MCC Grounding 3

[mikeengurs]

I have a 4160v MCC being fed by a feeder breaker in an other building. The feeder cable is 3 1/C #1000MCM, and is being run underground. The MCC equipment is locally grounded to a local ground in the building. But in addition to this, we have a grounding 4/0 5kv cable running with the 3 1/C #1000MCM from feeder breaker to ground the MCC. Is that going to create any issue? Does the 4/0 ground replace the local building equipment ground? Thanks!

 

[7anoter4]

The fault current Ifault=V/(Z1+Z2+Zo+3*Rground)
The touch voltage between a Human Body and a non-energized metallic part which could be energized in a fault case Vbody=Iground*Rground.
If resistance to Ground [Rground] will be very elevated the fault current will be very small so the voltage drop through the body will be very reduced. This will occur when the body and the faulted electric object are standing both on an insulated area from the Ground.
If the Area is not insulated from the Ground we have to reduce the nonliving metallic parts connection to the remote Ground to a such level that the remaining voltage drop on the body from the nonliving metallic parts touch in a fault case will be in the safety limits.
Only the part of the fault current flowing directly through the Ground will raise the Vbody potential.
If Iground will be equal to the fault current Vg could be still elevated. The grounding conductor will return the most of the fault current directly to the source and so will reduce the remaining Iground and in turn the Vbody.
The lower the Grounding Conductor impedance the lower the Vbody.
So, both System Grounding[local grounding]and Grounding Conductor are required.

 

[WoodrowJWeen]

I agree with 7anoter4. We usually spec the ground wire to have 600V insulation not 5kV.

 

[mikeengurs]

Thanks all. WoodrowJWeen, I couldn't find it anywhere in the NEC. I agree that 600V insulation for the ground wire is suffecient, but is that mentioned anywhere in the code, or it's just a common practice?

 

[dpc]

See NEC 250.180 through 250.191.

An equipment grounding conductor generally does not even have to be insulated.

 

[mikeengurs]

Thanks guys! This was helpful!