grounding of SDS

[T.M.Haja Sahib]

There appears to be contradiction of this

The connection to the grounding electrode at the building would not make a real change in the voltage to earth under fault conditions, except for a very small area very close to the building grounding electrode.

with this

..... the earth and metallic objects with a path to earth will remain at "earth" potential.

There is a concept called ground potential gradient......

 

[don_resqcapt19]

There appears to be contradiction of this


with this


There is a concept called ground potential gradient......

The IEEE Green Book shows that ~5' from the electrode you will have 85% of the voltage that is impressed on the electrode. That is you will bring the voltage of the earth around the electrode up to the voltage of the electrode when you are very close to the electrode, but as you move away from the electrode, you will have raised the voltage less becuase of the ground potential gradient.

 

[Gregg Harris]

I agree with you. Have to... it's Code.

However, how does one determine what electrode is or isn't at the building or structure? Is an SDS generator always a separate structure or always at the building? Is a metal water pipe electrode in the building considered at a generator that's 10' away from the building? Is that as near as practicable to, and in the same area, as the generator? If yes, what if the generator is 200' from the building? What if the water pipe enters on the other side of the building 100' away and there is no structural steel electrode? In practice, there are just too many possibilities, while the NEC has no basis of determination for what is or isn't at a building or structure. :happyyes:

Well I guess the electrician on site would need to determine if there is an electrical service, a water service, a metal frame of the building or structure, Concrete Encased Electrode, and if none are present at the building being served by the generator then install one or more of the grounding electrodes as specified in 250.52 (A) (4) through (A) (8)

 

[Smart $]

Well I guess the electrician on site would need to determine if there is an electrical service, a water service, a metal frame of the building or structure, Concrete Encased Electrode, and if none are present at the building being served by the generator then install one or more of the grounding electrodes as specified in 250.52 (A) (4) through (A) (8)

A GES is required for any building with an electrical system (except where only a single or multiwire branch circuit supplies the building). 250.30(C) doesn't change that, nor do I see where it requires an outside SDS (the generator in this case) to be connected to the GES of the building by a GEC.

Compare to the required grounding connection for an outside transformer under 250.24(A)(2). Does 240.30(C) require something different? If we condone the picture, and assuming it is a grounded system, there will be three ground conductors (grounded, grounding, and earthing) running between the generator and the building. With a service transformer, there is only one (grounded). What makes an outside SDS so special that it would require three???

 

[T.M.Haja Sahib]

What makes an outside SDS so special that it would require three???

Simple. The service transformer would not be handled by the building owner, whereas the SDS would be.

 

[T.M.Haja Sahib]

The IEEE Green Book shows that ~5' from the electrode you will have 85% of the voltage that is impressed on the electrode. That is you will bring the voltage of the earth around the electrode up to the voltage of the electrode when you are very close to the electrode, but as you move away from the electrode, you will have raised the voltage less becuase of the ground potential gradient.

I do not understand what your point is.
I stated that due to nature of the earth, there would be voltage between service equipment enclosure and ground as well as between metallic enclosure of SDS and ground during a ground fault in the building powered by the SDS. But this voltage can be made negligible by interconnecting the ground rods of the two sources, simply because of the uniform impedance of the connecting wire, the fault voltage gets so distributed along the wire length that voltage per unit length would become very small.

 

[don_resqcapt19]

I do not understand what your point is.
I stated that due to nature of the earth, there would be voltage between service equipment enclosure and ground as well as between metallic enclosure of SDS and ground during a ground fault in the building powered by the SDS. But this voltage can be made negligible by interconnecting the ground rods of the two sources, simply because of the uniform impedance of the connecting wire, the fault voltage gets so distributed along the wire length that voltage per unit length would become very small.

My point is that unless you are standing on the rods or the connecting cable there is a shock hazard when you are in contact with both anything connected to the electrical grounding system and the earth, under fault conditions.

 

[T.M.Haja Sahib]

Don, excellent exposition.

 

[T.M.Haja Sahib]

Don, in addition to your point, the following in favour of interconnection of the grounds of two sources as shown in the picture of the OP are worthy of note.

1)The increased magnitude of fault current due to reduced impedance of neutral and gronding conductor combination, the fault interruption time by the protective device is reduced.

2)The increased magnitude of fault current also causes reduced neutral to ground voltage due to increased voltage drop in the phase conductor and this reduced voltage around the structures may be found by computer simulation or by other means and may be made safe uhder all conditions by selecting suitable size of the grounding wire.

 

[Smart $]

Simple. The service transformer would not be handled by the building owner, whereas the SDS would be.

In many, if not most cases of pad-mount service transformers, the service point is the transformer's secondary terminals. As such, the secondary gets grounded per NEC requirements...???