Electrical Utility Neutral Return Wire

In summary: I think I finally have it!Thank you again for your patience and your help. I appreciate it.In summary, the home electrical system in the USA receives power from the utility company transformer through 3 wires: 2 hot wires and 1 neutral return wire. Inside the house Mains Panel, the hot wires are connected to separate buses and the neutral wire is connected to the neutral bus. The branch circuit hot wires are connected to the buses through circuit breakers and the return neutral wires are connected to the neutral bus. The current flow in branch circuits alternates between phases for 240v and through the neutral bus for 120v. In the case of unbalanced current flow, only the unbalanced current will flow back to
  • #1
twodotmike
4
0
I have questions based on my understanding on the home electrical system (USA). I am not a student, this is for my personal understanding.

My understanding (please bear with me):

Electrical power to the house Mains Panel comes from the secondary of the utility company transformer in the form of 3 wires: 2 phase (hot wires) and 1 neutral return wire (center tapped on the secondary and grounded). Inside the house Mains Panel the 2 separate phase wires are connected to the 2 separate hot buses and the neutral return wire is connected to the Mains Panel neutral bus. At the house Mains Panel this neutral bus is also bonded to ground.
In the house Mains Panel the branch circuit hot wires are connected to the buses through circuit breakers and the branch circuit return neutral wires are connected to the neutral bus.
Within the house Mains Panel branch circuits the current flow alternates from phase to phase for 240v and from phase to phase through the neutral bus for 120v. If the phase to phase current flow is balanced then virtually no current flows back to the transformer on the utility neutral return wire. If there is an unbalanced current flow then only this unbalanced current flows back to the transformer.

My questions:

1. Does this mean the return current from the house Mains Panel neutral bus to the utility transformer flows only in one direction and does not alternate (on the neutral return wire) as the current in the phase wires does?

2. If it is a one way flow, then, is this unbalanced current returned to ground at the transformer center tap ground point or is it somehow reintroduced into the phase wiring?

3. If the return current on the utility return neutral wire alternates, what keeps it from impeding the current flow of one of the phase wires? If the return current alternates, it seems like it would be "out of phase" with one of the 2 phase wire?

If you have made it this far, thank you for your patience.

Mike
 
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  • #2
here's an ancient thread on your subject.
https://www.physicsforums.com/threads/grounding-in-electrical-circuits.526008/

Try PF's search feature. dlgoff has posted some most excellent graphics and every picture is worth a thousand words.

Like this one, from
https://www.physicsforums.com/threads/ground-fault.506179/
proxy.php?image=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2Fhbase%2Felectric%2Fimgele%2Fgfault.gif
 
  • #3
jim hardy said:
Looks to me nobody explained there (or I missed it) what happens with voltages in 3-phase isolated systems when one phase shorts to the ground?
It's not just that asymmetry in the network present problem to proper operation of 3-phase motors and other devices , but also phase voltages of 2 healthy phases rise to line voltage (rise by 73%). That's not all. Voltage with respect to grounded parts may rise many times higher. Fault phase in many cases generates spark of relatively weak rms current due to presence of distributed capacity in the system. Behaviour of the spark is that it acts like nonlinear switch and charges whole the system as current repetitively turns on and turns off. The phenomenon is called "escalation of the voltage" and it doesn't have to be explained how bad and destructive that can be.
 
  • #4
zoki85 said:
Looks to me nobody explained there (or I missed it) what happens with voltages in 3-phase isolated systems when one phase shorts to the ground?
Hmmmm that's a good point. I'm so used to industrial systems where you never "float" anything i didn't think about it.
zoki85 said:
Behaviour of the spark is that it acts like nonlinear switch and charges whole the system as current repetitively turns on and turns off. The phenomenon is called "escalation of the voltage" and it doesn't have to be explained how bad and destructive that can be.

That's what brought about IEEE 142, the grounding standard.

Ferroresonance between distributed capacitance and inductance of equipment will raise the voltage enough to pierce insulation . It can wreck all the motors in a factory.
That's why one sizes the earthing resistor not more than Z of that distributed capacitance, to keep down Q..
 
  • #5
jim hardy said:
That's what brought about IEEE 142, the grounding standard.

Ferroresonance between distributed capacitance and inductance of equipment will raise the voltage enough to pierce insulation . It can wreck all the motors in a factory.
That's why one sizes the earthing resistor not more than Z of that distributed capacitance, to keep down Q..
Ferroresonance is another "fish in the pond" (also bad & dangerous nonlinear phenomenon which can lead to overvoltages). However, I was talking just about 3-phase ungrounded system where only nonlinear element is a spark between fault phase and ground. This is enough for escalation of the voltage to occur. Higher the nominal voltage of the system the voltage escalation more dangerous. It is interesting that in Europe, on rare occasions, one can still find cases of ungrounded 10 kV distribution networks! However, most of MV networks of today are earthed systems (resonantly earthed sometimes). And networks above 69 kV are not resonantly earthed. I've read once there were cases in North America of 138 kV network (or maybe 230 kV?) with resonantly earthed neutrals. That surprised me quite becouse overvoltages are also significant in phase-to ground faults in resonantly earthed networks.
 
  • #6
You're ahead of me on that one.

This was my first time hearing of "Resonant Earthing"

very clever !
http://www.oriongroup.co.nz/downloads/Technical_information.pdf
 
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  • #7
jim hardy said:
here's an ancient thread on your subject.
https://www.physicsforums.com/threads/grounding-in-electrical-circuits.526008/

Try PF's search feature. dlgoff has posted some most excellent graphics and every picture is worth a thousand words.

Like this one, from
https://www.physicsforums.com/threads/ground-fault.506179/
proxy.php?image=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2Fhbase%2Felectric%2Fimgele%2Fgfault.gif
jim hardy said:
here's an ancient thread on your subject.
https://www.physicsforums.com/threads/grounding-in-electrical-circuits.526008/

Try PF's search feature. dlgoff has posted some most excellent graphics and every picture is worth a thousand words.

Like this one, from
https://www.physicsforums.com/threads/ground-fault.506179/
proxy.php?image=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2Fhbase%2Felectric%2Fimgele%2Fgfault.gif
 
  • #8
Mr. Hardy,

Thank you for your time and your information. Following your links, I found another response of yours on a similar topic that I believe helps me more clearly "see" this return current flow to the transformer and that it is a one way return to the transformer (supply) and on to ground. I was trying to get a clear picture in my mind to explain this "flow" to my grandson. Again, thank you.

Mike
 
  • #9
Thanks for the kind words - makes an old fella feel less useless !Us Grampas got to stick together...
old jim
 
  • #10
jim hardy said:
This was my first time hearing of "Resonant Earthing"

very clever !
http://www.oriongroup.co.nz/downloads/Technical_information.pdf

Principles of resonant earthing and Petersen coils are known for a long time. For example, see:
http://www.hvpower.co.nz/TechnicalLibrary/RE+DS/Petersen Coils Basic Principle and Application.pdf
That works very well in MV networks. Hence I was surprised to find some 10 and 20 kV networks are still left floating today (no, they are not delta-delta). As always, pros and cons to be considered. Generally and technically, it is easier to deal with higher or smaller fault current than with TOV in the system so any grounding is better than no grounding. OTOH, there is financial aspect vs risk factor to be considered as well.
 
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  • #11
In three phase utility power transmission lines and distribution lines, there is no neutral wire. In a wye connected circuit, the wye is grounded. In delta connected circuits, there is no Earth or neutral at all. Since there are many ways to wire transmission, and since distribution circuits split from three phase to multiple single phase circuits, there is no single scheme for fault protection.

Protective relays detect faults by various means including combinations of overcurrent, undervoltage, apparent impedance, and unbalance.

To dig deeper, google "The Art And Science Of Protective Relaying", it is a free publication from General Electric that you can download.
 
  • #12
Hello ToDot - nice OP. Sorry if the thread is getting off topic with the 3 phase, transmission and distribution issues. I believe your question has to do with the basic nature of AC power as much as anything.

In typical US households the 240 / 120 is really split phase, by that I mean the 120 and 240 Circuits are in-phase with one another and there is no phase shift in the AC voltages supplied.

What I believe confuses most people is the idea of "flow" in general - they think of current only. Really we are dealing with POWER which requires a Voltage and Current - the direction of power flow, is determined by the relative polarity of the voltage and the current. So only talking about current can be difficult to understand especially when thinking about AC - "Wait the current flows "in" and then it flows "out" ? - How does that do anything?"

One way to help - think of your feet on the pedals of your bike - a basic bike rider pushes down. makes sense.. but a serious cyclist uses toe-clips and pushes and pulls... but the power delivered is always from their body to the bike. ( Foot pressure downward, pedal motion down = Delivered Power, with both "polarities" reversed foot pulls up, pedal moves up = delivered power.)
 
  • #13
anorlunda said:
In three phase utility power transmission lines and distribution lines, there is no neutral wire. In a wye connected circuit, the wye is grounded. In delta connected circuits, there is no Earth or neutral at all.

Maybe that depends on where one lives.
I grew up in South Florida, the lightning capital of the world.
There the neutral is run on top of the transmission towers. It acts both as neutral and as a lightning rod .
That's fairly typical in US, this image claims to be in Indiana.
http://www.betaengineering.com/en-us/projects.aspx
http://www.betaengineering.com/Portals/0/projects/Transmission%20Line%20Indiana.jpg

In my plant the delta fed medium voltage buses were impedance grounded through a wye transformer that made a
"virtual neutral".

IEEE 142 describes recommended practices for earthing, search on "IEEE Green Book".
University curricula ought to include a course on grounding. It is widely misunderstood.
 
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  • #15
Windadct,

Thank you for your comment. I appreciate your time. I believe I understand the basics about the "power" flow, although, as a non EE, the nuances of all the science involved in system designs is over my head.

Mike
 
  • #16
jim hardy said:
Maybe that depends on where one lives.
I grew up in South Florida, the lightning capital of the world.
There the neutral is run on top of the transmission towers. It acts both as neutral and as a lightning rod .
That's fairly typical in US, this image claims to be in Indiana.
http://www.betaengineering.com/en-us/projects.aspx
http://www.betaengineering.com/Portals/0/projects/Transmission%20Line%20Indiana.jpg

In my plant the delta fed medium voltage buses were impedance grounded through a wye transformer that made a
"virtual neutral".

IEEE 142 describes recommended practices for earthing, search on "IEEE Green Book".
University curricula ought to include a course on grounding. It is widely misunderstood.
Not quite. Those wires on top are called shield wires. They are grounded and their purpose is lightning protection. But they have no connection to the power conductors and can not act as a neutral for the power circuits in case of a fault.

By analogy, compare these shield wires on a transmission line to lightning rods on the roof of a building. Lightning rods are never connected to the power wiring of the building.
 
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  • #17
anorlunda said:
But they have no connection to the power conductors and can not act as a neutral for the power circuits in case of a fault.
Call it what you like
but it's earthed at same place as stepup transformer neutral
as well as at every tower along the way
 

Related to Electrical Utility Neutral Return Wire

1. What is an Electrical Utility Neutral Return Wire?

An Electrical Utility Neutral Return Wire, also known as a neutral wire or return wire, is an essential part of an electrical system that carries current back to the source to complete a circuit. It is typically a white wire that is connected to the neutral bus bar in the main electrical panel and is grounded to the earth.

2. Why is an Electrical Utility Neutral Return Wire necessary?

An Electrical Utility Neutral Return Wire is necessary for the safe and efficient operation of an electrical system. It helps to balance the electrical load in a circuit and provides a safe path for excess electricity to return to the source. Without a neutral wire, the circuit may become overloaded, and there is a risk of electrical shock and fire.

3. Can an Electrical Utility Neutral Return Wire be used as a ground wire?

No, an Electrical Utility Neutral Return Wire cannot be used as a ground wire. While both wires are connected to the earth, they serve different purposes in an electrical system. The neutral wire carries the return current, while the ground wire is designed to protect against electrical faults and ensures safety for people and equipment.

4. What is the difference between a neutral wire and a ground wire?

The main difference between a neutral wire and a ground wire is their function in an electrical system. The neutral wire carries the return current to complete the circuit, while the ground wire is a safety measure to prevent electrical shocks and fires. Additionally, the neutral wire is typically white, while the ground wire is green or bare copper.

5. What should I do if there is no Electrical Utility Neutral Return Wire in my electrical system?

If your electrical system does not have a neutral wire, you should consult a licensed electrician to assess the situation. They may recommend upgrading your electrical system to include a neutral wire, as it is an essential safety feature. Attempting to add a neutral wire on your own can be dangerous and should only be done by a trained professional.

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