"Electrically common" means any points in a circuit that have 0 voltage potential difference between them. So even if there's a 250V difference in potential between point A and ground, and a 250V difference between point B and ground, there is 0 difference in potential between points A & B; therefore they are electrically common.

If there was a 250V difference in potential between point A and ground and a 255V difference between point B and ground, there would be a 5V potential difference between points A & B, and they would not be common to each other.

 

This discussion is off on the wrong path. You cannot model the earth as a high resistance. The original telegraph and telephone systems used only one wire.
What you are ignoring is that the earth has huge capacitance. Hence regardless of how many amps you pump into it, it still remains at zero potential (relatively speaking).

thanks for the response MrChips! But I'm still confused. I just don't understand why the person in the diagram is safe. Why is there no current between ground and the man? Isn't there a complete circuit where current can flow from the negative terminal of the battery to the ground, to the man, then to the load and back to the positive terminal of the battery? I've been reading the e-book in order until I got stuck in chapter 3 on shock current paths. Is the earth acting as a giant capacitor the answer to why the man isn't shocked and why there is not current between the two?

 

If you think of the current flowing the other way, from the positive terminal, through the load, and then into the negative terminal, the current will bypass the person because the person represents a much more difficult path to ground than the direct wire. Current always flows along the path of least resistance.

If there was a resistor AFTER the person on the return wire to ground which had a higher resistance than the person, then the person would be in some danger.

 

"Electrically common" means any points in a circuit that have 0 voltage potential difference between them. So even if there's a 250V difference in potential between point A and ground, and a 250V difference between point B and ground, there is 0 difference in potential between points A & B; therefore they are electrically common.

If there was a 250V difference in potential between point A and ground and a 255V difference between point B and ground, there would be a 5V potential difference between points A & B, and they would not be common to each other.

No. This is incorrect. "Electrically common" is not about Point A and Point B being at the same potential. Point A and Point B can be at the same potential and still not be "Electrically common".

"Electrically common" means that there is a low impedance path between Point A and Point B.

In practical terms it means that there is zero resistance between Point A and Point B.

 

thanks for the response MrChips! But I'm still confused. I just don't understand why the person in the diagram is safe. Why is there no current between ground and the man? Isn't there a complete circuit where current can flow from the negative terminal of the battery to the ground, to the man, then to the load and back to the positive terminal of the battery? I've been reading the e-book in order until I got stuck in chapter 3 on shock current paths. Is the earth acting as a giant capacitor the answer to why the man isn't shocked and why there is not current between the two?

A person is in danger of being electrocuted if there is a path for electric current to flow through the body. A current as low as 15mA can be fatal. A potential difference between two parts of the body would be required for current to flow. In your diagram, if the circuit at which the hand comes into contact is at the same potential as where his foot touches the ground, there would be zero potential difference and hence zero current.

The problem with the diagram is that this person is NOT safe.
There is still current flowing in the bottom wire back to the power source. Since there is current flowing, there is a potential drop along this wire, albeit small. Hence there is a potential drop between his hand and foot.

 

I took the diagram earlier directly from the e-book on this site http://www.allaboutcircuits.com/vol_1/chpt_3/3.html. It's either wrong or that I'm leaving some very important context related to the diagram.

 

I wonder how much wiser the OP is about the concept of common potential at this stage of the discussion? One point that still may not have been grasped is that the current will flow (very) preferentially through a low resistance wire than through a body return to ground.

Typically the ground return resistance from an appliance should be low enough to save someone in our stick-man's situation from a fatal shock. Guaranteed cast-iron safety is hard to engineer, but in many cases the voltage rise along the ground return wire will not be enough to make a fatal shock likely.

Of course, the ground return may be excessively long, or made with thin-gauge wire. Stick-man may happen to be stark naked and up to his waist in sea water... but with any luck he won't.

 

I wonder how much wiser the OP is about the concept of common potential at this stage of the discussion? One point that still may not have been grasped is that the current will flow (very) preferentially through a low resistance wire than through a body return to ground.

Typically the ground return resistance from an appliance should be low enough to save someone in our stick-man's situation from a fatal shock. Guaranteed cast-iron safety is hard to engineer, but in many cases the voltage rise along the ground return wire will not be enough to make a fatal shock likely.

Of course, the ground return may be excessively long, or made with thin-gauge wire. Stick-man may happen to be stark naked and up to his waist in sea water... but with any luck he won't.

I think I finally feel more confident with this subject now.
To make sure however, in the picture earlier the man should be "safe" because although there is a current path through the ground and to the man, the current would be significantly less compared to the current that would be following the path straight to the load and back to the positive terminal (the path with least resistance). And this is why appliances are grounded?

Thanks for all the help everyone. If this is right and the reason then I think I'm covered for this topic.

 

JasonL,

Yes, I think you have gotten the message by now. I didn't say the stick man is either "shocked" or "not shocked" but that the man is NOT safe, i.e. there is still a potential hazard.

If that bottom wire (the return path) is a very long cable or a narrow gauge wire, it may have substantial resistance. This would present an electrical hazard to our man. So when we refer to "GROUND", ideally we would like a zero resistance path. Unfortunately, many times this is not the case. (This is why in many situations, industrial computer interfaces call for optical isolation... and that's a whole new thread!)

 

This discussion is off on the wrong path. You cannot model the earth as a high resistance. The original telegraph and telephone systems used only one wire.
What you are ignoring is that the earth has huge capacitance. Hence regardless of how many amps you pump into it, it still remains at zero potential (relatively speaking).

I'm not quite sure I understand the capacitance of the earth. I've drawn a picture of what I think you mean, but I'm not sure. I have the capacitance of the earth in parallel with the resistance of the ground. When a short circuit happens, the voltage across the ground resistor and the ground capacitor should increase, since the load no longer has a voltage drop. So although the ground has high resistance and would normally draw little current, the capacitor will draw high current until the potential across the capacitor equals that of the generator? Is this what you are saying?

 

john avery,

No, we tend to think of a capacitor as a pair of parallel plates but any object is a reservoir for electrons. The top globe of a van der graaf generator or the door knob on your door handle is capable of holding a charge. Thus the earth itself, because of its massive size can be considered an infinite sink for charge. Hence any point in a circuit that has a low impedance path to earth ground will be at earth potential - which we use as our zero-volts reference point.

 

The earth is an infinite sink, but in order for an electron to go through someone touching the hot line and into the ground, doesn't another electron have to flow back from the ground to the generator? Otherwise all the electrons of the powerline would be lost and not replaced.

So say someone touches the hot line and touches a piece of plumbing at the same time. An electron will go through that person and into the ground. At the same time, an electron near where the neutral is tied to ground wire (at the breaker panel) will flow into the neutral wire. So I thought that was like two parallel plates, one plate at where the hot wire faults to ground, and the other plate at the breaker panel. If one electron flows into one plate, another electron leaves the other plate.

Or something like that. I have no idea how to model the earth, and I'm confused whether the earth is a conductor or resistor. If you put an ohmeter in the soil, do you measure something high? Does it depend on the distance between the leads?

 

Apart from the special case of superconductors, all conductors nave some resistance, although not all of them have the linear V/I behaviour idealised in Ohm's Law.

Any practical earth connection will have some resistance, maybe not very linear but certainly significant. Considerable effort has to be made to construct "earths" which can handle large currents without developing an excessive local voltage with respect to the earth as a whole.

Putting small probe leads into the soil will show a relatively big resistance, the exact level depending on the area of the probe tips, the level of soil moisture and chemical content. Much of the resistance will be localised to the volume close to the probes, so past a certain distance the spacing of the probes may not have so much effect.

 

The earth is an infinite sink, but in order for an electron to go through someone touching the hot line and into the ground, doesn't another electron have to flow back from the ground to the generator? Otherwise all the electrons of the powerline would be lost and not replaced.

So say someone touches the hot line and touches a piece of plumbing at the same time. An electron will go through that person and into the ground. At the same time, an electron near where the neutral is tied to ground wire (at the breaker panel) will flow into the neutral wire. So I thought that was like two parallel plates, one plate at where the hot wire faults to ground, and the other plate at the breaker panel. If one electron flows into one plate, another electron leaves the other plate.

Or something like that. I have no idea how to model the earth, and I'm confused whether the earth is a conductor or resistor. If you put an ohmeter in the soil, do you measure something high? Does it depend on the distance between the leads?

Ok, I retract what I said about the earth being a big capacitor. It is a conductor, resistor, capacitor, inductor and a whole pile of s*** in between. The earth is obviously way too complex to model in simple terms.

Please don't anyone take me out of context. For the purpose of this thread, we can simplify the model of the earth as being a huge ground plane, i.e. a massive expanse of copper. (Ok, don't hassle me with this. I know that the resistivity of the earth is high compared to copper.)
But the model we are going to use is that there is zero potential at any and every point along the surface of the earth.)

Now see if this will work for you in your analysis of ground-faults.

 

Here is a true story. We moved into a new house one year after it was built. In one spare bedroom I had a computer plugged into one wall outlet. On the opposite wall I had a printer plugged in. Every time I connected the serial cable from the printer to the computer, the GFI in the breaker panel would trip.

I soon figured out that there was a fault in the house wiring somewhere. Through a successive approximation technique I was able to narrow it down to a wall outlet remote from the original bedroom but on the same circuit. Behind the culprit outlet the neutral line was touching the grounding wire. It turned out that the computer and the printer were on separate circuits and by connecting the computer to the printer I was completing a ground connection.

Why did the GFI not trip out in the first place? On further inspection, this was exactly what was happening when the electrical contractor closed the main breaker. I can only guess that it must have been close to quitting time. The simple solution for them was they cut the ground lead at the GFI thereby effectively disabling the safety function of the GFI that was clearly indicating that a fault was present.