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.
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.