GroundEd vs. GroundIng: Why Does One Trip the Breaker While the Other Doesn't?

[XPTPCREWX]

[Ground"Ed" vs. Ground"Ing"]

Why does the ground"Ing" wire in a circuit draw high amounts of current and trips the breaker when a ground"Ed" (neutral) wire does not?

 

[MATLABdude]

Why does the ground"Ing" wire in a circuit draw high amounts of current and trips the breaker when a ground"Ed" (neutral) wire does not?

Ah, I see you've read the APC Article! Both will, assuming you short these to the live wire. It's just that the ground"Ed" wire usually has the load between it and the live wire. And hopefully, your breaker trips in both situations due to the large amount of current through the live wire.

Well, a normal breaker will do that. A GFCI (Ground Fault Circuit Interrupter) will (hopefully) detect an inbalance in the supply and return wires and kill the circuit before an appreciable amount of current goes through you and the ground wire.

 

[Averagesupernova]

XPTPCREWX: I don't have time to post right now but I will try to post something later tonight to help you get it straight in your head. Judging from your posts in another thread I'm guessing you may need some more explanation.

 

[XPTPCREWX]

The Way I see it, is that the 1st drawing is what works...(Blue Line)

The second drawing depicts what should "theoredically" happen...(Red Line) because:

a. The Grounding wire offers a lower potential difference than does the Neutral, thus providing a greater 120Vac potential difference rather than a 119Vac potential difference in the neutral.

b. Since the potential difference is greater in the Grounding wire as opposed to Neutral it will therefore constitute, as a consequence, that current will be subjected primarily down this alternate path.

C. As a result of the two givens listed above I conclude that this should trip a breaker or pop a fuse because its like grounding out...

but what i don't understand is:

1. that nothing like this happens.
2. why a Hot grounding out is different from a neutral grounding out.
(I could swear that I have tripped many breakers by accidentally grounding out the neutral while the circuit was energized)

 

[XPTPCREWX]

It's just that the ground"Ed" wire usually has the load between it and the live wire.

The Load?...is this the only reason?

 

[dlgoff]

The safty ground is what is used to protect you in the event that the hot wire comes in contact with the case/box the electrical device is enclosed in. Instead of the current flowing through your body to the ground, it is diverted to the safty wire to ground.

 

[XPTPCREWX]

I think I'm getting there...

1. Since the Ground"ED" and Ground"Ing" wire equalize their potential difference at their junction, is it safe to say that they really do short out??

2. So where the Neutral taps into the Delta/Star configuration, is it safe to say this is one of the many faces of "ground".. 0 potential...?

3. Is it safe to assume the ONLY real difference between the grounding wire and the neutral wire is that the neutral wire is the path of "higher" resistance TO ground, and the grounding wire is the path of "least" resistance to ground under normal operating conditions?

4. Is it also safe to assume the Ground"ED" (neutral) and Ground"Ing" wires BOTH end up at "EARTH GROUND" regardless ...and that the only reason for providing a redundant dual path is if the "direct grounding conditions on that plot of land become unstable."...?

 

[MATLABdude]

The Load?...is this the only reason?

Yes, but this is also the most important reason! The resistance (let's assume that it's purely resistive) CONTROLS THE AMOUNT OF CURRENT that flows through the circuit (from the source to the live wire, through the load, and through neutral, assuming it's working properly). This was, IIRC, the initial question in the other thread. This fact is why your breaker does not trip when you use stuff, because you are not shorting (connecting through a low resistance path) live to neutral. Unless what you're using draws more current than what your breaker is rated for (e.g. a power saw using 20 A on a 15 A circuit--we used to do this all the time when we were cutting through thicker sections of metal).

I think I'm getting there...

1. Since the Ground"ED" and Ground"Ing" wire equalize their potential difference at their junction, is it safe to say that they really do short out??

2. So where the Neutral taps into the Delta/Star configuration, is it safe to say this is one of the many faces of "ground".. 0 potential...?

3. Is it safe to assume the ONLY real difference between the grounding wire and the neutral wire is that the neutral wire is the path of "higher" resistance TO ground, and the grounding wire is the path of "least" resistance to ground under normal operating conditions?

4. Is it also safe to assume the Ground"ED" (neutral) and Ground"Ing" wires BOTH end up at "EARTH GROUND" regardless ...and that the only reason for providing a redundant dual path is if the "direct grounding conditions on that plot of land become unstable."...?

Let's go into ideal land and assume 0 wire resistance (since, for the purposes of this explanation, they only complicate the matter without conveying the basics)--as stated previously, the load is what determines current in normal operation.

1) Yes.

2) Don't quite know what you mean by tapping into the DELTA/STAR connection, but if you mean the interface between what the power company gives you and your house, then yes, this point is ground (and with the above 0 wire resistance assumption, actually at Earth ground).

3) No. With the 0 wire resistance assumption, both the neutral and ground wires have the same resistance measured relative to ground: 0! (Not 0 factorial, which is 1, but zero!) I think your confusion in this regards is the same as many people starting out in the AC power biz (and myself, oh so long ago). So let me let you in on a little secret that'll make your life (or analysis, or both if you happen to be an engineer) so much simpler...

You think that there's something special about 'ground', that somehow, it's a power supply with semi-mystical properties. That it is both a supply and sink for electrons (or, the positive charges that Ben Franklin stuck us with). This last point is the one thing that "ground" actually is: a supply and sink of current. However, the amount of current sunk is the same as the amount of current sourced by ground (as it pertains to any circuit). In other words: IT'S A WIRE! :-D

The only thing special about this wire is that it's where we put one lead of our multimeter when we make measurements. Well, that and the fact that it's a really, really big wire, and really easy to connect to: you just jab a 4' copper probe into the (wet) earth, and voila!

4) No, see (1) and (3). Also remember: ground wire shouldn't be carrying current. And if it is: something's wrong (usually, live's connected to earth, or is touching the case, which is also connected to earth).

In regards to the pictures you posted in post 5 of this thread, the actual current flow is most like 1. However, it's like 2 if you use the power of point (3) above, and join the two grounds together, and have the current flowing back to the negative terminal of the power supply. In other words, picture 1 (with part of the wire being earth, instead of your copper metal). Let me end on a pseudo-philosophical point and say: everything goes full circle, current can only flow in loops.

 

[Averagesupernova]

XPTPCREWX: I think you need to step back a bit and take a look at it from a different point of view. Let's set up a hypothetical case using DC. It is really no different in terms of what shocks you and what does not. Suppose you have two 120 volt batteries that are wired in series out in the middle of your front lawn. The negative of one is hooked to the positive of the other, but that is all. No other connections are made. You can grab any battery terminal you want as long as you grab ONLY one and you will not receive a shock. There is no path for the electrons to get back to the opposite terminal of the battery so no current will flow through your body. Current, AKA electrons in motion, is what you actually feel and what hurts you during an electrical shock.
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Now suppose that you drive a ground rod and hook it to the connection that is connecting our hypothetical batteries in series. So now we have one battery that has its negative terminal grounded and the other battery has its positive terminal grounded. Now suppose that we run three wires from our two batteries a short distance away and the terminate inside a small lawn shed. One wire comes from each terminal. This would be the typical way power enters a residence except the wire that is hooked to the pair of battery terminals that are wired together (we'll call this neutral) would be the center tap of a secondary transformer winding and the other two wires would come off the ends of the secondary winding. But let's just stick with DC for this hypothetical case for now.
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Ok. We have a small lawn shed with three wires entering it. We can touch only one of the wires and not receive a shock. This would be the one I named neutral in the above paragraph. DO YOU KNOW WHY THIS IS? I think this part right here is the key to your trouble. I'll wait for your reply before I post any more.

 

[XPTPCREWX]

DO YOU KNOW WHY THIS IS? I think this part right here is the key to your trouble. I'll wait for your reply before I post any more.

Simply because you are not completing the circuit...You would have to touch neutral with one of the 120V feeds... or both 120V feeds...or either one and stand in a puddle of water in front of your shed.

 

[Averagesupernova]

Ok. You have it correct. What I have described is basically what our residential power consists of. Let's take it a bit farther. We have three wires entering our shed. Let's throw in a breaker box, hook the neutral to the neutral bar and the two other wires to the power lugs. There is a screw that connects the neutral bar to the chassis of the breaker box which connects the GROUND bus-bar to the neutral. Now all of our branch circuits connect in the normal way with the the live wire to the breaker, the neutral to the neutral bar and the ground wire to the ground bar. Do you see any reason now why touching the neutral would shock you? I'm having a hard time seeing why you had trouble understanding this in the other thread. If you understood my last post and you understand this post I would say you have your head wrapped around it.

 

[XPTPCREWX]

Averagesupernova:

I was under the asumption that "Earth Ground" physically sucked all current...
I was taught that one of electricitys main properties was to reach ground...no matter what.
...but it seems that this property only exists because the return to the supply of the original "source" is connected to Earth Ground.
and NOT because electricity naturally inherently does this.

 

[Averagesupernova]

I was under the same impression long ago too. I think this is more of a safety thing to keep people out of trouble.

 

[XPTPCREWX]

Let me rephrase these questions...I can see how they can be misunderstood.

1. Since the Ground"ED" and Ground"Ing" wire equalize their potential difference at their junction, is it safe to say that they really do short out??

2. So where the Neutral taps out from, is it safe to say that this is at "ground" potential since both the Ground"Ing" and Ground"Ed".. Equalize...?

3. Is it safe to assume the ONLY real difference between the grounding wire and the neutral wire is that the neutral wire is the path TO THE SUPPLY under normal operating conditions,
and the grounding wire is the path under abnormal operating conditions? but BOTH are returns to the supply?

4. Which offers a lower/higher resistance?

 

[XPTPCREWX]

Can I conclude then...Electricity ONLY goes to ground if its source return is grounded
and NOT because electricity naturally inherently loves Earth Ground so much.

 

[XPTPCREWX]

Can anyone tell me...in which direction the current is traveling for both legs in the drawing...if this is even possible...any why.

 

[Averagesupernova]

Let me rephrase these questions...I can see how they can be misunderstood.

1. Since the Ground"ED" and Ground"Ing" wire equalize their potential difference at their junction, is it safe to say that they really do short out??

This would be a choice of which words to use. The term 'short' usually refers to a connection that is NOT SUPPOSED TO BE THERE. A 'short circuit' is just what it sounds like. The current no longer takes the long path around the circuit, instead it takes a path where it is normally not supposed to. The circuit has been shortened. In the case of the neutral and ground I would not use the term 'short'.

2. So where the Neutral taps out from, is it safe to say that this is at "ground" potential since both the Ground"Ing" and Ground"Ed".. Equalize...?

I think it is safe to say that.

3. Is it safe to assume the ONLY real difference between the grounding wire and the neutral wire is that the neutral wire is the path TO THE SUPPLY under normal operating conditions,
and the grounding wire is the path under abnormal operating conditions? but BOTH are returns to the supply?

Yes this is exactly right.

4. Which offers a lower/higher resistance?

Neither should offer any resistance at all. In the real world this is not possible but each should be as low as possible.

 

[Averagesupernova]

Can anyone tell me...in which direction the current is traveling for both legs in the drawing...if this is even possible...any why.

Conventional theory would say that electricity flows from positive to negative. Electron theory states the opposite.
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Assuming both resistors are the same NO current will flow in the wire between what you have labeled 'tap' and the node where the 2 neutrals join together and join up to the tap. It can be considered one big series circuit with no need for the tap. However, if one load is different than the other, suppose one is drawing 10 amps and the other 15, the neutral will be carrying the difference. BTW, you have one of the loads mislabeled polarity-wise.

 

[XPTPCREWX]

Things are starting to make sense...

BUT...why don't we just connect the metal box in a receptacle to neutral instead of ground...if its eventually going to the same place?

 

[Averagesupernova]

Things are starting to make sense...

BUT...why don't we just connect the metal box in a receptacle to neutral instead of ground...if its eventually going to the same place?

I always know it's starting to make sense when I get that question. The reason is because if there were a poor connection on the neutral on the way back to the source then the chassis become slightly energized relative to the earth. At one time, clothes dryers were one exception to that. The code allowed the neutral to connect to the chassis of the dryer. In clothes dryers the heating element runs on 240 volts and the motor runs on 120 volts. So the neutral in a clothes dryer circuit does actually carry current. It is more than just a safety ground. However, in the last 10 to 15 years the code has required that all clothes dryers have a four prong plug. Now there is a wire dedicated to chassis ground. Mobile homes have always had stiffer grounding rules. At one time one of my friends lived in a mobile home and it had the neutral and ground reversed in the breaker box for the dryer circuit. You may ask what the difference is, but in mobile homes, the neutral bus-bar and the ground bus-bar are not connected. The ground bus bar goes to the ground rod. The neutral goes back the same way as in any other residence. The reason for this is that mobile homes literally plug in. They have a four prong plug with a dedicated ground. It is not shared as in my hypothetical example earlier. So, there is a much greater chance of having a poor connection on the neutral which could potentially make a chassis of an appliance partially hot. So in my friends mobile home clothes dryer situation, he had a clothes dryer that would turn over very slowly if at all. I also wonder though, that there could have been other problems in what his house plugged into. At one point or another I would think that the ground should eventually connect up to the neutral with wire and not just rely on the Earth to pass current during a fault. I also recall getting a shock from the aluminum siding on his house. Probably related.

 

[XPTPCREWX]

Another thing... I know that A good grounding system will improve the reliability of equipment and reduce the likelihood of damage due to lightning or fault currents.

If there are Hazardous currents present...how does the current get dissipated? especially with lighning...

A source can only return what it can supply...so where does this "Floating" current dissipate into?

 

[Averagesupernova]

By floating I assume you mean current that comes from lightning. A lightning strike is like a large capacitor discharing. In cloud to ground lightning the ground is considered on plate of a capacitor and the cloud is considered the other plate. The current is not dissipated, power is dissipated generally in the form of heat in the arc and of course melting of various appliances, wires, etc. if your house happens to get hit.