Objectionable current

[dahualin]

NEC 250.20 (B)(2) requires grouding for utility transformer where the system is 3-phase, 4-wire, wye connected in which the neutral is used as a circuit conductor. It is not clear if the grounding conductor for transformer is allowed to connected to electrical grounding system for main service. If it is allowed (and it is done at most cases), I think that will result in objectionable current in electrical grounding system between main service panel and utility transformer when any unbalanced current existed in neutral cable.

That is the same idea as grounding for Separately derived systems.

Is it a problem to tie the grounding conductor for utility transformer to electrical grounding system? Any comments are welcome. Thanks.

 

[LarryFine]

Is it a problem to tie the grounding conductor for utility transformer to electrical grounding system?

David, the simple answer is that the grounded neutral does double-duty as the service EGC, and there shouldn't be an EGC run. If you're talking about electrodes at both the transformer and the service, that's not considered a paralleled connection.

The earth is not considered conductive enough to qualify as a conductor. This is similar to the soon-to-be-extinct exception that permits a 3-wire feeder to a separate building as long as there is no other grounded conductive pathway between them.

 

[bob]

NEC 250.20 (B)(2) requires grouding for utility transformer where the system is 3-phase, 4-wire, wye connected in which the neutral is used as a circuit conductor. It is not clear if the grounding conductor for transformer is allowed to connected to electrical grounding system for main service. If it is allowed (and it is done at most cases), I think that will result in objectionable current in electrical grounding system between main service panel and utility transformer when any unbalanced current existed in neutral cable. That is the same idea as grounding for Separately derived systems.
Is it a problem to tie the grounding conductor for utility transformer to electrical grounding system? Any comments are welcome. Thanks.

The NEC does not apply to utility electrical systems. The utility does ground
a transformer installation by driving a ground rod and installs a #6 cu conductor from the rod to the utility system neutral. The transformer secondary neutral is bonded to the primary neutral. When a service is installed, the service neutral is also bonded to the system neutral and is also grounded at the main switch by bonding it to the GEC. You would have any unbalanced current returning to the transformer over the service neutral. This is not considered as objectionable current.
I believe that this is the way its done every day.

 

[coulter]

Yes, sometime bonding and grounding the neutral at the xfm and again at the first disconnect is a problem. Even though it is done every day, as bob says, it still damn poor science.

Take an industrial installation where there is a ground mat under the entire area, substation xfm and process area. The transformer primary disconnect is at the xfm and secondary disconnect is 500 feet away in the process area.

Example 1:
The utility owns the xfm and the service is at the xfm secondary.

The utility bonds and grounds the xfm to the ground mat. Conductors to the service disconnect are 4W Y. NEC demands the service is bonded and grounded at the disconnect - again to the ground mat.

Whoops parallel connection of the neutral. More neutral current flows over the ground mat as over the neutral. Bad science - code correct.

Example 2:
User buys the transformer and service point is at the xfm primary. Still using secondary metering. Oh-oh SDS. The code now mandates that the neutral is bonded and grounded either at the xfm or the disconnect, but not both. Conductors from xfm to disconnect are now 5W Y. Good science - meets code.

What possible difference could it make on who owns the xfm?

It would appear the only code compliant option available for Ex 1 is to saw the ground mat into between the xfm and the process area. Dumb idea - poor science

Oh, I know. Next time I run into Ex 1 and the AHJ squawks about me wanting to run a 5W Y and not bonding at the panel, I'm going to get the utility to agree to the service point is on the primary, they still own the xfm and primary disconnect.

Yes this has come up. Fortunately the AHJ understood the physics, did not want a code compliant, substandard installation and okayed the engineering request to treat like an SDS.

carl

 

[dahualin]

The NEC does not apply to utility electrical systems. The utility does ground
a transformer installation by driving a ground rod and installs a #6 cu conductor from the rod to the utility system neutral. The transformer secondary neutral is bonded to the primary neutral. When a service is installed, the service neutral is also bonded to the system neutral and is also grounded at the main switch by bonding it to the GEC. You would have any unbalanced current returning to the transformer over the service neutral. This is not considered as objectionable current.
I believe that this is the way its done every day.

I am concerned about the unbalanced current returning to the transformer over the grounding (bonding) conductor bonded between ground rod for transformer and ground electrode system for the building not the part of unbalanced current over the service neutral. NEC considers those unbalanced current over grounding conductor or grounding path objectionable current that is dangerous. I think it is the same situation as Separatedly Derived System. If a power system has more than two grounding bonding (neutral to ground) points, an objectionable current will flow between the two points. And it is dangerous. Any thoughts? Thanks.

 

[LarryFine]

I am concerned about the unbalanced current returning to the transformer over the grounding (bonding) conductor bonded between ground rod for transformer and ground electrode system for the building not the part of unbalanced current over the service neutral.

Again, there need not, and should not, be a separate EGC between the utility and the service.

NEC considers those unbalanced current over grounding conductor or grounding path objectionable current that is dangerous. I think it is the same situation as Separatedly Derived System. If a power system has more than two grounding bonding (neutral to ground) points, an objectionable current will flow between the two points. And it is dangerous.

The system neutral will indeed be earthed at both places, but eartyh current is not considered parallel current.

 

[dahualin]

Again, there need not, and should not, be a separate EGC between the utility and the service.

The system neutral will indeed be earthed at both places, but eartyh current is not considered parallel current.

From electrical circuit theory, I don't understand why the utility sevice transformer is not Separately Derived System such as 480v to 208/120v step down transformer. They are both power sources. NEC doesn't allow more than one system bonding jumper for Separately Derived System to prevent objectionable current. What is the difference between the earth current mentioned in your post and objectionable current? The enclosures of utility transformer and main service equipment that connected to grounding system are exposed.

 

[LawnGuyLandSparky]

What is the difference between the earth current mentioned in your post and objectionable current? The enclosures of utility transformer and main service equipment that connected to grounding system are exposed.

The difference is, the earth current @ the utility transformer, and the earth current @ a disconnect or service, are bonded TO earth, which causes whatever potential to exist at that point to BE what we call "0 volts."

"Ground" is not actually a complete lack of voltage. "Ground" is actually a degree of potential or voltage which we only reference as zero, and call it zero, because everything is either nonconductive, or at that potential we call zero, and even if that potential is 1000 volts, since everything ELSE around is also at that same 1000 volt potential, we are perfectly safe.

If you think of the secondary windings on a transformer that is not referenced to ground - touching any conductor to ground will not short out the transformer, because with no ground reference the secondary windings are isolated from anything we would call ground. Everything would, as referenced to that transformer's secondary windings, become the same potential.

 

[dahualin]

The difference is, the earth current @ the utility transformer, and the earth current @ a disconnect or service, are bonded TO earth, which causes whatever potential to exist at that point to BE what we call "0 volts."

"Ground" is not actually a complete lack of voltage. "Ground" is actually a degree of potential or voltage which we only reference as zero, and call it zero, because everything is either nonconductive, or at that potential we call zero, and even if that potential is 1000 volts, since everything ELSE around is also at that same 1000 volt potential, we are perfectly safe.

If you think of the secondary windings on a transformer that is not referenced to ground - touching any conductor to ground will not short out the transformer, because with no ground reference the secondary windings are isolated from anything we would call ground. Everything would, as referenced to that transformer's secondary windings, become the same potential.

I don't know if we are talking about the same earth current. I am talking about the possible unbalanced current flow from service panel to transformer. The grounding conductor has resistance, the neutral cable has resistance and the earth has resistance. Even we have grounding system at both transformer and service equipment, it will have potential difference between transformer grounding point and service equipment grounding point if a unbalanced current is existing.

 

[LarryFine]

The grounding conductor has resistance, the neutral cable has resistance and the earth has resistance.

Again, there should be no separate grounding conductor run with the service conductors.

 

[LawnGuyLandSparky]

I don't know if we are talking about the same earth current. I am talking about the possible unbalanced current flow from service panel to transformer. The grounding conductor has resistance, the neutral cable has resistance and the earth has resistance. Even we have grounding system at both transformer and service equipment, it will have potential difference between transformer grounding point and service equipment grounding point if a unbalanced current is existing.

If I connect a conductor from the neutral in a service panel to a transformer's XO, and a ground rod at the pole to XO, and a ground rod at the service neutral, how much resistance will the conductor put against that current flow that the ground rods driven into earth would also force the earth to carry some of that current?

 

[dahualin]

Again, there should be no separate grounding conductor run with the service conductors.

Sorry, Larry. I think I have used wrong term "grounding conductor". There is no separate grounding conductor run with the service conductors. I have a service grounding detail that shows the grounded neutral of utility transformer bond to transformer enclosure, then bond to grounding ring around transformer, then bond to ground electrode system for the service equipment. So the grounded neutral of utility transformer is bond to service equipment neutral bar and ground bar through all those bonding connections.

 

[dereckbc]

There is no separate grounding conductor run with the service conductors. I have a service grounding detail that shows the grounded neutral of utility transformer bond to transformer enclosure, then bond to grounding ring around transformer, then bond to ground electrode system for the service equipment. So the grounded neutral of utility transformer is bond to service equipment neutral bar and ground bar through all those bonding connections.

That is just a poor design. In this case there is no difference between your grounded service conductor and EGC at the service. Both are going to carry unbalanced load current, there is no way around it other than change the poor design.

The simple solution is to use a different GE for the service ground, rather than the common one used as the utility ground.

 

[LarryFine]

I have a service grounding detail that shows the grounded neutral of utility transformer bond to transformer enclosure, then bond to grounding ring around transformer, then bond to ground electrode system for the service equipment. So the grounded neutral of utility transformer is bond to service equipment neutral bar and ground bar through all those bonding connections.

The connection in bold should not be made. The service neutral conductor (from X-0 to meter/disco neutral bar) should not be paralleled by the bonding conductor.


In other words:

The simple solution is to use a different GE for the service ground, rather than the common one used as the utility ground.