Grounding to earth question

[GerryB]

I was watching a Mike Holt video where he was talking about the danger of having for example a ground rod for a solar array not run back to the main GEC. He explained a difference in potential and the possibility of high current flowing between the two, (and through the house). He had an easy way to think of it, "everything above the ground should be bonded together, everything below the ground should be bonded together, and there should only be one spot where they are tied together. That is how I understood it, which made me think of a detached garage. Isn't a detached garage treated as a sub panel but ground rods should be added at the garage? Am I understanding this correctly? And wouldn't that be two separate ground points.

 

[GoldDigger]

I think you do.
At some point the separation between the closely associated electrical networks becomes great enough that it needs to be treated as two separate systems (except for the necessary EGC connection as a fault current path.)
For want of a better way of specifying that, the NEC makes the distinction whenever you have separate buildings.
Which leads to the question of how you should treat GEC requirements for ground mount PV arrays. And I don't have a solid answer.

Sent from my XT1585 using Tapatalk

 

[al hildenbrand]

"everything above the ground should be bonded together, everything below the ground should be bonded together, and there should only be one spot where they are tied together. That is how I understood it, which made me think of a detached garage. Isn't a detached garage treated as a sub panel but ground rods should be added at the garage? Am I understanding this correctly? And wouldn't that be two separate ground points.

So, the garage ground rod is bonded to the garage branch circuit EGC, which, in turn is bonded back at the service center to the house Grounding Electrode System. That is the "below ground" bonding. The "above ground" is in the garage neutral, which, at the garage is NOT tied to the EGC, but, instead relies on the single Main Bonding Jumper (MBJ) back at the service.

When you ask, "wouldn't that be two separate ground points?" the question should be "two separate MBJ points."

If having more than one "ground points" was an issue we wouldn't be required to have a supplemental ground rod for a metal water pipe in contact with earth, or TWO ground rods, etc.

 

[electrofelon]

So why is a detached structure set up "safe", and the controversial PV auxiliary electrode setup "not safe"?

 

[al hildenbrand]

The "above ground" is in the garage neutral, which, at the garage is NOT tied to the EGC, but, instead relies on the single Main Bonding Jumper (MBJ) back at the service.

I didn't say this at all well. The "above ground" that is bonded together ALSO is tied to the garage branch circuit EGC. That does create the appearance that two bonding systems are tied together, but, as I say, I think you'll be able to check that Mike is saying there is only one Main Bonding Jumper tying the neutral to the GES and the EGCs.