690.47(B) Grounding Electrodes Directly to Frames

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Rex Hungerford

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I am designing electrical onelines for residential homes that contain Hifi and Video equipment. I am very conscious the grounding and design a single point where all grounding electrodes on site (concrete encased, rods, cold water, chemical, etc) all come to a single bonding point. I have a lot of customers wanting to add solar now.

NEC 490.47(B) in short says additional grounding electrodes shall be permitted to be connected directly to the PV module frame or support structure.

In my mind, this is a parallel ground path to earth. The Frames and support structures are bonded to the primary grounding electrode system on the property. And now to a new rod or plate in the earth.

I am assuming this is a safety design for some installations. Say your only existing grounding electrode is a Ufer. If a lightening bolt hit the array, the energy would go to the Ufer and could flash heat the concrete surrounding the rebar ground in the footer and shatter the footer. An additional rod from the frames to earth might take some of the energy and save the homes foundation.

This is my assumption why this might be done. Is this correct?

Are there other reasons why a solar installer might want to land additional grounds on the property and only connect them to the frames or support structure of the solar array?
 
You can put in as many auxiliary rods as you want, pretty much wherever you want, as long as they only connect to the EGC and not to the grounded conductor. 250.54.

the bottom line is they don't really serve much purpose, but some people think they do.
 
Are you sure you are stating that correct Peter. I though you were only to attach grounding electrodes to a grounding electrode conductor per 250.66. Not to a EGC per 250.122. And that is my rub. 490.47(B) seems to say they can connect it to an equipment grounding conductor as that is by my understanding, the conductor that is bonding the frame and PV modules. Maybe I am misunderstanding. Solar is not my specialty.
 
Rex Hungerford said:
Are you sure you are stating that correct Peter. I though you were only to attach grounding electrodes to a grounding electrode conductor per 250.66. Not to a EGC per 250.122. And that is my rub. 490.47(B) seems to say they can connect it to an equipment grounding conductor as that is by my understanding, the conductor that is bonding the frame and PV modules. Maybe I am misunderstanding. Solar is not my specialty.
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Look at 250.54.

Where do you connect the grounding electrode system at a separate structure?
 
There used to be a stupid requirement for auxiliary electrodes on solar arrays. It was removed in 2017 but I guess they left the permissive language in there for those who (incorrectly) think it's a good idea.

See
 
Thanks Jaggedben. I did not at all like the idea of 490.47(B). I had not seen this particular video, but I have seen others where Mike talked about auxiliary electrodes driven at CNC machinery. I am looking for some backup in case I run into old school guys who think more electrodes all over is better.
 
Keep in mind that the NEC does not really cover lightning protection, that's NFPA 780. So none of the grounding and bonding is intended to protect anything from lightning, but it can't hurt, probably. The NEC also does not care about how many aux grounding electrodes are installed. Your site can look like a pincushion and the NEC is a-okay. Just don't try to use the earth between two aux grounding electrodes as a replacement for equipment bonding. I think aux grounding electrodes are a waste of money but boy do some people love them.
They can be a problem in areas with a lot of lightning. If there is a potential difference between two points on the ground then current will go up the aux electrode, through the equipment ground, and to another grounding electrode at a lower potential. If the electrodes were bonded together into a grounding electrode system then that would not happen.
 
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Rex Hungerford said:
Thanks Jaggedben. I did not at all like the idea of 490.47(B). I had not seen this particular video, but I have seen others where Mike talked about auxiliary electrodes driven at CNC machinery. I am looking for some backup in case I run into old school guys who think more electrodes all over is better.
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For the record, 690.47. (Not 490).
 
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