the material between you and ground will add to your total resistance.

Some typical values for different materialsOhms)

Rubber gloves or soles More than 20 M
Dry concrete above grade 1-5 M
Dry concrete on grade 0.2-1 M
Leather sole, dry 100-500 k
Leather sole, damp 5-20k
Wet concrete on grade 1-5 k
(Resistance values for equal areas (130 cm 2) of various materials)

 

[charlie b]

Let me offer the following for your consideration:

• When I discuss electrical safety with others (as I did with my company over lunch last Friday), I usually say that 0.1 amps is sufficient for the shock to be fatal. Let's work with that number for now.
• If a person receives a shock of 0.1 amps from a 120 volt source, then one could conclude that that person's resistance was 1200 ohms at that time.
• There have been cases in which people have died from shock from a 120 volt source.
• We can thereby infer that the human body can have a resistance of 1200 ohms.
• We can further infer that the 3200 and above values do not apply to all humans at all times.

 

[Dennis Alwon]

Let me offer the following for your consideration:

• When I discuss electrical safety with others (as I did with my company over lunch last Friday), I usually say that 0.1 amps is sufficient for the shock to be fatal. Let's work with that number for now.
• If a person receives a shock of 0.1 amps from a 120 volt source, then one could conclude that that person's resistance was 1200 ohms at that time.
• There have been cases in which people have died from shock from a 120 volt source.
• We can thereby infer that the human body can have a resistance of 1200 ohms.
• We can further infer that the 3200 and above values do not apply to all humans at all times.

Okay here is what I am trying to get at. I know that if I get in series of a load side conductor coming from some source with, say, 1200 ohms as you stated, then can I get the same shock or expectation just from touching a 120V source. In both cases there is --using your example, 1200 ohms, but in one case you are completing a circuit and in the other your body is the load.

It seems hard to believe I can get killed just touching a 120V source thru my body without any load other than the resistance of my body. I can understand getting killed when my body tries to complete the circuit. Do you all see what I am thinking--- not sure it is getting across.

 

[GoldDigger]

Okay here is what I am trying to get at. I know that if I get in series of a load side conductor coming from some source with, say, 1200 ohms as you stated, then can I get the same shock or expectation just from touching a 120V source. In both cases there is --using your example, 1200 ohms, but in one case you are completing a circuit and in the other your body is the load.

It seems hard to believe I can get killed just touching a 120V source thru my body without any load other than the resistance of my body. I can understand getting killed when my body tries to complete the circuit. Do you all see what I am thinking--- not sure it is getting across.

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

You are overthinking things Dennis.

Completing a circuit which has a heavy load (say 20A, or 6 ohms from 120V) will not put any greater v thatoltage on your body than just touching 120V directly. Putting a load resistance in series just reduces the current (although not by much); it does not increase it. The presence of that six ohm resistor in series does not magically cause more current to flow though the series circuit.
Now if you are holding both ends of the wire that you are separating while the circuit is live and the load is being driven could cause some inductive kick effects that would drive the voltage across you much higher than the line voltage. And that could be really bad.
But if the wires start out separated (no current through the load) then there can be no inductive effect worth mentioning when you complete the circuit. And whatever effect there is would reduce the voltage across you anyway in that case.

 

[LMAO]

Okay here is what I am trying to get at. I know that if I get in series of a load side conductor coming from some source with, say, 1200 ohms as you stated, then can I get the same shock or expectation just from touching a 120V source. In both cases there is --using your example, 1200 ohms, but in one case you are completing a circuit and in the other your body is the load.

It seems hard to believe I can get killed just touching a 120V source thru my body without any load other than the resistance of my body. I can understand getting killed when my body tries to complete the circuit. Do you all see what I am thinking--- not sure it is getting across.

it all depends how you are getting shocked; i.e., the path through your body. If your hand is touching a grab rail and comes into contact with 120V you'll probably never going to die from electrical power (it can however interfere with your heart signal that propagates through your heart chambers) because the path of least resistance is through your hand. After a minutes or two you'll cook your hand though.

But if you stand up and touch the 120V in a way that the only path available to current is along your body to earth then you'll get knocked out in a couple of seconds.

In general, you should not just be looking at current, resistance or voltage. You should consider power dissipated through your body which is a function of 2 of those 3 variables.

Also, your body is never the main load; say a 120V source is feeding a 120W bulb. The current through your body is roughly the same no matter you are in series with 120W load or in parallel. This about it....

 

[charlie b]

It seems hard to believe I can get killed just touching a 120V source thru my body without any load other than the resistance of my body. I can understand getting killed when my body tries to complete the circuit. Do you all see what I am thinking--- not sure it is getting across.

You have me confused, Dennis. It seems to me that in your first case you would be in parallel with that other load (if there is one), and in your second case you are in series with the load.

For example, in the first case you might have touched an energized light switch with the light bulb turned on. There will be current flowing from the source through the light bulb and back to the source via the neutral wire. There will also be current flowing from the source to the light switch, to your hand, through your body, into the dirt, through dirt to the ground rod, and up the GEC back to the source. Those are two parallel paths. The current through your body does not depend on the resistance of the other path, and it would not matter if there even was another path (e.g., the light bulb had previously burned out and it was an open circuit - no load).

 

[charlie b]

The current through your body is roughly the same no matter you are in series with 120W load or in parallel.

That is an accident of mathematics, not a general rule. A light that uses 120 watts in a 120 volt circuit will have a resistance of 120 ohms. Let's use my earlier estimate of 1200 ohms for the resistance of a human body (pick another value, if you prefer).

• Parallel case: Current in your body = 120 volts divided by 1200 ohms, or 0.10 amps.
• Series case: Current in your body = 120 volts divided by (1200 + 120) ohms, or .09 amps.

So yes, the two results are fairly close to each other. But if you were talking about a 12 watt exhaust fan, the parallel case would still be 0.10 amps, and the series case would be 0.05 amps. In other words, the smaller the wattage value of the series load, the higher its resistance, the higher the total resistance (i.e., resistance of your body added to resistance of series load), the lower the shock current, and the greater your chance of survival.

 

[Dennis Alwon]

You have me confused, Dennis. It seems to me that in your first case you would be in parallel with that other load (if there is one), and in your second case you are in series with the load.

For example, in the first case you might have touched an energized light switch with the light bulb turned on. There will be current flowing from the source through the light bulb and back to the source via the neutral wire. There will also be current flowing from the source to the light switch, to your hand, through your body, into the dirt, through dirt to the ground rod, and up the GEC back to the source. Those are two parallel paths. The current through your body does not depend on the resistance of the other path, and it would not matter if there even was another path (e.g., the light bulb had previously burned out and it was an open circuit - no load).

I was talking about a series shock vs a parallel shock. In one case I am getting shocked by completing the circuit after the load while the other I am just touching 120V source with no load. In both cases I am using my 2 hands. The load, for argument sake has the same resistance as my body.

I was trying to say if the resistance of my body is the same as the load used in the series connection thru my body would the effect be different.

 

[GoldDigger]

I was talking about a series shock vs a parallel shock. In one case I am getting shocked by completing the circuit after the load while the other I am just touching 120V source with no load. In both cases I am using my 2 hands. The load, for argument sake has the same resistance as my body.

I was trying to say if the resistance of my body is the same as the load used in the series connection thru my body would the effect be different.

OK, but if you reread your words carefully you should see that you are implying that if the load resistance is low enough the series result will be *worse* than the parallel result.

 

[Dennis Alwon]

OK, but if you reread your words carefully you should see that you are implying that if the load resistance is low enough the series result will be *worse* than the parallel result.

That is what I am asking and it would be my guess that the series one would be worse-- maybe not

 

[GoldDigger]

That is what I am asking and it would be my guess that the series one would be worse-- maybe not

100% definitely not!
Line voltage drop aside, the closer the series resistance gets to zero the closer the series current gets to the parallel case current. But it is never greater.

 

[Dennis Alwon]

100% definitely not!
Line voltage drop aside, the closer the series resistance gets to zero the closer the series current gets to the parallel case current. But it is never greater.

Thank you for your efforts to help me on this. So the reason this went thru my little brain is because I would have thought more people would be hurt from a parallel shock then there seems to be if all were equal. I get shocked all the time and other then the time I had my boot in water it never affected me much. I guess the resistance in most of our bodies is rarely high enough to cause much damage

 

[GoldDigger]

Thank you for your efforts to help me on this. So the reason this went thru my little brain is because I would have thought more people would be hurt from a parallel shock then there seems to be if all were equal. I get shocked all the time and other then the time I had my boot in water it never affected me much. I guess the resistance in most of our bodies is rarely high enough to cause much damage

Make that "rarely low enough" and I will agree with you.
And it is mostly the skin resistance that counts.

 

[Dennis Alwon]

Make that "rarely low enough" and I will agree with you.
And it is mostly the skin resistance that counts.

Yes , of course-- my bad

 

[charlie b]

That is what I am asking and it would be my guess that the series one would be worse-- maybe not

To add to GoldDigger's response, allow me to mention that resistors in series add up to a higher value (i.e., resulting in a lower current) than either resistor alone. By contrast, resistors in parallel add up to a lower value (i.e., resulting in a higher total current) than either resistor alone. However, looking at only one of the parallel resistors (regardless of how many there are), the current through any one of the resistors is completely independent of the presence, or absence, or resistive values, of any and all others. So indeed, the series one would be less severe. But it may still be severe enough to make for a very bad day.

 

[LMAO]

To add to GoldDigger's response, allow me to mention that resistors in series add up to a higher value (i.e., resulting in a lower current) than either resistor alone. By contrast, resistors in parallel add up to a lower value (i.e., resulting in a higher total current) than either resistor alone. However, looking at only one of the parallel resistors (regardless of how many there are), the current through any one of the resistors is completely independent of the presence, or absence, or resistive values, of any and all others. So indeed, the series one would be less severe. But it may still be severe enough to make for a very bad day.

impedance of the load is typically much much smaller than the resistance of human body so whether you are in series or in parallel with the load makes virtually no meaningful difference.

 

[Sahib]

I get shocked all the time and other then the time I had my boot in water it never affected me much. I guess the resistance in most of our bodies is rarely low enough to cause much damage

Perhaps not only that; the expectation of electric shock keeps the nervous system of the body alert, thereby lessening the effect of electric shock. Incidentally, how many convicts executed on electric chair, using around 4kV, were killed instantly under identical conditions? Answer: Not many (and as a result, electric chair execution is no longer practised in any state) .

 

[edlee]

I can image your bone is almost a perfect insulator while less rigid parts of your body are better conductors.

FYI, it is counter-intuitive but human bone conducts current. It may classed as a semi-conductor though rather than a conductor. And, interestingly, bone demonstrates the piezoelectric effect. It is the fastest growing tissue in the body and there is a lot of biological activity happening. Plenty of info online if you do a quick search.

Interesting discussion.......

 

[GoldDigger]

FYI, it is counter-intuitive but human bone conducts current. It may classed as a semi-conductor though rather than a conductor. And, interestingly, bone demonstrates the piezoelectric effect. It is the fastest growing tissue in the body and there is a lot of biological activity happening. Plenty of info online if you do a quick search.

Interesting discussion.......

Now you've done it!
I am picturing a primitive tribesman building a crystal radio receiver out of bone.....