EGC working in parallel with Grounding grid

[jsim]

Greetings,

From what I read in previous forum threads, I understand a grounding grid can't be used to replace the equipment grounding conductor in a installation. (Primarily due to the non cancellation of magnetic fields due to the separation of the source and return conductors)

But, can the EGC work in parallel with a grounding grid? For example: imagine a main AC Panel, electrically joining the neutral with the ground bus bar and the ground bus bar connected to a grounding grid. Then that AC Panel feeds another sub-panel elsewhere, giving it phase, neutral and EGC, connecting that EGC to this subpanel's ground bar, then connnecting that ground bar to the grounding grid.

In the event of a ground fault after the sub-panel, current would flow from the phase conductor through a normally non-current carrying metallic case, then through the EGC into the subpanel then at the sub-panel it will encounter two paths:
- The EGC to the main panel then to the main panel's neutral and,
- Another path through the grounding grid into the main panel's ground bus bar to the main panel's neutral

https://imgur.com/a/XQvsFLw

(Please check the attached image)

- Is that a valid situation? Does that somewhere violate a section of the NEC?

I tend to believe it is a valid situation, as with simple resistances having two resistances in parallel results in the worst case scenario (or more specifically, in the greatest resistance scenario) in a resistance equal to the smallest of the resistances in the parallel. So the current that triggers the overcurrent protection in that case wouldn't be negatively affected from that second path, it could only be equl or greater

But I know impedances make the exercise more complex (pun intended) as not necessarily the magnitude of the smallest impedance will be the maximum magnitude the resultant impedance may have.

- Do you know of any restrictions by the NEC on this scenario?
- Have you seen installations where this happens, and if so, everything works fine?

Thanks in advance

 

[infinity]

What you've depicted is fine. The connection to the grounding grid from the sub-panel is not required because it won't really do much but it isn't prohibited either.

Hundreds or thousands of parallel EGC paths are common especially in large buildings.

 

[petersonra]

There should be very little current flow in any EGC or grounding electrode which is what the ground grid is.

You cannot use a grounding electrode as part of an EGC because the code does not allow it. Not for physics reasons.

The code does not prohibit connecting a grounding electrode to any EGC at any point you feel like it.

Incidentally, you can use an EGC as both an EGC and a grounding electrode conductor if it meets the requirement for both.

 

[jsim]

You cannot use a grounding electrode as part of an EGC because the code does not allow it. Not for physics reasons.

Sorry, I didn't mean to imply it was for physics reasons alone, certainly the primary reason is to comply with the NEC. But as I understand it, the reason behind the NEC prohibiting the use of a grounding electrode as the EGC, in the case of a grounding grid (which at first thought seems to be a pretty good path, being some thick copper pretty much all the way), is due primarily to the (non) cancellation of the magnetic fields between the source and return conductors as they are too far apart, which in turn increases the path's reactance and in consequence the total impedance, reducing the fault current that should trigger the overcurrent protection device. But if I'm being mistaken at any point of this paragraph please let me know