Two Equipment Ground Conductor Paths

[akazici]

Two questions about the Exhibit from the NEC 2008 handbook,

The remote building is attached to the main building service ground bar with the egc in the same conduit with the feeder cables. It has also seperate grounding electrode in the remote building. So we have two ground paths totally and in a case of fault, most of the current will use the low impedance path which would be the grounding electrode in the remote building.

The Exhibit notes the grounding electrode required but could not exactly point the article.

Any help will be appreciated.

 

[charlie b]

. . . in a case of fault, most of the current will use the low impedance path which would be the grounding electrode in the remote building.

Not true. The lower impedance path will use the equipment grounding conductor (green wire in the image) that connects the ground bus in the remote building to the ground bus in the main panel. That has a resistance well under one ohm, and a pair of ground rods with dirt in between them will have a resitance of perhaps 50 ohms or even more. Dirt is not used to clear a fault, unless you are talking about substation voltages in the tens of thousands or higher.

 

[charlie b]

The Exhibit notes the grounding electrode required but could not exactly point the article.

Try 250.32(A).

 

[petersonra]

why is there even a requirement for a GE in a remote building? it does not appear to serve any real purpose. The only thing I can think of is if the earth potential rises suddenly at the remote building (such as a nearby lightning strike) it would tend to keep the grounded parts and the earth at a similar potential.

 

[charlie b]

why is there even a requirement for a GE in a remote building? it does not appear to serve any real purpose.

For the same reasons that there are grounding electrode(s) in the main building. :happyyes:

 

[Speedskater]

What's the "Isolated Neutral" in the drawing?

 

[eprice]

What's the "Isolated Neutral" in the drawing?

Since the panel in the remote building is not a main disconnect, the neutral is required to be isolated from the enclosure just as in any other panel down stream from the main disconnect.

 

[raider1]

What's the "Isolated Neutral" in the drawing?

I believe what they are trying to point out with the "Isolated Neutral" in the drawing is that the neutral bus in the remote building will be isolated from the equipment grounding conductors at the building.

Chris

 

[roger]

I believe what they are trying to point out with the "Isolated Neutral" in the drawing is that the neutral bus in the remote building will be isolated from the equipment grounding conductors at the building.

Chris

And I agree.

Roger

 

[Flapjack]

What's the "Isolated Neutral" in the drawing?

If the neutral wasn't isolated, then neutral current could flow through the ground and cause nuisance tripping in devices protecting from ground faults

 

[fmtjfw]

By isolating the neutral, effectively splitting it from the grounding wire, you force the current carried by the neutral to stay on the neutral and not be split with the grounding wire. Except under fault conditions, there should be essentially no current in the grounding wire. This allows it to stay at the same potential (0V) everywhere. Let's assume you have a bolted fault at the end of a long two-wire circuit, then the voltage at the fault end of the neutral and of the hot would be the same. In a 120V circuit about 60V. With the grounding conductor separated, the grounded box or box cover would remain at about 0V from "ground". If the grounding and neutral were connected together, you would get a voltage somewhat below 60V.

(Lots of simplification and hand waving about, but I'm trying to convey the concept.)

 

[akazici]

So as I understand the grounding electrode in the second building is not meant to carry the fault current. Is it for surges or transients ?

 

[don_resqcapt19]

So as I understand the grounding electrode in the second building is not meant to carry the fault current. Is it for surges or transients ?

No grounding electrode on a under 600 volt system it meant to carry fault current. At a second building their purpose is mainly lightning protection.