Why won't ground reroute the general home circuit?

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Old 08-27-09, 04:36 PM
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Why won't ground reroute the general home circuit?

For general house wiring.

As I understand it, the ground will carry away harmful power when there is a fault in the neutral lets say a wall outlet that has a load plugged into it.

What keeps power from going to ground normally? Is the ground only available when a load is plugged into the wall outlet thus adding the ground to complete a mini circuit within the load?
If so, why doesn't current just flow to ground initially rather than only to neutral; why no power to both paths in such cases as well?

Thanks in advance.
 
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Old 08-27-09, 05:24 PM
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There's no direct connection from hot to ground (through a load or not) anywhere in the circuit, except in the panel. The ground is for user protection. Only things that are NOT in the circuit get grounded - device straps, fixture shells, metal electrical boxes - things that are NOT in the circuit but are metallic and could be touched by users.

The only connection is in the panel, where the ground and neutral are bonded. And I guess since the ground wire actually runs directly into the earth (via the water pipe/ground rod), there is less resistance on the neutral going back to the transformer on the pole.
 
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Old 08-27-09, 05:35 PM
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Since you said "outlet", I'm going to reply regarding an equipment ground which is the bare wire included in branch circuits throughout the building. This is different than the grounding electrode, which is a metal rod or pipe driven into the earth.

Originally Posted by foolios View Post
As I understand it, the ground will carry away harmful power when there is a fault in the neutral lets say a wall outlet that has a load plugged into it.
That would only be the case if the neutral was broken in such a way that it came in contact with exposed metal like a device box or appliance frame. The same would be true for a hot wire.

What keeps power from going to ground normally?
In a branch circuit there is no connection between the neutral and the ground or between the hot and the ground, therefore no path exists for current to flow.

If so, why doesn't current just flow to ground initially rather than only to neutral; why no power to both paths in such cases as well?
In a properly installed circuit it does not. If the circuit was wired with an illegal connection "bootleg ground", there would be current on the ground wire which is an unsafe situation as I believe your question implies.
 
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Old 08-27-09, 05:39 PM
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The ground does not "carry away harmful power", it carries current only during a fault in order to operate an overcurrent device. (fuse/breaker) The equipment ground wire bonds (attaches to) any metal part of equipment that may become energized if a hot (ungrounded) wire comes in contact with it.

The only place where the neutral and ground are connected together is in the main electrical panel. After that, they are always (or should be) separate.
 
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Old 08-27-09, 06:46 PM
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Thank you for the responses.

Ok, so neutral comes in from the power lines outside the house > connects through a meter box > then through to the main breaker box > then throughout the entire house > then back to a neutral bus bar on the main breaker and then to where? What is the neutral bus bar tied to? Is it the same grounding rod or pipes that the ground is connected to?
 
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Old 08-27-09, 06:57 PM
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There have been many books written on the subject of grounding and most of the books have more than 100 pages. It is subject that even many electricians are confused about.

Here is a 12 part series on grounding and bonding. It is one of the best, yet still understandable, works I have seen.

Grounding vs. bonding.
 
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Old 08-27-09, 07:21 PM
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Ok, so neutral comes in from the power lines outside the house > connects through a meter box > then through to the main breaker box > then throughout the entire house > then back to a neutral bus bar on the main breaker and then to where? What is the neutral bus bar tied to? Is it the same grounding rod or pipes that the ground is connected to?
Not quite. Your hots and neutral come from outside from the power companies lines. The two hots go through the meter and the neutral is just spliced there. Then then go into the house (if your meter is outside) and the hot terminate to the main breaker/fuses. The neutral terminates to the neutral buss which is not connected to the main breaker what so ever. Each branch circuit needs two wires to make the circuit complete. One hot and one neutral for a 120 volt circuit. Two hots for a 240 volt circuit. Each circuit will also have one equipment ground. (or just ground to keep it simple) The load (anything that uses power) is in between the hot and neutral wires of a circuit. When the equipment (the load) is turned on, current flows between the hot and neutral wires.

Heres kind of the kicker where people get a little goofed up:
Electricity in homes is AC. Alternating Current. Current does not just flow from the hot, through the load, and back on the neutral to home. It changes direction 120 times per second (60 cycle or Hertz (Hz))

With electricity, it is all about difference of potential. Between a hot wire and neutral there is 120 volts difference of potential. The water pipe ground and the ground rod and grounded (bonded) to the service to make sure there no difference of potential between them (water pipe/earth) and the electrical systems ground. These grounds are all tied to the neutral buss. The neutral is also called the "grounded conductor" because it is grounded in the power companies transformer outside.

Also, electricity is not consumed like gasoline. It is only the flow of electrons through the wire. The power company is just a pump that is making the electrons move back and forth REALLY fast.

I'm really shooting from the hip here and may not be the best explaining how this all works. You may want to do a little searching on electrical theory for more info.
 
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Old 08-30-09, 07:28 AM
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Tolyn, that was by far the best explanation I have run into so far. I have been trying to get my head around how power is flowing in and out of the house. Thanks so much for helping to make that so much clearer. I will ponder it some more and come back with more questions.

Thanks again so much! And thank you furd for the link!
 
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Old 08-30-09, 09:49 AM
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If you think of it like this it may help...a completed circuit begins at the transformer and ends at the transformer over two wires. 120 volt needs a neutral and ungrounded (hot) to complete the trip to the transformer. A 240 volt circuit also needs two wires both are ungrounded (hots) and no neutral. Current flows back and forth to the transformer on those wires.

Also key is remembering that current is always seeking its source (transformer) it will use any path it can find to get there including the earth. However it will return to the source on the lowest resistance/impedance path available to it. Copper and aluminum wire ....it does'nt get much better than that. Current will not flow to your ground rods and other paths connected to earth because the resistance is to high trying to travel through the earth to get back to the transformer...so it takes the wire path to get to the transformer. 120 volt circuits get back to the transformer via the service neutral..it's the only low impedance path back... other than the high resistance earth. Even though an earth path is available current will not take it when given a "better low resistance" path to the transformer center tap. Same with 240 volt branch circuits only they don't use the service neutral to get back they use the two hot wires to return current to each end of the transformer winding.

The ground wire in both 120 and 240 volt circuits is a parallel path with the branch circuit current carrying wires but is not connected to them (this being neutral or hot wires). It joins the the neutral wires at the neutral bar in the panel in both 240 volts circuits and and 120 volt circuits. This is because we intentionally only give fault current on ground wires and neutral current from 120 volt branch circuits one path back to the transformer...and that is over the service neutral.
this path for your equipment grounds in the branch circuits is called the "effective ground fault path". It is required to allow enough current to flow thru the breaker during a fault to get it to trip out. ON the other hand if we were to open the service neutral then the only path back to the transformer is thru earth because the earth path is made available due joining of all these paths at the neutral bar of the main panel (service equipment). IN this case the low resistance path has been taken away from the fault or neutral current to flow to the transformer so it takes the only path it has ....to earth...high resistance or not. Problem is at the voltages of 120 and 240 there is not enough force pushing the current to get the earth path to allow enough current to flow back to the source to trip the breaker..... so the fault will not clear the breaker will no trip and all metal on the fault path will become energized.

Some images to help you understand in next post
 
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Old 08-30-09, 10:23 AM
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Images for your understanding ..hope these help 1st is a 120 volt branch circuit. Notice the bonding to earth at the neutral in the panel. Current will not take that path to earth. Some might but will be insignificant.




This image below shows both the 240 volt circuit using two breakers on opposite legs and a 120 volt circuit using a single breaker and the neutral/grounded conductor . The neutral is grounded because it is intentionally connected to earth. We get the 120 volts from the transformer by the tapped 'midpoint' of the 240 volt winding of the transformer. We earth it to help stabilize voltages in the event of a open neutral and for high voltage events. Also notice the parallel path of the neutral/grounded conductor and equipment ground in the branch circuits. Then notice they join a the big N and at that point the only way back to the transformer over a low impedance/resistance path is the service neutral.



The two images below are showing the effective fault path. the last one shows the fault at a receptacle and shows how the fault current gets back to the transformer... not the earth.
Notice the neutral and the fault path are parallel and join at the neutral bar of the service equipment and then use the service neutral to get to the source transformer..

The path to earth is what we construct to protect property from high voltage events like lightning and downed power lines during storms. It is not the path we want current in the electrical system of your home to follow.

The equipment ground is for human safety and facilitates the low impedance/resistance path for current to get back to the transformer to trip the breaker. Remember during ground faults the load is by-passed and essentially there is no load to restrict current flow. So massive current flow results as it returns to the transformer over just a wire and bonded metal path. It's this massive current flow thru the circuit breaker due to this low impedance/resistance that makes the breaker trip. Imagine 100 amps flowing thru a 15 amp breaker...it likely will trip....



 

Last edited by Bruto; 08-30-09 at 12:44 PM.
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Old 08-30-09, 10:25 AM
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Bruto do have few wonderfull drawings he made in this fourm and other fourms as well and he do expain it very clear and he is more than welcome to post the drawing anytime.

If other question realted to the drawing just holler one of us will help you with it.

I know it can be compiated subject but with the drawing what Bruto made here it will really make it more clear.

Along the side note as well anytime you mount the subpanel this item is get critical here as you see why the NEC /CEC code get strict to run four conductor to the subpanel like example if the netural conductor fail the grounding conductor serve as back up for short circuit protection.

Yeah with long distance we know we have to size up the conductor to larger one to compreised with voltage drop however.,, nice gotcha with very long runs we have to increase the grounding conductor as well { I allready deal with that with parking lot luminaires that is most common issue there } the reason why I mention that with long distance the restanice amout of the conductor actally increase with long run so size up the conductor will able get the OCPD to work if not done in first place some case the OCPD may not function at all.


Merci,Marc
 

Last edited by french277V; 08-30-09 at 10:33 AM. Reason: add info plus edit it
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Old 08-30-09, 10:40 AM
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Originally Posted by french277V View Post
Bruto do have few wonderfull drawings he made in this fourm and other fourms as well and he do expain it very clear and he is more than welcome to post the drawing anytime.

If other question realted to the drawing just holler one of us will help you with it.

I know it can be compiated subject but with the drawing what Bruto made here it will really make it more clear.

Along the side note as well anytime you mount the subpanel this item is get critical here as you see why the NEC /CEC code get strict to run four conductor to the subpanel like example if the netural conductor fail the grounding conductor serve as back up for short circuit protection.

Yeah with long distance we know we have to size up the conductor to larger one to compreised with voltage drop however.,, nice gotcha with very long runs we have to increase the grounding conductor as well { I allready deal with that with parking lot luminaires that is most common issue there } the reason why I mention that with long distance the restanice amout of the conductor actally increase with long run so size up the conductor will able get the OCPD to work if not done in first place some case the OCPD may not function at all.


Merci,Marc
Hello friend

Nice to see you are up and about...hope the images were not inappropriate. I don't believe there are any errors that would be misleading or a safety issue...if you see something I will gladly correct it. Have a good day Frenchy. Beautiful day down here in the Midwest....
.
 
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Old 08-30-09, 10:50 AM
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The photo came out really good Bruto and I am sure that will really help alot of readers in here to clear up some of the confusing situation related to this topic.

Again thanks for your time to post the drawing here.

If you have time can you make third drawing with subpanel set up ? the reason why I ask due we get pretty good percentage of readers here will install subpanel so that drawing will clear up any misunderstooding with this.

Again thanks for your wonderfull work with the drawing.

Offtopic here it pretty cool up northeast wisconsin and yeah the trees is changing colours now.

Merci,Marc
 
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Old 08-30-09, 02:06 PM
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Thanks so much for the images. They definitely explained a lot.

I have a question about the comment:
Originally Posted by Bruto View Post
If you think of it like this it may help...a completed circuit begins at the transformer and ends at the transformer over two wires. 120 volt needs a neutral and ungrounded (hot) to complete the trip to the transformer. A 240 volt circuit also needs two wires both are ungrounded (hots) and no neutral. Current flows back and forth to the transformer on those wires.
I am confused as to why a 240v circuit doesn't need a neutral as opposed to the 120v. We have two 120v wires coming off of our xformer along with the neutral into the meter box outside the house via a service drop, so 3 wires coming in. I know that if those two 120v connections touch each other that the voltages will buck and an explosion could occur. How is it that they can be combined without this problem and even be used minus having a neutral?

Wow, just WOW!
 
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Old 08-30-09, 02:49 PM
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Originally Posted by foolios View Post
I am confused as to why a 240v circuit doesn't need a neutral as opposed to the 120v. We have two 120v wires coming off of our xformer along with the neutral into the meter box outside the house via a service drop, so 3 wires coming in. I know that if those two 120v connections touch each other that the voltages will buck and an explosion could occur. How is it that they can be combined without this problem and even be used minus having a neutral?
The 240 volt circuit does not need a neutral because it is using the entire winding of the transformer where the 120 volt only uses half. Some 240 volt circuits do require a neutral, such as new 4 wire range circuits, but that is due to the range using 120 volts for the clock and other electronics and 240 volts for the elements. That also falls under the "ground is not to carry anything other than fault current".

The reason you can use 240 volts on equipment is because the equipment (the load) is between the two wires. The equipment is designed to handle/utilize the higher voltage. You will still get an explosion if you touch a hot and a neutral (120 volts) directly, it is just not as large as one you get with 240 volts. The greater the the difference of potential (voltage), the bigger the boom!
 
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Old 08-30-09, 03:07 PM
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If the load is between the two 120v's without a neutral. How is it that the load receives 240 instead of just the 120v. It would seem to me that 120v is going to come in along one wire and then out the other, but then when the cycle reverses because these two 120v wires are the same phase, that the current will then switch directions coming back through from the one wire through the load and out the other wire. This appears to me that there is only 120v traveling through the load at this point, even with 2 120v wires connected to it as there has to be a return path.

I don't see how we can have two 120v go in at the same time with nowhere else to go back out. I mean if both phases being similar and are pushing(at one stage of the cycle), how can one pull back to the power plant while that's happening?

in 120 and neutral, on the push, the voltage has a path to return, on the pull, the voltage has a path to pull from. It would seem that in 240v, with 2 wires, that pushing would mean they are coming in against each other and pulling against each other during the cycling that happens with A/C.
Gah, I'm not sure if I know how to ask this question.

I can assume that the equipment is designed to accommodate a neutral then? Is that why?
 
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Old 08-30-09, 04:15 PM
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If you study Bruto's 2nd picture you will see the 240 volt load between the two wires from the transformer. The two wires are both 120 volts from neutral but if you look close the top wire is +120 volts and the bottom is -120 volts in relation to the neutral which is 240 volts from each other. The + and - is the parts of the sine wave. Current will flow from the top wire to the bottom and then reverse with the sine wave 120 times per second.

The only reason 240 volt equipment needs a neutral is because it has some 120 volt components. Other equipment that doesn't need neutrals is: Air conditioners, Water heaters, Base board heaters. Another item that needs 120/240 4 wire are dryers. Again, that is because of the electronic timers and such.
 
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Old 08-30-09, 05:02 PM
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Originally Posted by foolios View Post
Thanks so much for the images. They definitely explained a lot.

I have a question about the comment:


I am confused as to why a 240v circuit doesn't need a neutral as opposed to the 120v. We have two 120v wires coming off of our xformer along with the neutral into the meter box outside the house via a service drop, so 3 wires coming in. I know that if those two 120v connections touch each other that the voltages will buck and an explosion could occur. How is it that they can be combined without this problem and even be used minus having a neutral?

Wow, just WOW!
Not easy to explain but lets try....

To explain the neutral. Voltages are about potential and in particular... differences of voltage potential. Current flows if I have a difference of potential. If potentials are the same current cannot flow. Look at the diagram of the 120 volt circuit. Notice the neutral is connected to the transformer winding right in the middle. Now notice that the 'potential' difference between the end of the transformer winding (+ to -) is 240 volts with the midpoint of the winding (the neutral point) at 0 volts. 120 volts of potential between either hot wire at the ends of the winding and the neutral point of the transformer.

Now take the neutral wire and move it up to the +120 volt end of the transformer. Now run it out and connect it to the load where the neutral is connected in the drawing. You now have a wire connected to each end of the load that is the same potential or +120 volts. Current cannot flow because both voltages are the same and push current against each other equally. If you stand in front of me and we both push against each other equally we won't go anywhere.

Now leave the hot wire attached to the end (+120) and start moving the other wire towards the the midpoint of the transformer winding. As we move towards zero our potential difference increases and until once at the midpoint we are at 120 volts. As the drawing shows and knowing current will only flow to points of potential difference if we turn on the load current will flow in response to the 120 volt potential difference (+120 to 0 ) And because of the nature of AC systems current and voltage will turn around at the neutral point and respond to the same 120 volt potential difference the other direction (0 to +120). This tapping of the middle of the transformer winding is how we get the 120 volts of potential. So think of it as a bulldozer at the +120 end of the winding and the load dictates how much current it can push to the neutral point or zero volt point. Once the bulldozer gets there it is thrown into reverse and back you go. And of course this trip back and forth happens rather fast as you know.

Now take a look at the 240 volt circuit notice as opposed to the 120 volt circuit it is +120 to - 120 passing thru the neutral point or 240 volts of potential between the 2 hot wires. Since both wires are connected to the ends of the winding there is no neutral connection. If we bring a neutral out and a load as in the second drawing and tap one of the hot wires we are able to change the potential and make ourselves an 120 volt circuit along with a 240 volt circuit. the big difference is potential ...so how do I get that 240 volt circuit to work like a 120 volt circuit with a neutral. This is done by offsetting the the two sine waves (long story here) by 180 degrees. At any given time each hot wire is opposite in polarity but the same in amplitude. I looks like this (below) in sine wave form and this relationship allows both voltages and currents to exist on the circuit at the same time.



E represents the voltage for each opposite leg of the service from your transformer. When voltage is above the neutral line polarity is positive and below negative. Notice that at all times the potential difference is maintained between the two hots (legs) at the peaks which is 240 volts and of course this alternates the direction of current and voltage from one end of the winding to the other. So it is sort of a push me pull me relationship.

If I stick a neutral in that 240 volt branch circuit I get what is called a multiwire branch circuit and I then have 2 120 volt branch circuits sharing a neutral with current canceling in the neutral due to the opposite legs of the hot wires. this is a common wiring method. It looks like this..



hope this helped you
 

Last edited by Bruto; 08-30-09 at 06:20 PM.
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