Equipment Grounding Conductors in utility plant

[don_resqcapt19]

I hope I am not being repetitive. The NESC does apply here and Section 9 must be followed. When outside electricians go into a power generation station where only the NESC is applied, they find massive amounts of "Code" violations. However, they are NEC violations, not NESC violations.

Many utilities have sold there generation to non-regulated comanies. If this is the case, it is my opinion that these generation plants are covered by the NEC and not the NESC as they are no longer utility owned. I am not saying that this is the case in this thread, but it is possible.

 

[don_resqcapt19]

I work in power plants occasionally and, though most work I see and do has EGC's, the grounding situation is otherwise as you describe. However, one thing that most NEC-conscious electricians fail to realize is that the redundant GEC-type grounding also provides 'networked' paths to the source.... so when a fault does occur, current does not just take one path back to the source... it takes all available paths. Being 'networked' paths, the impedence is quite low and taking all available paths minimizes the potential at any one point.
FWIW, all the grounding grid/rings/bonds I've worked on have been 4/0 Cu.

Smart,
It is my understanding that these remote paths do little to decrease the impeadance of the fault return path for high current faults. The fact that the supply and return are physically separate increases the impedance of the remote return path. They are effective in reducing step and touch potential and is often the reason they are used.

 

[charlie]

I have a client who is a power generation utility, and thus is not bound by the NEC. (This is the 1st sentence of the OP)

I am making the assumption that the OP knows that this is a power generation utility that is recognized by the public service commission, utility commission, or other regulatory agency having jurisdiction for such installations. :smile:

 

[crossman]

Am I correct in assuming that this violates NEC requirements and poses an increased hazard to workers

But the Op asked the question above at the end of his post, which is why we are talking about the NEC regulations for EGCs.:smile:

 

[charlie]

Am I correct in assuming that this violates NEC requirements and poses an increased hazard to workers (i.e. touch voltage)?

OK Gene, the answer is yes for the first question and no for the second question in the sentence.

 

[Bentworker]

Just a rant by me.

They may just be worried about ground potential rise, and nothing else since they are tying everything to the grid. Not that I can quote anything but it seems that every electric utility has their own theory on how to best protect their equipment and personnel. Sometimes the way things are done by one company is the exact opposite of what another is doing. Since I have been on the job I have NEVER seen anyone from any regulatory body review any of our prints or inspect any of our work (and I've probably seen 60+million dollars of equipment installed or replaced). The occasional county inspector shows up and wonders when he gets issued a high voltage suit, they may look around then rubber stamp whatever the project may be. Some of these systems have to be designed for fault duties of over 100,000 amps, at transmission voltages (60+kV), so I hate to say it but I think the attitude of many companies is that they just interpret the rules however they choose, and in this case it probably means that they are ignoring all of the "small" stuff that is not generating them income.

 

[charlie]

Just a rant by me.

I hope we can disagree and still be friends?

 

[Bentworker]

I hope we can disagree and still be friends?

Yep. Nothing to worry about, in this business everybody disagrees sometime.

 

[charlie]

Yep. Nothing to worry about, in this business everybody disagrees sometime.

Cool :smile:

 

[quogueelectric]

The grounding grid or the grounding electrode should never be part of the fault clearing path. The path should be via the EGC to the main bonding jumper to the grounded supply conductor. With large current circuits a remote fault clearing path (EGC not run with the circuit conductors) will have a much higher impedance as a result of inductive reactance and may not be able to flow enough current to operate the OCPD in the instantaneous range or even in the short time range.

I dont understand for a second how an engineered grounding grid in parallel with the egc will increase the total impedance of the fault current please enlighten me.

 

[charlie]

I dont understand for a second how an engineered grounding grid in parallel with the egc will increase the total impedance of the fault current please enlighten me.

Not in parallel with the grounding conductor but in place of the grounding conductor. If not engineered correctly, Don is correct. :smile:

 

[quogueelectric]

Not in parallel with the grounding conductor but in place of the grounding conductor. If not engineered correctly, Don is correct. :smile:

My bad I realize now he said no egc he is correct it is unanimous. 2 screaming todlers and 1 screaming wife sometimes I miss the small print sorry.

 

[msteiner]

As the original poster, thanks to everyone for the input! I'm not clear right now on whether or not the specific "utility" I'm referring to is actually a regulated utility, so I can't say for certain whether the NEC or NESC applies. Not having much familiarity with the NESC, can anyone chime in as to whether or not the grounding methodology I describe would be NESC-compliant?

 

[tryinghard]

As the original poster, thanks to everyone for the input! I'm not clear right now on whether or not the specific "utility" I'm referring to is actually a regulated utility, so I can't say for certain whether the NEC or NESC applies. Not having much familiarity with the NESC, can anyone chime in as to whether or not the grounding methodology I describe would be NESC-compliant?

The NEC is only authored for two reasons for the use of electricity: personal and property protection. It is true a powerhouse is not regulated by the NEC but this is not meant to give free rain rather it means more stringent codes apply. I say this because commonly electricians (or other installers) will rationalize an installation by stating "the NEC does not count in a powerhouse", ironically this statement is never made in defense of a more stringent application rather it is always made to rationalize a simpler installation.

Most powehouses are utility owned and most installations to existing are not inspected by a non-bias electrical inspector, because of this most installations are left to the installers interpretations for correctness. This format is really "design/build" and if the design/build is not drawn it loses the benefit of yet another form of critique allowing unsafe installations.

The electrical trade is very large and there are many important things to consider for each application, it is very easy for a qualified journeyman electrician to miss an item especially with design/build, and with the use of electricity this most often means the application is unsafe, it is rarely an esthetical mistake.

With all this said I recommend you find out from your client what type of agency they are. With the may facets to the electrical trade I recommend an electrical engineer who's niche is powerhouses this type of engineer is often not real familiar with construction electrical so involve another electrical engineer who's niche is industrial electrical preferably in powerhouses. Make sure the grounding grid system is qualified by the EE for powerhouse generation, and you'll find at minimum the bonding throughout the powerhouse will be NEC compliant.

I believe you'll be using both.

 

[Smart $]

Smart,
It is my understanding that these remote paths do little to decrease the impeadance of the fault return path for high current faults. The fact that the supply and return are physically separate increases the impedance of the remote return path. They are effective in reducing step and touch potential and is often the reason they are used.

Sorry for the delay in getting back to you... been busy and your response didn't make the newest posts that I checked in on

I'm thinking you have a misconception to the type of GEC-grid I'm referring to in that you say the supply and return are separate. In my experience they are not... this structure's grid is tied to every structure's grid throughout the whole facility. If it's metal, it's bonded... sometimes twice, and that's not counting the conduit or EGC, or incidental contact with other bonded equipment or material.

However, I'm open to correction because I am not part of the design team, so my information is second hand at best... some perhaps just rational speculation

You are correct though about these ["remote"] paths doing little to reduce the impedance. This is the intended result. The theory is, "add a path, reduce the impedance just a bit more", just like adding another resistor to a parallel resistor network, and when done consistently, each reduction in impedance is more minute as the progression continues...

 

[mivey]

As the original poster, thanks to everyone for the input! I'm not clear right now on whether or not the specific "utility" I'm referring to is actually a regulated utility, so I can't say for certain whether the NEC or NESC applies. Not having much familiarity with the NESC, can anyone chime in as to whether or not the grounding methodology I describe would be NESC-compliant?

If it is designed properly, yes.

NESC Section 9:
093.Grounding Conductor and Means of Connection
093A.Composition of Grounding Conductors...shall be made of copper...Metallic electrical equipment cases or the structural metal frame of a building or structure may serve as part of a grounding conductor to an acceptable electrode.

096 Ground Resistance Requirements
Grounding Systems shall be designed to minimize hazard to personnel and shall have resistances to ground low enough to permit operation of circuit protection devices...

096A. Supply Stations
Supply stations may require extensive grounding systems consisting of multiple buried conductors, grounding electrodes, or interconnected combinations of both. Grounding systems shall be designed to limit touch, step, mesh, and transferred potentials in accordance with industry practices.

NESC Section 11:
123. Protective Grounding
123A. Protective grounding or Physical Isolation of Non-Current-Carrying Metal Parts. All electric equipment shall have the exposed non-current-carrying metal parts, such as frames... effectively grounded or physically isolated...

123B. Grounding Method. All grounding...shal be made in accordance with the methods specified in Section 9 of this code

NESC Section 16: Conductors
160.Application...Conductors shall have ampacity that is adequate for the application.

 

[don_resqcapt19]

I'm thinking you have a misconception to the type of GEC-grid I'm referring to in that you say the supply and return are separate. In my experience they are not... this structure's grid is tied to every structure's grid throughout the whole facility. If it's metal, it's bonded... sometimes twice, and that's not counting the conduit or EGC, or incidental contact with other bonded equipment or material.

However, I'm open to correction because I am not part of the design team, so my information is second hand at best... some perhaps just rational speculation

You are correct though about these ["remote"] paths doing little to reduce the impedance. This is the intended result. The theory is, "add a path, reduce the impedance just a bit more", just like adding another resistor to a parallel resistor network, and when done consistently, each reduction in impedance is more minute as the progression continues...

I know exactly what type of installation you are talking about. No matter how many "remote" or "separate" paths are installed most of the fault current will flow on the EGC that is run with the circuit conductors. I am not sure that lowering the impedance of the fault return path is the goal of this type of installation. I think it is an attempt to reduce the step and touch potential.

 

[mivey]

I forgot this NESC section:
093C.Ampacity and Strength
093C5.Grounding Conductors for Equipment...
093C5a.Conductors. The grounding conductors for equipment, raceways,...shall have short time ampacities adequate for the available fault current and operating time of the system fault-protective device...

These standard sections sound like what you are talking about:

And from IEEE Std 665-1995 (Generating Station Grounding):
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4.1 c) The equipment ground ensures a low impedance return path for ground current should an electrical fault occur between the live conductors and the equipment enclosure. If such a return path is ensured, the circuit protection will be able to trip the faulted circuit in a short time.
​

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5.5.3.3 Grounding with only safety grounding conductor run with conductors run to the local grounding grid.​

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The main advantage of this arrangement is that it minimizes the touch potential, since it connects together the equipment and the structures on which personnel may stand. For this reason, it is a suggested arrangement for systems with ground fault alarm. This arrangement is also used for grounding the station structures, auxiliary buildings, and all noncurrent-carrying accessible conductive materials that may be subjected to electrostatic or electromagnetic induction... The grounding conductor is run directly to the closest grounding electrode or to the ground grid connecting these electrodes. The use of dedicated grounding conductors is recommended.
...​

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The use of structural metal frames as grounding conductors is appealing due to their large size. This is
acceptable provided that the following conditions are met:
a) The cross-section is adequate for the ground fault currents that may occur.
b) All junctions are bonded with adequately sized connections...​

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[Smart $]

I know exactly what type of installation you are talking about. No matter how many "remote" or "separate" paths are installed most of the fault current will flow on the EGC that is run with the circuit conductors.

An expected response from someone as NEC-savvy [and conditioned] as yourself. The reality is two low-impedance paths will handle current twice as well. Three would be, well, three times as good... and so on. Current is very promiscuous (for lack of a better word, at present).

I am not sure that lowering the impedance of the fault return path is the goal of this type of installation. I think it is an attempt to reduce the step and touch potential.

Why not all of the above... are redundancy and reliability not on the table as considerations, too?

 

[don_resqcapt19]

Smart,
Yes, all parallel paths reduce the total impedance, however it is still my understanding that the impedance of any remote path is significantly higher than that of the EGC that is run with the circuit conductors and these remote paths do not make much of a change in the total parallel impedance. They are not redundant paths for the purpose of fault clearing as they are not suitable for clearing high current faults as a result of their high impedance.