Why will 240 volt condenser kill me but not 7,000 volt electric fence?

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Summary:

Why will an electric shock of 240 volts of AC current kill me but an electric shock from a 7,000 volt electric fence won't kill me?

Main Question or Discussion Point

I used to attend an HVAC program at a trade school. One time my instructor and i were working on either the outdoor unit of a split-system heat pump or the condenser of a split-system straight air-conditioner. Even though the unit we were working on was either an "outdoor unit" of a heat pump or a condenser, the outdoor unit or condenser was indoors, inside the trade school. My instructor & i were discussing the contactor. The outdoor unit/condenser was live with 230 volts. I put my finger on the metal contacts of the contactor to point out to my instructor what i was talking about. My instructor said the following to me: "The only reason you are still alive is that this concrete floor beneath us provided enough resistance to ground that the current did not want to go through your body." My instructor said that if i touched the contactor with my bare fingers on a normal outdoor unit while standing on the bare earth, the electric shock would have killed me.

But i have been shocked by a 7,000 volt electric fence on the leg once. One guy i know told me that one time his arm brushed a live electric fence, and the electric current went through his arm. He said the current hurt his arm briefly, but he is still alive. Perhaps the fact that the current only went thru my leg and not thru my heart explains why the electric fence did not kill me. But why did the shock from the electric fence not kill the guy whose arm brushed against electric fence? To me, it seems like if you get shocked on your arm, the current would go thru your heart on the way to the ground.

One possibility i thought of is maybe the electric fence is powered by DC current, which i have heard is not as dangerous as AC current.

Why would an electric shock from a mere 230 volts of AC current kill me, but a shock from a 7,000 volt electric fence not kill me?
 

Answers and Replies

  • #2
anorlunda
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It's more complicated than just voltage.

You should read this
https://en.wikipedia.org/wiki/Electrical_injury
In addition to voltage it depends on skin and body resistance, and point of entry and several other factors.

1586379035311.png


Likely the highest voltage we may ever experience is the static shock that we might get after walking across a carpet in winter. I used to get static sparks from the door handle on my car in winter. Electrostatic shocks can be as much as 10 or 20 KV. They can be dangerous to certain people, but usually not.
 
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  • #3
phinds
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Summary:: Why will an electric shock of 240 volts of AC current kill me but an electric shock from a 7,000 volt electric fence won't kill me?
I remember in high school I used to get hit with about 30,000 or 40,000 volts every time the physics teacher turned the Van De Graaff generator on. I loved that thing.
 
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  • #4
Janus
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Voltage doesn't kill, its the amperage passing through your body that does. The 230 volt condenser had enough stored energy to be able to deliver enough amperage to kill you under the right conditions. ( luckily for you they were not.)

However electric fences are specifically designed with a current limit. No matter what, they cannot deliver enough current to kill you.

When I was much younger than I am now, I built a simple DC to DC converter. It was a simple circuit that "pumped up" a 9 volt battery to 120 v or so. It would give you a bit a shock if you touched the ends.

Now the 9 v battery providing the power could only deliver maybe 5 ma of current even if directly shorted out.
That's a wattage of something in the order of 45 mw.

If I pumped the output voltage up by a factor of 10, I still can't increase the power the battery can provide. This means, even if shorted out, the max current my convertor could supply would be 0.5 ma. It takes around 1/10 of a amp to be fatal and 0.0005 amp comes way short of that.
 
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  • #5
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Voltage doesn't kill, its the amperage passing through your body that does.
This isn’t strictly true, although I acknowledge that it’s hard to encapsulate a complex subject in one sentence. The thing that kills you is energy, E=VIT. You do mention energy in the very next sentence. In mains power situations, there is essentially limitless energy available, so mains safety focuses on current. I think that’s where the confusion lies.

For example, a VdG generator shock will deliver a very large current through you, but it decays so quickly that an insignificant amount of energy is transferred. Defibrillator shocks are dispensed in joules, too.

The voltage must be high enough to drive sufficient current (say 30mA) through the heart, and be maintained long enough for sufficient energy transfer.

Like the VdG generator, your electric fence had enough volts and amps, but can’t deliver that for enough time. The shock was also through your leg, not heart. As for your condenser shock, the path through you and the concrete and back to the supply transformer was too high in resistance to allow enough current.
 
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  • #6
russ_watters
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I remember in high school I used to get hit with about 30,000 or 40,000 volts every time the physics teacher turned the Van De Graaff generator on. I loved that thing.
We formed chains to get to the teacher...
 
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  • #7
russ_watters
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....I acknowledge that it’s hard to encapsulate a complex subject in one sentence. The thing that kills you is energy, E=VIT... In mains power situations, there is essentially limitless energy available, so mains safety focuses on current. I think that’s where the confusion lies.

For example, a VdG generator shock will deliver a very large current through you, but it decays so quickly that an insignificant amount of energy is transferred. Defibrillator shocks are dispensed in joules, too.
But there still has to be a time limit for the energy transfer, right? Like, it has to be delivered in a certain fraction of a heart beat?

For reference, google gives me common defibrillator shocks in the range of a few hundred Joules. A mid-sized residential air conditioner (3 Ton) pulls about 4,000 J/s, and the circuit would be capable of much more for a short time before the circuit breaker blows.
 
  • #8
anorlunda
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But there still has to be a time limit for the energy transfer, right?
Look at the curves in post #2.
 
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  • #9
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As for your condenser shock, the path through you and the concrete and back to the supply transformer was too high in resistance to allow enough current.
I thought that if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would just go through my body to the earth, and the earth would absorb all the electric current, and the current would stop at the Earth.

Your post implies that i was mistaken. So if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would go through my body and through the ground (dirt or mud) and then back to the supply transformer?

If so, what does the concrete floor have to do with why i was not electrocuted? Does a square foot of concrete contain substantially more resistance than a square foot of the dirt soil on the bare earth?

‐--‐---------
P.S. when you respond to me, please keep in mind that i do understand the concept of a normal circuit for, say, a condenser fan motor. I understand that for a normal circuit you have to have a power supply and a line 1 going from power supply to the load and then a line 2 from the load back to power supply.

However, i thought that when a person's body gets an electric shock, i thought that is a "short to ground" and different than a normal circuit. I thought that in a short to ground, the current just takes the shortest path to the earth, and the current just stops at the earth.
 
  • #10
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But there still has to be a time limit for the energy transfer, right? Like, it has to be delivered in a certain fraction of a heart beat?

For reference, google gives me common defibrillator shocks in the range of a few hundred Joules. A mid-sized residential air conditioner (3 Ton) pulls about 4,000 J/s, and the circuit would be capable of much more for a short time before the circuit breaker blows.
Yes, as per the curves in the graph above. As I say, it’s hard to encapsulate this complex subject in one sentence, but at least E=VIT gives a better understanding than ‘it’s not the volts, it’s the amps’.
 
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  • #11
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I thought that if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would just go through my body to the earth, and the earth would absorb all the electric current, and the current would stop at the Earth.

Your post implies that i was mistaken. So if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would go through my body and through the ground (dirt or mud) and then back to the supply transformer?

If so, what does the concrete floor have to do with why i was not electrocuted? Does a square foot of concrete contain substantially more resistance than a square foot of the dirt soil on the bare earth?

‐--‐---------
P.S. when you respond to me, please keep in mind that i do understand the concept of a normal circuit for, say, a condenser fan motor. I understand that for a normal circuit you have to have a power supply and a line 1 going from power supply to the load and then a line 2 from the load back to power supply.

However, i thought that when a person's body gets an electric shock, i thought that is a "short to ground" and different than a normal circuit. I thought that in a short to ground, the current just takes the shortest path to the earth, and the current just stops at the earth.
The path through you and to ground is as normal a circuit as any. Current does not stop at the earth, but continues through it and back to the supply transformer up its earth/ground wire. Look up ‘earthing systems’.

If you were isolated from the ground, floating in mid-air, you may feel a small tingle from capacitive coupling. This is not a ‘normal’ circuit, of course.

I’m not sure of the resistivity of concrete vs earth, but in buildings, concrete tends to be damp-proofed and so drier. Certainly not a reliable way of preventing shock!
 
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  • #12
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The path through you and to ground is as normal a circuit as any. Current does not stop at the earth, but continues through it and back to the supply transformer up its earth/ground wire. Look up ‘earthing systems’.

If you were isolated from the ground, floating in mid-air, you may feel a small tingle from capacitive coupling. This is not a ‘normal’ circuit, of course.

I’m not sure of the resistivity of concrete vs earth, but in buildings, concrete tends to be damp-proofed and so drier. Certainly not a reliable way of preventing shock!
I am not necessarily saying that Guineafowl is wrong, but this does not parse with my understanding of what happens when a person is electrically shocked.

Can any electrical engineers or physicists give me a second opinion or a third opinion?
 
  • #13
Dr_Nate
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I am not necessarily saying that Guineafowl is wrong, but this does not parse with my understanding of what happens when a person is electrically shocked.

Can any electrical engineers or physicists give me a second opinion or a third opinion?
I don't see anything wrong with it. Can you be more specific what you disagree with?
 
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  • #14
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I don't see anything wrong with it. Can you be more specific what you disagree with?
Guineafowl's first paragraph in post #11, describing what happens when a person is electrically shocked. I contend that the current goes to the Earth and stops. Guineafowl says otherwise.
 
  • #15
Dr_Nate
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I thought that if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would just go through my body to the earth, and the earth would absorb all the electric current, and the current would stop at the Earth.

Your post implies that i was mistaken. So if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would go through my body and through the ground (dirt or mud) and then back to the supply transformer?

If so, what does the concrete floor have to do with why i was not electrocuted? Does a square foot of concrete contain substantially more resistance than a square foot of the dirt soil on the bare earth?

‐--‐---------
P.S. when you respond to me, please keep in mind that i do understand the concept of a normal circuit for, say, a condenser fan motor. I understand that for a normal circuit you have to have a power supply and a line 1 going from power supply to the load and then a line 2 from the load back to power supply.

However, i thought that when a person's body gets an electric shock, i thought that is a "short to ground" and different than a normal circuit. I thought that in a short to ground, the current just takes the shortest path to the earth, and the current just stops at the earth.
If your power supply emitted electrons and the electrons just go into the ground and stay there, what would that means for the electric charge on your power supply and the electric charge of the ground? Where do you think the electrons 'want to go'?

I think you can look up the resistivity of soil and concrete yourself.

I think it also would be informative if you did calculations on two circuits. Both circuits have some high voltage. In the first circuit, you are the only resistor between the high voltage and ground. In the second circuit, add a concrete in series between you and the ground. Calculate the current in both cases. Perhaps you have heard that the voltage drop across a component is because it dissipates energy. After finding those two currents calculate the power that gets deposited in your body in each case.
 
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If your power supply emitted electrons and the electrons just go into the ground and stay there, what would that mean for the electric charge on your power supply and the electric charge of the ground?
If the electrons just went into the ground & stayed there, i did not think it would make the earth excessive negatively charged because the Earth is so massive that the extra electrons would be negligible (perhaps i was wrong about that). The power supply would be the electrical power plant. If electrons just went into the ground & stayed there, i did not think that it would make the power supply (the electrical power plant) excessively positive charge because i thought the power plan was so big that the effect would be negligible.



Where do you think the electrons 'want to go'?
The electrons want to go to any positively charged area.

I think you can look up the resistivity of soil and concrete yourself.
Maybe another day. Today i can only access the internet on my cell phone, not my computer. That would be something i would only do on my PC with its larger screen and keyboard.

I think it also would be informative if you did calculations on two circuits. Both circuits have some high voltage. In the first circuit, you are the only resistor between the high voltage and ground. In the second circuit, add a concrete in series between you and the ground. Calculate the current in both cases. Perhaps you have heard that the voltage drop across a component is because it dissipates energy. After finding those two currents calculate the power that gets deposited in your body in each case.
Thats a bit above my skill level.
 
  • #17
Dr_Nate
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The electrons want to go to any positively charged area.

Thats a bit above my skill level.
They may not be the same electrons, but the electrons in ground certainly push on electrons nearby and the positive charge of the power supply pulls on electrons near it. In the end there is a flow of electrons from the ground back to your power supply.

I have a hard time believing it is above your skill level. It's a very simple circuit. Pretend it's DC and just use ##V=IR## and ##P=I^2R##.
 
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  • #18
russ_watters
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If the electrons just went into the ground & stayed there, i did not think it would make the earth excessive negatively charged because the Earth is so massive that the extra electrons would be negligible (perhaps i was wrong about that). The power supply would be the electrical power plant. If electrons just went into the ground & stayed there, i did not think that it would make the power supply (the electrical power plant) excessively positive charge because i thought the power plan was so big that the effect would be negligible.
You're not wrong, but for the purpose of circuit analysis, the circuit must be complete/balanced, a bunch of electrons are discharging from a power circuit to ground in one place, the same number has to be absorbed in another place otherwise the number of electrons in the circuit would be changing.

The difference is in looking at the Earth vs looking at the circuit.

The other side of the coin is lightning, where the Earth and sky basically act like two opposite plates in a capacitor, and there is a net loss of charge in Earth during the development of a thunderstorm that gets equalized by lightning.
 
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  • #19
russ_watters
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Look at the curves in post #2.
Thanks...it was too complicated-looking to read the first time I scrolled through the thread. Per my reasoning, the S-curve where the red zone gets more severe happens between 0.1 and 1.0 seconds, which are on the order of the duration and spacing of heartbeats.
 
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  • #20
phinds
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@timmeister37 I really have nothing to add to what has aleady been said, but since you asked me specifically to comment on this post, here it is:

I thought that if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would just go through my body to the earth, and the earth would absorb all the electric current, and the current would stop at the Earth.
No, it returns to the fence charger. If the fence charger is not properly grounded, the fence won't work.

Your post implies that i was mistaken. So if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would go through my body and through the ground (dirt or mud) and then back to the supply transformer?
Correct.

If so, what does the concrete floor have to do with why i was not electrocuted? Does a square foot of concrete contain substantially more resistance than a square foot of the dirt soil on the bare earth?
Concrete is NOT a path to ground (unless it's wet everywhere and you are standing in a puddle that has a path to ground around the concrete). see post 23
 
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  • #21
Averagesupernova
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... the current would just go through my body to the earth, and the earth would absorb all the electric current, and the current would stop at the Earth.
The above is basically gibberish. Can't make any sense of it at all.
Your post implies that i was mistaken. So if i was standing on the bare earth and touched the contactor on the condenser and got shocked, the current would go through my body and through the ground (dirt or mud) and then back to the supply transformer?
Yes
P.S. when you respond to me, please keep in mind that i do understand the concept of a normal circuit for, say, a condenser fan motor. I understand that for a normal circuit you have to have a power supply and a line 1 going from power supply to the load and then a line 2 from the load back to power supply.

However, i thought that when a person's body gets an electric shock, i thought that is a "short to ground" and different than a normal circuit. I thought that in a short to ground, the current just takes the shortest path to the earth, and the current just stops at the earth.
No. It in some manner is tied back to the source. Usually ground rod or several. Also a ground to a water pipe. You keep asking for an engineer or physicist to weigh in here. Just what do you think an engineer or physicists post would look like compared to the posts you've been reading?
 
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  • #22
Averagesupernova
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@timmeister37 I really have nothing to add to what has aleady been said, but since you asked me specifically to comment on this post, here it is:

No, it returns to the fence charger. If the fence charger is not properly grounded, the fence won't work.

Correct.

Concrete is NOT a path to ground (unless it's wet everywhere and you are standing in a puddle that has a path to ground around the concrete).
Concrete IS a path to ground. Look up concrete encased electrode.
 
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  • #23
phinds
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Concrete IS a path to ground. Look up concrete encased electrode.
o:) I'll be darned. Did not know that. o:)

I was thinking of dry concrete on dry ground, neither of which condition is likely except for thin slabs in the desert.
 
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  • #24
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Concrete IS a path to ground. Look up concrete encased electrode.
Then why did my HVAC instructor say that the only reason i am still alive after touching the live contacts on the contactor is/was the concrete floor that i was standing on?
 
  • #25
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The above is basically gibberish. Can't make any sense of it at all.
It is crystal clear.


No. It in some manner is tied back to the source. Usually ground rod or several. Also a ground to a water pipe. You keep asking for an engineer or physicist to weigh in here. Just what do you think an engineer or physicists post would look like compared to the posts you've been reading?
I don't know.
 

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