Ground Grids

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Flapjack

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In industrial settings, the ground grid is tied to everything from motors, transformers, every large piece of equipment in the electrical room, skids, etc.

Take for example a motor fed from a 100 amp circuit breaker in a MCC that is not a service entrance, nor does it have the first OCPD in a SDS. The motor should be grounded by an equipment grounding conductor per 250.112(C) and the EGC should be 8 AWG per Table 250.122. The ground grid is 4/0 AWG everywhere. There is a 4/0 AWG ground from the ground grid going to the motor frame and a 4/0 AWG ground going to the MCC enclosure. The MCC ground bus is bonded to the enclosure. If there is a fault to the motor frame, the current will split. The breaker still opens cause the ground grid is tied to everything and the effective ground-fault current path is satisfied by the 8 AWG EGC. Does the code only care that the effective ground-fault current path exists and is not the earth, even though the ground grid will take a decent amount of fault current? Does 250.54 (Auxiliary Grounding Electrodes) make the scenario I described acceptable by the NEC?

In reading through 250, I think that the ground grid connections to the MCC and the motor frame in the example above are not required and are not ideal, but allowed by code.
 

charlie b

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The motor should be grounded (my emphasis) by an equipment grounding conductor per 250.112(C) and the EGC should be 8 AWG per Table 250.122.
Please allow me to split a hair here. The EGC does not cause the motor to be "grounded." It establishes a low resistance path for fault current to make its way back to the source. It is not directly connected to planet Earth, so the word "grounded" does not apply.
In industrial settings, the ground grid is tied to everything from motors, transformers, every large piece of equipment in the electrical room, skids, etc.
I have no idea why that would be required in industrial settings.
In reading through 250, I think that the ground grid connections to the MCC and the motor frame in the example above are not required and are not ideal, but allowed by code.
I agree that they aer not required, and that they are allowed. I don't know what you mean by "not ideal." I agree that should an energized wire internal to the motor come into contact with the motor frame, there will be fault current flowing through the grid, up to the time that the breaker trips to terminate the event. Are you concerned about this causing a voltage spike on other equipment for which their frames are also bonded to the same grid?

 

Flapjack

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EE
Please allow me to split a hair here. The EGC does not cause the motor to be "grounded."

I will allow it. Bonded.


I have no idea why that would be required in industrial settings.

I have no idea why it would be required either. There are electricians and even on-site engineers that insist that motor frames, MCCs, switchgear, etc. are tied to the ground grid because that is how they do it. I should have clarified "in some industrial settings."

I don't know what you mean by "not ideal."

Ideal would be the fault travels on the equipment grounding conductor, because that is the effective ground-fault current path, and not on both the EGC and ground grid.
 

don_resqcapt19

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The code requires that an EGC be run with the circuit conductors. There is no provision in the NEC that would permit a connection to the "ground grid" to serve in place of the required EGC.

There is no issue with equipment that has both an EGC and a connection to the ground grid. Yes, under fault conditions there will be current on both paths, but that is not a real issue.
 
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