Grounding Grid Vs Ground Conductor

[jarrodp]

We are constructing a generating facility on a customers site. This facility contains a below grade grounding grid that connects to all of our generators, transformers, electrical enclosures, and to the building structure. The grid is designed to provide effective ground-fault protection and is made of bare copper cable and grounding rods.

Our electrical inspector is requiring, in addition to the grid, that all our phase carrying conduits also contain a grounding conductor. One example where this becomes an issue is our connection between our 480 switchgear and step-up transformer. This connection consists of 8 conduits.

I have always tried to eliminate ground loops to prevent noise in my systems. To have 9 loops created here, I must question this requirement.

Does the grounding grid not satisfy the requirements of Article 250.4 of the NEC 2002? Is there something else I am missing?

 

[dereckbc]

Re: Grounding Grid Vs Ground Conductor

Is he requiring an EGC? EGC serves a different purpose than a GEC bonding conductor. The EGC provides a fault clearing path for the associated phase circuits. GEC bonding conductors are to clear HV faults, and minimise the step potential problems associated with HV faults.

You are crossing over into NESC rules which are different than NEC rules.

[ October 18, 2004, 10:47 AM: Message edited by: dereckbc ]

 

[jarrodp]

Re: Grounding Grid Vs Ground Conductor

The inspector really has no reason other than ensuring all the equipment is at the same potential and that they are provided an effective ground-fault current path accourding to NEC, Art 250.4(3).

I believe the inspector is looking to find something that says the grounding grid can be used to satisfy the NEC requirement by itself.

 

[don_resqcapt19]

Re: Grounding Grid Vs Ground Conductor

The ground grid is a grounding electrode and it in no way takes the place of the required equipment grounding conductors. The inspector is correct. The raceways require equipment grounding conductors.
Don

 

[dereckbc]

Re: Grounding Grid Vs Ground Conductor

You cannot use the grounding grid (ground electrode) to clear LV (low voltage faults = >600 volts), you have to provide a EGC. Again the grounding grid is for HV faults and equalizing step rise potential differences during a HV fault. The NEC will permit multiple connections to the ground electrode system, but will not permit the GES to be used as a fault clearing path on LV, that is why you are required to run a EGC on low voltage systems.

In reality the grid will assist in clearing the LV faults.

 

[jarrodp]

Re: Grounding Grid Vs Ground Conductor

Is this stated in any portion of the NEC, about the GES not being able to satisfy the same requirements of the EGC?

If the Generators are 'Y' with their Neutrals tied to their frames, and their frames tied to the GES, which is then tied to everything else in the plant; are we not satisfying the requirments of Art 250.4?

 

[bphgravity]

Re: Grounding Grid Vs Ground Conductor

Originally posted by jarrodp:
Is this stated in any portion of the NEC, about the GES not being able to satisfy the same requirements of the EGC?

Yes, 250.4(A)(5): "the earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path."

 

[rbalex]

Re: Grounding Grid Vs Ground Conductor

250.134 Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed) ? Grounding.
Unless grounded by connection to the grounded circuit conductor as permitted by 250.32, 250.140, and 250.142, non?current-carrying metal parts of equipment, raceways, and other enclosures, if grounded, shall be grounded by one of the following methods.
(A) Equipment Grounding Conductor Types. By any of the equipment grounding conductors permitted by 250.118.
(B) With Circuit Conductors. By an equipment grounding conductor contained within the same raceway, cable, or otherwise run with the circuit conductors.

 

[don_resqcapt19]

Re: Grounding Grid Vs Ground Conductor

300.3(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductorsall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (4).

Don

 

[don_resqcapt19]

Re: Grounding Grid Vs Ground Conductor

If the Generators are 'Y' with their Neutrals tied to their frames, and their frames tied to the GES, which is then tied to everything else in the plant; are we not satisfying the requirments of Art 250.4

The inductive reactance caused by the remote fault return path increase the impedance of the fault return path to the point that 250.4 is not satisfied and the OCPD may not open.
Don

 

[charlie]

Re: Grounding Grid Vs Ground Conductor

We ground everything to a grounding grid and have no problems. However, the primary is 34.5 kV and higher plus the secondary is between 4.16 kV and 138 kV. We can do it but you can't because your voltages are not high enough. We have no trouble opening our overcurrent devices but our secondaries are not 600 volts and below. Of course we also ground everything to our system neutrals and static wire as well.

 

[jarrodp]

Re: Grounding Grid Vs Ground Conductor

Thanks for the information. We will be including the EGC.

But I still fail to see how adding a EGC in parallel with the burried copper cable of the GES will provide any significant change in the impedence. If our plant were larger, this may be an issue, but we have everything contained within a 36' x 40' space.

I'm not a power guy so I'm learning, this was just something that did not make sense to me. And I'm not designing this system, just looking across the office. If you care to go into more depth write on.

 

[don_resqcapt19]

Re: Grounding Grid Vs Ground Conductor

My understanding is that under the high currents from ground faults, even a few inches of separation causes a significant increase in the impedance of the circuit.
Don

 

[karl riley]

Re: Grounding Grid Vs Ground Conductor

Don, that's what I have read also. But since I don't get into the math, I often wonder if anyone can come up with an ohm number. Say a 200A fault current and the EGC separated from the hot by 1 foot for 50 feet of run. Ohms? Anyone have the math?
Karl

 

[don_resqcapt19]

Re: Grounding Grid Vs Ground Conductor

Karl,
The IEEE Green Book has some information on this issue. I think that a 200 amp fault is much too small to see much effect. I have been told that the inductive reactance does not play much of a part until the circuit size is 300 amps or more. That would give a fault current of 3000 amps or more. The Green Book shows a table of the voltage drop change in an EGC when it is separated from the phase conductor. They used a fault current of 5500 amps, a 500 kcmil phase conductor, a 4/0 EGC and a 200' length. They show a voltage drop of 87 volts on the EGC at 2" of separation, 114 volts at 8" and 143 volts at 30".
Don

 

[dereckbc]

Re: Grounding Grid Vs Ground Conductor

Originally posted by jarrodp:
But I still fail to see how adding a EGC in parallel with the burried copper cable of the GES will provide any significant change in the impedance.

Don eluded to it in his previous post. When you run a EGC with the associated circuit conductors and have a fault, a magnetic field is built up around the associated phase conductor. At the same time a equal and opposite field is built up around the EGC in the return portion. When the two are closely coupled the two magnetic fields cancel each other out thereby reducing the overall imepdance. If you separate the two you loose the magnetic coupling and field cancellation.

Same principle applies for the grounded circuit conductor (neutral). You would never run a neutral in separate raceways.

 

[jarrodp]

Re: Grounding Grid Vs Ground Conductor

Now, I see the light!

Thank you very much for exploring this topic.

 

[karl riley]

Re: Grounding Grid Vs Ground Conductor

Thanks, Don. That is the first time I have seen numbers on this effect.
Karl