Noob question: touching 2 electrically common points

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Joined Nov 28, 2018
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Hello everyone, i'm new here and i'm trying to learn about electronics from scratch.

At the end of this page of the primer https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/common-sources-hazard/ , i've read this sentence that is confusing me

Therefore, a person standing on the ground between those two points will be in danger of receiving an electric shock by intercepting a voltage between their two feet!
So in general , if you touch 2 electrically common points of a circuit with your hands or feet, like in the example, you are going to get shocked, right?

But how to calculate the voltage passing through your body, based on the distance between your feet/hands , your body resistance, the length of the wire you are touching and the voltage through its ends ?
 

dl324

Joined Mar 30, 2015
14,458
Welcome to AAC!

Whether you'll get shocked depends on the different in potential, the resistance of your body, and the path any current takes.
 

wayneh

Joined Sep 9, 2010
17,168
Hello everyone, i'm new here and i'm trying to learn about electronics from scratch.

At the end of this page of the primer https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/common-sources-hazard/ , i've read this sentence that is confusing me



So in general , if you touch 2 electrically common points of a circuit with your hands or feet, like in the example, you are going to get shocked, right?

But how to calculate the voltage passing through your body, based on the distance between your feet/hands , your body resistance, the length of the wire you are touching and the voltage through its ends ?
Calculating an accurate number is very difficult. Well, it would be easy if you had all the impedance values for all the various paths that might allow current to flow, but you never do. You have to estimate the impedance of the victim’s shoes in the grass - is it wet? - his socks, his skin, and so on. You also need to know the voltage gradient in the soil between his feet.

The important thing is to identify the potential risk and avoid it, not to quantitate it.
 

Thread Starter

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Joined Nov 28, 2018
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Welcome to AAC!

Whether you'll get shocked depends on the different in potential, the resistance of your body, and the path any current takes.
So in that example, i would get shocked just because my body resistance is lower than the soil's resistance, therefore all the current would "choose" to pass through my body.
Right?
What if i wear very thick rubber boots,
so that my resistance would be higher than the dirt's one?
In that case IMO the current would "choose" to pass through the soil underneath my feet, not through my body.
 

dl324

Joined Mar 30, 2015
14,458
So in that example, i would get shocked just because my body resistance is lower than the soil's resistance, therefore all the current would "choose" to pass through my body.
Right?
Your resistance doesn't need to be lower than the soil. The important factors are that the potential difference be large enough to conduct a current that could hurt you.
What if i wear very thick rubber boots,
so that my resistance would be higher than the dirt's one?
In that case IMO the current would "choose" to pass through the soil underneath my feet, not through my body.
Can't say. Where are you making contact with high voltage? If it's at one point (a hand, foot, or other body part), no current would flow. If you made contact with both hands, the fact that you're wearing rubber boots wouldn't help because the current would flow between your hands.
 

wayneh

Joined Sep 9, 2010
17,168
It’s important to not get hung up on the “path of least resistance”. Like water running off a mountain, current will follow every path available to it. Trickle or torrent, it doesn’t care. If you are part of a path, you’re at risk. It only takes a trickle to hurt you.
 

profbuxton

Joined Feb 21, 2014
419
In the circumstances shown the chances of experiencing a shock are vanishingly small. Note that there is NOT a direct circuit path where one can stand on the current passing through the ground. The return current from the fallen cable will be distributed over the whole of the ground between the fallen cable and the return point. Even if the return path was a single cable on which you were standing the chances of you feeling a shock would be next to zero.
 

MrSoftware

Joined Oct 29, 2013
2,027
In the picture, the guys feet are not on an electrically common place, maybe this is the source of your confusion. The difference in voltage between his feet is 250V, and that difference is what will push current through his body, in one foot and out the other, and cause a shock.
 

Thread Starter

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Joined Nov 28, 2018
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In the picture, the guys feet are not on an electrically common place, maybe this is the source of your confusion. The difference in voltage between his feet is 250V, and that difference is what will push current through his body, in one foot and out the other, and cause a shock.
My source of confusion is that in previous pages of the primer it says that a bird can rest on a high voltage power line cable without being shocked.
And a bird has 2 feet, like the guy.
Then i suppose the bird doesn't get shocked because it's on 2 electrically common points (one for each feet).

Now the guy is in the same situation as the bird, isn't it? Then why is voltage between his feet?

@nsaspook The wikipedia page seems to have an answer, thanks. I was just trying to understand what's the difference with a bird standing on a power cable
 
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MrSoftware

Joined Oct 29, 2013
2,027
The bird and the guy are in 2 totally different situations. The key difference is the conductivity of the surface that they're standing on. The birds feet are close together and on a very conductive surface, the voltage between the birds feet is relatively identical and can be considered common. There is no potential to push current through the bird and it is not shocked. The man is standing on a poor conductive surface, the potential difference in voltage between his feet is great and the two points are not electrically common. There is a large voltage difference between his feet, enough to push current through his body and shock him. The key difference is the metal that the bird is standing on is a good conductor, so the voltage between the birds feet is small. The surface the man is standing on is a very poor conductor, so the voltage between his feet is great.

Think of it like this; Assuming the resistance of aluminum wire is 1ohm per thousand meters, and assume 100A flowing (just making numbers up). If the birds feet are 0.1m apart (big bird) then the potential between the birds feet is 0.01v, not much. Assume the resistance of dirt is 1Mohm per thousand meters and the mans feet are 1m apart, and only 1A flowing. . The difference in potential between the mans feet would be 1000V, enough to shock him. In the real world the ground is not linear like a wire, but this explains the general idea.
 

WBahn

Joined Mar 31, 2012
26,398
My source of confusion is that in previous pages of the primer it says that a bird can rest on a high voltage power line cable without being shocked.
And a bird has 2 feet, like the guy.
Then i suppose the bird doesn't get shocked because it's on 2 electrically common points (one for each feet).

Now the guy is in the same situation as the bird, isn't it? Then why is voltage between his feet?

@nsaspook The wikipedia page seems to have an answer, thanks. I was just trying to understand what's the difference with a bird standing on a power cable
You keep using the term "electrically common" and I think that's the source of your problem. The phrase "electrically common" means that they are at the same electrical potential. But in practice two points are seldom truly at exactly the same potential -- one is almost always at a higher potential than the other. So whether we call them "electrically common" depends on whether that potential difference is enough to matter for the discussion at hand. We might be able to consider two points "electrically common" when talking about one effect but not be able to if we are talking about another.

What you are talking about is known as "step potential" -- namely the voltage potential that appears between your feet as you take a step, usually across a conducting path that has current flowing in it. In general, the voltage that appears across a step is a function of the resistance of the path, the current that is flowing, and the distance involved. For the bird, all three are pretty small -- they are standing on a really good conductor, the currents are kept small (which is why we use high-voltages in the first place), and the distance between the bird's feet is pretty small. So the developed voltage difference is very low and doesn't result in significant current flowing through the bird's body -- hence we can consider the two points at the bird's feet to be "electrically common". But the case of the person on the ground can be very different. The resistivity of the ground usually pretty high and if they are near the point where a failed electrical line is touching the ground then the current between the two points can be very high as well. If they have a large step, then the voltage difference that results can be high enough to cause problems -- keep in mind that there only has to be 50 mA ot 100 mA or so of current through their body to be fatal, so only a tiny, tiny fraction of the total current flowing through the ground has to be diverted to be a problem.

Yes, as you increase your total resistance (thicker shoes, drier skin, whatever) you increase the amount of step potential required to get a troublesome current to flow through your body. But if your current total resistance would result in 1 A (say out of 1000 A of total current flowing). Then increasing your total resistance by a factor of ten would still leave about 100 mA of current flowing through your body with quite possibly fatal consequences.

I don't know how much step potential is a real issue in the case of a downed power line because the currents involved are still relatively low since they spread out in the ground very quickly. But the extremely high currents produced by lightening strikes can result in lethal step voltages very readily. There are videos that have caught the effects. One of the most impressive is of a soccer match where lightening hits the field and many of the players drop where they are because of the step voltages produced. It think all of the players recovered in that case, fortunately.
 
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