Neutral to earth voltage

[Twoskinsoneman]

Trying get a better understanding of NEV.

I understand the voltage drop on the neutral causes a potential difference between the neutral and the earth where the utility is bonded to earth.

What I don't understand is why that voltage is supposed to be present even though the neutral at the service (and utility pole) is re-referenced to earth.

If the neutral is re-reference to earth (zero volts) then it seems like the only potential should be the voltage drop on the neutral since it was last referenced to the earth.

Any thoughts to make it more clear?

 

[mivey]

I understand the voltage drop on the neutral causes a potential difference between the neutral and the earth where the utility is bonded to earth.

It can't. They are essentially the same point.

What I don't understand is why that voltage is supposed to be present even though the neutral at the service (and utility pole) is re-referenced to earth.

Not sure what voltage you mean.

If the neutral is re-reference to earth (zero volts) then it seems like the only potential should be the voltage drop on the neutral since it was last referenced to the earth.

I think what you are saying here sounds right to me. I don't think I'm completely understanding what points and voltage your are referencing.

 

[Smart $]

Trying get a better understanding of NEV.

I understand the voltage drop on the neutral causes a potential difference between the neutral and the earth where the utility is bonded to earth.

What I don't understand is why that voltage is supposed to be present even though the neutral at the service (and utility pole) is re-referenced to earth.

If the neutral is re-reference to earth (zero volts) then it seems like the only potential should be the voltage drop on the neutral since it was last referenced to the earth.

Any thoughts to make it more clear?

Without getting into the possible differences in resistance to earth at grounding points, the neutral to earth bonding closest to the source always wins in the battle of voltage drop. The resistance of earth between grounding points is most assuredly higher than the resistance of the neutral conductor between grounding points.

 

[Twoskinsoneman]

It can't. They are essentially the same point.

Well there can be.... I do understand that. E= I * R Current flow on the conductor causes a voltage drop.

If the the neutral was only referenced to ground at the substation I could understand it perfectly.

What I don't understand is the fact the neutral is re-grounded at the service and therefore should reference earth again which would prevent the potential difference between the neutral conductor (and subsequently all metal parts connected to EGC) and the earth.

 

[don_resqcapt19]

Well there can be.... I do understand that. E= I * R Current flow on the conductor causes a voltage drop.

If the the neutral was only referenced to ground at the substation I could understand it perfectly.

What I don't understand is the fact the neutral is re-grounded at the service and therefore should reference earth again which would prevent the potential difference between the neutral conductor (and subsequently all metal parts connected to EGC) and the earth.

While it is reconnected to earth at many points that does little to change the voltage to "remote" earth (defined as earth outside the sphere of influence of a grounding electrode). The only change in the voltage is the small reduction in voltage drop that is the result of the additional parallel path through the earth. The voltage does not go away when you make the connection to earth...you just have another path for the current to travel on. All of the items that are directly connected to the electrical grounding system will be at the same voltage, however that voltage will not be zero with respect to "remote" earth. If you measure to "remote" earth you will be measuring the voltage drop on the utility primary neutral.

 

[mivey]

I thought when you said "...where the utility is bonded to earth..." you meant at the bonding point. They are essentially the same there (bonding resistance notwithstanding)

Well there can be.... I do understand that. E= I * R Current flow on the conductor causes a voltage drop.

Then you are good to go. Don't forget the earth is a conductor also.

 

[hurk27]

This is simply because the grounding electrodes we install don't always provide a low enough impedance connection to earth and can not possibly bring the two back to a zero point reference. you will see this in long runs from the transformers, we had a very bad problem with this with a house with a pool, and didn't have a right away to bring the transformer closer to the house, so the POCO worked with us, resolved it by running an extra grounding wire leaving the MBJ at the transformer, and separating the grounds and neutral at the service. it was fun trying to find a 320 can that the neutral wasn't bolted right to the can, but we did and the stray voltage at the pool was all gone.
Unfortunately we still had to bond the electrodes to the neutral, so we just buried them so they weren't easily touchable, the voltage at highest neutral load was only about 5-8 volts so it wasn't dangerous but very noticeable when stepping out of the water grabbing the hand rail.

Think about this one, what if your were to loose the neutral connection at the transformer with this same pool? could be deadly

 

[LarryFine]

What I don't understand is why that voltage is supposed to be present even though the neutral at the service (and utility pole) is re-referenced to earth.

It's not that there is "supposed to be" a bit of neutral-to-earth voltage, we just accept that there often is some. It's not so different than current on water-pipe GEC's in all-metal water-piping neighborhoods.

The re-grounding does, to a degree, minimize neutral-to-earth voltage, but it doesn't matter what the voltage is between the sub-station and our house. Think of it as a giant equi-potential grid. The voltage difference matters more than the absolute voltage.

Besides, who defines what absolute zero voltage really is? Maybe there's a large difference between the east-coast and west-coast grid ground. It doesn't really matter, since nobody has a wiggy with leads that long.

I wonder what the voltage difference between the earth and the moon is?

 

[Smart $]

If the the neutral was only referenced to ground at the substation I could understand it perfectly.

What I don't understand is the fact the neutral is re-grounded at the service and therefore should reference earth again which would prevent the potential difference between the neutral conductor (and subsequently all metal parts connected to EGC) and the earth.

Yes it is re-grounded at the service... but you have to remember the service starts [typically] at the local transformer at the utility end of the service drop or lateral, which BTW is considered the source of the system and where the neutral is first grounded. When bonded to earth at the service entrance, that is actually the second point the neutral is earth grounded.

The rest is all E=IR

 

[don_resqcapt19]

Yes it is re-grounded at the service... but you have to remember the service starts [typically] at the local transformer at the utility end of the service drop or lateral, which BTW is considered the source of the system and where the neutral is first grounded. When bonded to earth at the service entrance, that is actually the second point the neutral is earth grounded.

The rest is all E=IR

It is more that just the secondary side neutral...it is also the voltage drop on the primary neutral as the primary and secondary neutrals are bonded together.

 

[don_resqcapt19]

This is simply because the grounding electrodes we install don't always provide a low enough impedance connection to earth and can not possibly bring the two back to a zero point reference. ...

No matter how low the impedance of the grounding electrode is, you can't get rid of the neutral to ground voltage. You can reduce it some because the earth is a parallel path that reduces the voltage drop that is the source of the neutral to ground voltage, but you can't get rid of it unless you bring the voltage of the earth up to that of the neutral. That is just not going to happen. A neutral that carries current will always have a voltage as measured to remote earth. One thing that would go a long way towards getting rid of this problem would be to require that all utility transformers be connected line to line on the primary side and not line to neutral as many are now. This would get rid of the primary neutral voltage drop issue. Yes there will still be some on the secondary side, but that should not be as much and the use of larger secondary side grounded conductors would help.

 

[ptonsparky]

Are tranformers connected l-n (primary) for just single phase or is it also done for three phase? I am picturing pole top banks.

 

[LarryFine]

Are tranformers connected l-n (primary) for just single phase or is it also done for three phase? I am picturing pole top banks.

The only L-L primary connections I have seen are on older transformer banks, like the three cans on a raised platform between two poles. All the recent ones I've seen are L-N, single-bushing units, whether 1ph or 3ph.

 

[charlie]

The only L-L primary connections I have seen are on older transformer banks, like the three cans on a raised platform between two poles. All the recent ones I've seen are L-N, single-bushing units, whether 1ph or 3ph.

I agree with you Larry. The closest we come to L-L are in a wye primary, delta secondary configuration where we float the neutral on the primary. This only applies to pole mounted transformers since we use five legged cores on our pad mounted, three phase transformers.

 

[LarryFine]

The closest we come to L-L are in a wye primary, delta secondary configuration where we float the neutral on the primary.

I've never seen that. I can't imagine you float the cans, so you must be using dual-bushing units with one end of each tied together.

Or, since the floating-neutral should be somewhat close to zero, do you interconnect the small, side-mounted bushings? I've never seen that, either.

 

[charlie]

. . . I can't imagine you float the cans . . .

. . . you must be using dual-bushing units with one end of each tied together. . .

. . . do you interconnect the small, side-mounted bushings? . .

1. The tanks are grounded to the MGN.
2. We do exactly that.
3. I don't follow the question. I think the bushings you are talking about are the secondary bushings and they are connected into a delta configuration. On our system, it is normally 120/240 V, 3?, 4w.

:smile:​