Utility and Main service disconnect

[mikehughes8]

An equipment grounding conductor isn't required between the utility transformer and the main service disconnect. The nuetral is bonded and grounded at the service and the service disconnect. Is it OK to include an equipment grounding conductor also?


Secondly, on a seperately derived system why is it that only the generator or service disconnect are installed with a grounding electrode but not both. It also seems that on these systems an EGC is required. Why the difference?

Any help would be appreciated. I haven't been in the electrical realm long and I'm trying my best to learn.

 

[George Stolz]

An equipment grounding conductor isn't required between the utility transformer and the main service disconnect. The nuetral is bonded and grounded at the service and the service disconnect. Is it OK to include an equipment grounding conductor also?

If you did, it would be a parallel conductor to the neutral. Parallel conductors are addressed in 310.4.

It might be more cost effective to simply upsize the neutral, if you are interested in providing a lower-resistance ground-fault clearing path.

Secondly, on a seperately derived system why is it that only the generator or service disconnect are installed with a grounding electrode but not both.

They are both required to be connected to the Grounding Electrode System, see 250.30.

It also seems that on these systems an EGC is required. Why the difference?

The NEC is geared towards providing a ground-fault clearing path that is normally not current-carrying. Since installations governed by the NEC almost always have a starting point outside their control (as in, a serving utility on the line side of the service point), they have to write the NEC with that in mind.

Therefore, since the utility does all their bonding with the neutral conductor, the NEC is written to take that into consideration and start a seperate bonding system at the earliest opportunity, the service disconnect.

Hope that helps,

 

[mikehughes8]

If you did, it would be a parallel conductor to the neutral. Parallel conductors are addressed in 310.4.

It might be more cost effective to simply upsize the neutral, if you are interested in providing a lower-resistance ground-fault clearing path.



They are both required to be connected to the Grounding Electrode System, see 250.30.


The NEC is geared towards providing a ground-fault clearing path that is normally not current-carrying. Since installations governed by the NEC almost always have a starting point outside their control (as in, a serving utility on the line side of the service point), they have to write the NEC with that in mind.

Therefore, since the utility does all their bonding with the neutral conductor, the NEC is written to take that into consideration and start a seperate bonding system at the earliest opportunity, the service disconnect.

Hope that helps,

Thanks, that makes sense. I appreciate it.

 

[iwire]

An equipment grounding conductor isn't required between the utility transformer and the main service disconnect. The nuetral is bonded and grounded at the service and the service disconnect. Is it OK to include an equipment grounding conductor also?

I agree with George that this is generally prohibited by 310.4 and IMO 250.6(A).

But the real question is why would you want to run this extra conductor?

What do you see as a benefit to running this additional conductor?

Secondly, on a separately derived system why is it that only the generator or service disconnect are installed with a grounding electrode but not both.

All separately derived systems require grounding electrodes but not all generators are separately derived systems.

 

[mikehughes8]

I agree with George that this is generally prohibited by 310.4 and IMO 250.6(A).

But the real question is why would you want to run this extra conductor?

What do you see as a benefit to running this additional conductor?



All separately derived systems require grounding electrodes but not all generators are separately derived systems.

In my situation, we have a 16 megawatt diesel generator power plant. We are installing distribution using 11KV 500MCM underground feeders connected into primary switching centers. From here we run 2/0 to transformers which in most cases are installed in place of previously existing generator. In a cost saving effort. The grounding grid left from the generator to include the EGC is being re-utilized with the transformer. They could simply not connect the EGC. But for some reason they have been. I've just kinda gone along with it. No point in rocking the boat and I didn't see any harm in doing so.

 

[iwire]

II didn't see any harm in doing so.

250.6(A)?

250.6 Objectionable Current over Grounding Conductors.
(A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, and conductive non?current-carrying materials and equipment shall be installed and arranged in a manner that will prevent objectionable current over the grounding conductors or grounding paths.

IMO having the unbalanced neutral current traveling through the grounding grid is objectionable.

 

[mikehughes8]

250.6(A)?




IMO having the unbalanced neutral current traveling through the grounding grid is objectionable.

Well... I have a couple more questions.

Objectionable current is created in the grounding conductors between the service and service disconnect if an EGC is used in parallel with the grounded conductor (neutral). However, a generator on a seperately derived system has an EGC along with grounded conductor. Wouldn't this create objectionable current also. This point is a source of confusion for me.

At what point does current in the grounding conductors become objectionable?

 

[dahualin]

If you did, it would be a parallel conductor to the neutral. Parallel conductors are addressed in 310.4.

I don't really agree with this. The extra grounding conductor can be bonded to utility transformer enclosure and bond transformer enclosure to neutral. If it is in this situation, grounding conductor cannot bond with neutral conductor at service disconnect.


Mikehughes8,

The neutral conductor sometimes has unbalanced current. If the grounding system doesn't bond correctly, it is very likely get objectionable current run in grounding system that is very dangerous. Because all exposed conduits and other metal enclosure can be the path to person who touches those exposed metal and got shocked.

As my understanding, one power distribution system (with one power source) should have only one bonding point between grounding conductor and grounded conductor (neutral) to provide effective ground-fault path. If you bond them twice, the objectionable current will likely run from bonding point 1 to bonding point 2. The best bonding point is the power source (Service utility transformer, Separately derived system) and the second best bonding point is the service disconnent. Since the service utility transformer is controlled by utility company (electrical engineer cannot ask them to bond anything), so NEC reqires the bonding between grounding conductor and neutrol conductor in service disconnect and upsizing the neutral from service disconnet to service transformer as effective ground-fault path, no grounding conductor is needed between service disconnect and service transformer. Since Separately derived system is under our control (NEC can cover it), NEC allow you to bond them at either the power source (Separately derived system) or service disconnect (NEC250.30). But not both.

 

[mikehughes8]

I don't really agree with this. The extra grounding conductor can be bonded to utility transformer enclosure and bond transformer enclosure to neutral. If it is in this situation, grounding conductor cannot bond with neutral conductor at service disconnect.


Mikehughes8,

The neutral conductor sometimes has unbalanced current. If the grounding system doesn't bond correctly, it is very likely get objectionable current run in grounding system that is very dangerous. Because all exposed conduits and other metal enclosure can be the path to person who touches those exposed metal and got shocked.

As my understanding, one power distribution system (with one power source) should have only one bonding point between grounding conductor and grounded conductor (neutral) to provide effective ground-fault path. If you bond them twice, the objectionable current will likely run from bonding point 1 to bonding point 2. The best bonding point is the power source (Service utility transformer, Separately derived system) and the second best bonding point is the service disconnent. Since the service utility transformer is controlled by utility company (electrical engineer cannot ask them to bond anything), so NEC reqires the bonding between grounding conductor and neutrol conductor in service disconnect and upsizing the neutral from service disconnet to service transformer as effective ground-fault path, no grounding conductor is needed between service disconnect and service transformer. Since Separately derived system is under our control (NEC can cover it), NEC allow you to bond them at either the power source (Separately derived system) or service disconnect (NEC250.30). But not both.

I appreciate the explanation. I think I have a much better understanding now.

Thank You All

 

[George Stolz]

If you did, it would be a parallel conductor to the neutral. Parallel conductors are addressed in 310.4.

I don't really agree with this. The extra grounding conductor can be bonded to utility transformer enclosure and bond transformer enclosure to neutral. If it is in this situation, grounding conductor cannot bond with neutral conductor at service disconnect.

Unfortunately, 250.24(B) does not allow us that option. A main bonding jumper is required to be installed in every service; there are no exceptions for a service supplied with an EGC run with the service conductors.

There has been talk about sending a proposal their way for just such a circumstance, but there has not been any such action to date, that I'm aware of.

The neutral conductor sometimes has unbalanced current. If the grounding system doesn't bond correctly, it is very likely get objectionable current run in grounding system that is very dangerous. Because all exposed conduits and other metal enclosure can be the path to person who touches those exposed metal and got shocked.

What you've written confuses me, particularly what I've underlined. A neutral conductor always carries unbalanced current. If the neutral conductor is not securely terminated and suddenly has a high resistance or opens, then the unbalanced neutral current will begin making use of higher-resistance paths that normally carry minimal current (ie conduit, through people, through the earth, to the utility grounding electrode system(s), to the source).

A grounding electrode system will likely not bring the voltage down much in an open neutral scenario, it would depend on the particulars.

The best bonding point is the power source (Service utility transformer, Separately derived system) and the second best bonding point is the service disconnent.

Theoretically, why?

In the real world, has this ever been done? I've never seen an EGC with service conductors.

Perhaps you are grouping SDSs and services in your post - would it be all right with you to focus solely on services for now, and perhaps return to SDSs later?

 

[dahualin]

George,

People ask question here because they don't understand how it works even they have NEC or some other reference books.

In my previous post, I tried to explain it to Mike in my own words.

Here is the simple response to the highlight in your post:

1. "If the grounding system doesn't bond correctly" is my own words. It shall be "Improper Neutral-to-Case Bond" as per NEC. That will cause objectionable current. I believe you know the danger of objectionable current.

2. "The best bonding point is the power source" is my own words and my opinion. The main bonding jump that bonds the equipment grounding conductor at service equipment to the grounded nertral service conductor is to provide Effective Ground-Fault Current Path. But the service equipment is not the power source. If electrical contractor installed rigid metal conduit between service transformer and service equipment (assuming the conduit connect to the enclosure of transformer and service equipment), then objectionable current is going to flow from the service equipment to transformer because we have main bonding jumper in the service equipment and utility company put solid grounded system bonding jumper in their transformer. If you look at the diagram of a power system, the grounding conductor (effective ground fault current path) shall be separated from grounded neutral conductor except the power source (service transformer). That is why I said the best bonding point is the power source. Unfortunately, NEC cannot really cover service transformer, so it mandates to install main bonding jumper in service equipment in case some utility company forget bond their transformers. That is my opinion.