Grounded conductor (neutral) and earth voltage

[Bro8898]

I’ve posted here before. You all have been wonderful and have given me so much to study and understand. I’ve made some progress based on your teachings and still have a ways to go. I’m grateful for the information I have received here.

How exactly is the neutral conductor “referenced” to earth? This question is from information I received in a previous question I posted.

I can understand that neutral is grounded (via ground rod) at the meter of every house, and often times grounded to the plumbing pipes. I also understand that the utility transformer is grounded, in many locations throughout the system.

How is earth zero volts? Or is it zero volts? Is it because of the grounding points I mentioned above? I can see how the neutral tap at the transformer is zero (or is it?) and I can see how a 120v circuit will flow in a path from hot (120) to neutral (0).

But I’m having trouble understanding where the earth itself comes into play. I also understand it’s for lightning protection purposes.

My struggle is in the case of an open neutral. I’m assuming that since the path has been lost, now the unbalanced neutral current is trying to find a way back to the ground rod at the transformer. It could flow through the house ground rod and through the earth and ultimately to the ground rod at the transformer but this is a poor path due to high resistance so it flows on the other 120v conductor and creates dangerous voltage differences that could cause lights to flicker and damage to appliances. I’m also aware that the plumbing piping system could “mask” the case of an open neutral by allowing the current to flow into the city main and through someone else’s neutral and back to the transformer (thank you all again for teaching me that years ago).

So if neutral is lost AND the plumbing system is not connected to everyone else’s (let’s say a water meter breaks the connection) and you have a situation where voltage ups and down are causing damage to appliances, if I were touching the pipes (that were grounded) or I went outside and grabbed the ground wire - wouldn’t current flow through me? It seems like I would make a good parallel path (in addition to the ground rod and piping system) from a grounded connection through my body through the earth and back to the ground at the transformer.

Are all the neutral connections to the earth causing the earth to be at zero volts? Are you basically making the earth zero when otherwise it would be nothing (floating basically)? I know that neutral is called the grounded conductor but in the case of an open neutral is it now ungrounded and trying to find a way back to the transformer through the ground which we have intentionally made zero volts?

You all have taught me that I have to be touching two things with a voltage difference between them in order for current to flow through my body.

I think I’m trying to understand the theory behind the idea of grounding the neutral (aside from lightning protection) and how the grounded conductor is referenced to earth. Or what that even means exactly.

Sorry for the long post and thank you in advance any input. Maybe there’s a book or some information out there on the internet that you all can direct me to. I’ve read these forums for hours and I would like to understand the theory behind this because I think it will help me to be more effective at what I do. This seems to be one of the most basic ideas and is key to understanding before moving on or even being able to have a relatable conversation with someone on the topic.

Thank you

 

[Strathead]

How exactly is the neutral conductor “referenced” to earth? This question is from information I received in a previous question I posted.

I can understand that neutral is grounded (via ground rod) at the meter of every house, and often times grounded to the plumbing pipes. I also understand that the utility transformer is grounded, in many locations throughout the system.

How is earth zero volts? Or is it zero volts? Is it because of the grounding points I mentioned above? I can see how the neutral tap at the transformer is zero (or is it?) and I can see how a 120v circuit will flow in a path from hot (120) to neutral (0).

But I’m having trouble understanding where the earth itself comes into play. I also understand it’s for lightning protection purposes.

I will tackle this part. Voltage is the potential difference between two points. Without defining both points, voltage can't be measured. We generally reference ourselves to the surrounding earth and call that zero volts for simplicity. It doesn't mean anything scientifically, and while I haven't seen research on this, I am pretty sure, different points on the earth would have different potential, meaning there would be voltage between them. Does that make sense? Once we bond the neutral and the earth together, we make those two points have 0 volts between them, at that point in the earth. But not really because there is always resistance between the grounding electrode and the surrounding earth, so once again voltage.

 

[Bro8898]

I will tackle this part. Voltage is the potential difference between two points. Without defining both points, voltage can't be measured. We generally reference ourselves to the surrounding earth and call that zero volts for simplicity. It doesn't mean anything scientifically, and while I haven't seen research on this, I am pretty sure, different points on the earth would have different potential, meaning there would be voltage between them. Does that make sense? Once we bond the neutral and the earth together, we make those two points have 0 volts between them, at that point in the earth. But not really because there is always resistance between the grounding electrode and the surrounding earth, so once again voltage.

Thank you for the quick response. Bonding neutral and earth together sounds to me like a ground rod making contact with the earth, so there would be 0 volts between the rod and the earth at that point.

But as you get farther from the rod will you read a higher potential if you were to touch the rod with one lead and stick the other lead in the earth a few feet away? If I’m on the right track, why does that difference exist? Is there a potential difference there because there is another ground rod located farther away at the transformer that the current is trying to flow to?

You also make me think of equipotential bonding around a pool. Sounds like the goal is to make all of the earth in the area as close to the same potential as neutral as possible. But in the case of a pool the connection to ground is much more sophisticated than a single ground rod connection.

Thanks

 

[wwhitney]

But as you get farther from the rod will you read a higher potential if you were to touch the rod with one lead and stick the other lead in the earth a few feet away?

Only if there is current flowing through the ground rod into the earth. If there is current flowing, then the resistance of the earth between the point of contact of the rod and the point of contact of your other voltmeter lead will cause a corresponding voltage drop, according to V=IR.

E.g. suppose the POCO transformer's secondary neutral is earthed at the pole, and you take a 120V (to earth/neutral, our chosen 0V reference) ungrounded conductor and fault it directly to the ground rod at the house. Now measure the voltage between the ungrounded conductor and the neutral conductor (which is not part of the current path), and say it's 120V. That 120V is all dropped along the current path of the ground rod at the house, the earth, and the ground rod at the transformer. If you put an ammeter on the circuit and it reads 1 amp, then you know that that current path has a resistance of 120 ohms (V=I*R). So if you stick your earth probe into the ground half-way between the two ground rods, and keep the voltmeter lead on the house ground rod, you might read 60V (if the two ground rods have identical earth resistances). Very close to the house ground rod, you might read closer to 0V.

The exactly spatial pattern in the earth by which that measured voltage would change is complicated. If you measure from the ground rod to a point 3 foot away, I'm not entirely sure whether you'd be more likely to measure 10V or 50V. I'm thinking 50V is more likely, as I think most of the resistance is attributable to the limited amount of soil that is very close to the ground rod, and that as you get farther away, the portion of the soil carrying the current gets much larger, so the additional resistance of traveling additional distance is quite small.

Cheers, Wayne

 

[Bro8898]

Only if there is current flowing through the ground rod into the earth. If there is current flowing, then the resistance of the earth between the point of contact of the rod and the point of contact of your other voltmeter lead will cause a corresponding voltage drop, according to V=IR.

I think that’s where my question comes into play. Rather than fault a 120v conductor to a ground rod and measure voltage between it and the transformer neutral (which it sounds to me like in this case the earth becomes the “wire” and completes the circuit), what about when unbalanced neutral current has nowhere to go because of an open neutral. Now you have return current trying to get back to the rod at the transformer flowing through the earth. Is this case of an open neutral any different than the scenario you described?

Thanks for your earlier response

 

[Speedskater]

A very basic universal definition for the lower case word 'ground' is:
A common reference point (or path to that point)

So in a local 120/240 Volt system, the point would be the power company transformer Neutral.
So thru Planet Earth could be a path to that common point.

There are also several upper case definitions of Ground.

 

[Bro8898]

A very basic universal definition for the lower case word 'ground' is:
A common reference point (or path to that point)

So in a local 120/240 Volt system, the point would be the power company transformer Neutral.
So thru Planet Earth could be a path to that common point.

There are also several upper case definitions of Ground.

That makes sense. I can see what whitney said about connecting a hot to the ground rod and the earth being the path. What about an open neutral where the current trying to return to the transformer has no other path to the transformer neutral but through the earth? Although the earth is a poor conductor. Would there be a measurable difference in potential between the ground rod at the house and the grounded neutral at the transformer, and also at certain points in the ground in between?

 

[retirede]

That makes sense. I can see what whitney said about connecting a hot to the ground rod and the earth being the path. What about an open neutral where the current trying to return to the transformer has no other path to the transformer neutral but through the earth? Although the earth is a poor conductor. Would there be a measurable difference in potential between the ground rod at the house and the grounded neutral at the transformer, and also at certain points in the ground in between?

In reality, in a panel with diverse loads, an open neutral will cause the L-N voltages to vary wildly in proportion to the load. It basically becomes a 240V-only circuit with the 120V loads in series. The impedance of the dirt is too high to conduct what would have been the neutral current. Step potential in the dirt is the least of your problems.

There are cases where this won’t happen (like in areas with a metallic municipal water system). In that example, your (and your neighbor’s) pipes carry the neutral current. Still little to no current in the dirt.

 

[Bro8898]

The impedance of the dirt is too high to conduct what would have been the neutral current. Step potential in the dirt is the least of your problems.

So because the impedance of the dirt is so high most, if not all, of the current returns on the other 120v side making the whole circuit 240. Is there any way someone that touches the ground wire to the rod or a plumbing pipe that’s bonded could become part of a parallel circuit back to the transformer through earth? Or is the resistance so high that all of the current will return on the other 120v side?

I keep coming back to the idea of being shocked between a hot and the earth, ie touching a bare hot wire and standing on wet earth with bare feet. My question is how is unbalanced neutral return current that has nowhere to go (because the main neutral has been broken) any different than a hot ungrounded conductor?

 

[jaggedben]

A very basic universal definition for the lower case word 'ground' is:
A common reference point (or path to that point)

That's not a *universal* definition. Far from it. 'ground' means different things in different contexts. Notably, another basic definition of ground is the one the NEC uses: the earth.

 

[jaggedben]

So because the impedance of the dirt is so high most, if not all, of the current returns on the other 120v side making the whole circuit 240. Is there any way someone that touches the ground wire to the rod or a plumbing pipe that’s bonded could become part of a parallel circuit back to the transformer through earth? Or is the resistance so high that all of the current will return on the other 120v side?

The entire circuit can be modeled mathematically as a combination of impedances in series and parallel. Done correctly, you'll find that the higher the resistance of the neutral connection back to the transformer, whether through ground or the neutral wire, the more negligible the effect of the neutral becomes. That is, with zero neutral conductance (open and ungrounded neutral), the loads behave as a 240V series circuit. With a proper low impedance neutral connection, the hots remain closely balanced with each near 120V to ground. In between, say a loose but not totally open neutral, you can have an in between situation where the voltage imbalance is worse than it should be but not like having a totally series circuit. Having only ground rod connections between service neutral and transformer neutral is high enough resistance that it's closer to having no neutral than a poorly connected one.

I keep coming back to the idea of being shocked between a hot and the earth, ie touching a bare hot wire and standing on wet earth with bare feet. My question is how is unbalanced neutral return current that has nowhere to go (because the main neutral has been broken) any different than a hot ungrounded conductor?

Your concern is theoretically valid. An open neutral means that the voltage of the load neutral to ground is no longer predictable. It also could create a voltage gradient around a grounding electrode that creates a shock hazard within arms length of the electrode. This would be worse if the connected load is unbalanced enough that one of the hot conductors becomes much closer to 240V to ground.

 

[Bro8898]

It also could create a voltage gradient around a grounding electrode that creates a shock hazard within arms length of the electrode. This would be worse if the connected load is unbalanced enough that one of the hot conductors becomes much closer to 240V to ground.

Thank you for this. I feel like I’ve read this multiple places on the forum. It confirms what I was thinking, specific to my situation. When you say “could create a voltage gradient around a grounding electrode that creates a shock hazard within arms length of the electrode” - if I’m touching the ground rod or the ground wire what other thing do I have to be touching if current were to flow through me? Would it be a good connection to the ground, ie knee or bare feet? If I were to touch another object that is bonded to the grounding system like a plumbing pipe, I would be touching something at equal potential which no current would flow through me. Does this sound right?

Thanks so much for your reply.

 

[winnie]

I think that you are very much on the right track with your thoughts, and that if you sit down and go through the math of different possible scenarios the concepts will gel.

'What you would need to be touching to get a shock' depends on the voltage present, the insulation values of your skin, and just how good the contact is.

Soil is not a particularly good conductor, but there is a huge amount of it. You can approximate a circuit with multiple ground electrodes as if the entire earth were a perfect conductor but each electrode has its own resistance. Obviously this is an approximation that falls apart if the electrodes are close together, but it is pretty good if the electrodes are more than their own size apart.

Look up the term 'step potential' for what to expect near an energized ground electrode.

Jonathan

 

[Bro8898]

step potential

Thanks for the reply Jonathan. I’ve thought about that before. I went back and researched it after you suggested it. So it sounds like there is a difference in voltage potential at different distances from the ground rod and if you have each foot at a different potential there is a possibility for current to flow through you. I mentioned earlier about equipotential bonding around a pool. Is this when an elaborate bonding structure is created underground to minimize or eliminate the possibility of step potential occurring around a pool area?

I think it relates to what I’m working to learn because I’m interested in neutral current (in the case of an open neutral) trying to find a path back to the transformer through the earth. If you were to create many grounding points like in pool bonding you would be keeping the the potential of the earth at the same potential as neutral and eliminating step potential. In my particular case there is no elaborate grounding system but a normal ground rod and sometimes the plumbing pipe is also a part of the grounding path. I’m trying to learn what two things I need to be touching in order for current to flow through me (or anyone) in the case of an open neutral.

 

[winnie]

You are correct that equipotential bonding is done to eliminate voltage differences between two points of contact with soil (or two different points in water in contact with concrete in contact with soil), and is done in pool areas because people at pools are wet and wearing little by way of protection.

The other place where equipotential bonding is seen is at places such as substations, to reduce 'step potential' in the event of a major high power fault to earth. There the power levels are high enough that step potential voltages might be large even for normal shoes.

In the case of a plumber dealing with an open neutral situation? I'd be concerned about contact between any metal that is bonded to the electrical system neutral and bare skin in contact with wet soil or metal that is independently in contact with soil. This might be someone who cuts the incoming water pipe (so now you have an electrically bonded part of the pipe and a separate length of pipe in the soil), or someone working on plumbing next to a non-bonded metal drain pipe.

In an open neutral scenario, the 'source voltage' is variable, anything from 0 to full hot voltage. The risk of shock changes with the 'source voltage' that is pushing current though the person.

-Jonathan

 

[wwhitney]

The entire circuit can be modeled mathematically as a combination of impedances in series and parallel. Done correctly, you'll find that the higher the resistance of the neutral connection back to the transformer, whether through ground or the neutral wire, the more negligible the effect of the neutral becomes.

An aside for some algebra; here's what I'm getting:

If the source is a voltage V from L1 to N and N to L2, and if the load is an impedance R1from L1 to N and an impedance R2 to N to L2, the current in the neutral is V * (1/R2 - 1/R1). That should be familiar, as it is just the difference of currents, and I = V/R. We can also write this as V*(R1-R2)/R1R2.

If we insert an impedance R0 in the neutral conductor, the current in the neutral turns out to be V*(R1-R2)/(R1R2+R0R1+R0R2). The voltage difference between the source neutral point and the load neutral point is just the voltage drop across the impedance R0, which is R0 times this current, or V*(R1-R2)/(R1R2/R0 + R1 + R2).

So take an example where V=120V, the load R1 is 12 ohms (i.e. 10A load), and the load R2 is 6 ohms (i.e. 20A load). If the neutral impedance is 0, we obviously get 0 volts between the load midpoint and the source midpoint. If the neutral impedance is infinite, then we get 120*(12-6)/(12+6), or 40V. And if the neutral impedance is 100 ohms, which would be a pretty good path through the earth, the voltage would be 38V. Even just 6 ohms, same as the heavy load, gives a voltage of 24V.

I think it's fair to say that when the neutral impedance significantly exceeds the load impedances, the voltages involved will be very close to a truly open neutral.

Cheers, Wayne

 

[Bro8898]

I'd be concerned about contact between any metal that is bonded to the electrical system neutral and bare skin in contact with wet soil or metal that is independently in contact with soil.

In an open neutral scenario, the 'source voltage' is variable, anything from 0 to full hot voltage. The risk of shock changes with the 'source voltage' that is pushing current though the person.

This is perfect. Thank you so much for teaching me. In an open neutral in a plumbing situation, would you say there’s a chance that someone (not necessarily a plumber) touching a kitchen sink or a metallic shower valve/head could feel a tingle or a shock even if they weren’t on bare ground, like standing on a finished floor for instance? Maybe if the conditions were right as in your last statement if the source voltage is very high due to a highly unbalanced load.

I remember you all teaching me years ago about neutral current on a plumbing pipe flowing through someone else’s neutral and “masking” a serious open neutral. And I appreciate you hearing the difference and understanding what I was asking in this situation. That was very thoughtful of you to see it in another way and go through possible scenarios that could be different than the plumbing pipe being used as the only neutral return path. Thank you for taking the time to listen and respond.

Also as someone commented earlier if the load was that highly unbalanced and lights were flickering and other strange things going on then the plumbing would probably be the last of your concerns!

 

[winnie]

This is perfect. Thank you so much for teaching me. In an open neutral in a plumbing situation, would you say there’s a chance that someone (not necessarily a plumber) touching a kitchen sink or a metallic shower valve/head could feel a tingle or a shock even if they weren’t on bare ground, like standing on a finished floor for instance? Maybe if the conditions were right as in your last statement if the source voltage is very high due to a highly unbalanced load.

There is one scenario that IMHO is quite plausible, especially for someone in a shower.

Metal supply pipes are pretty much universally bonded to the electrical system.

But I think people generally ignore drain pipes.

Lots of old houses have cast iron drain stacks leading to underground cast iron pipes, and these don't get bonded.

I could imagine someone standing in the shower, with their feet in water that is in contact with a grounded metal pipe, feeling a jolt from a metal shower head connected to an electrical system with a broken neutral.

-Jonathan