Latest interpretation of NEC rules for PV system grounding

SolaRevolution

Photovoltaic system grounding meathods are some of the most complex subjects when trying to figgure how to install a code compliant system.

This report was prepared by John Wiles for the Solar America Board for Codes and Standards and released in October of 2012.
http://www.solarabcs.org/about/publications/reports/systemgrounding/pdfs/SystemGrounding_studyreport.pdf

Understanding this report should make you well armed for most PV installation work. (I still have some questions for interpreting grounding of battery systems which I feel are not fully addressed here.)

Unfortunately, many AHJs have their own interpretations of the rules which may differ from jurisdiction to jurisdiction. To make matters worse, some individual inspectors within a given area may disagree about the proper meathods for any given installation. I've seen plans which were approved by a building department only to be rejected by their own field inspectors.


-Alex Aragon

niel

Re: Latest interpretation of NEC rules for PV system grounding

thanks for posting this. i will attempt to digest this when i get further time to.

vtmaps

Re: Latest interpretation of NEC rules for PV system grounding

Thanks for the link. They seem well aware of all the DC GFP problems that have been discussed on this forum (especially by BB.)

From another publication at SolarABCs.org:

This safety issue—undetected faults in grounded PV array conductors—came to light during investigations into two well-publicized PV system fires.
<snip>
More seriously, the activation of the ground-fault fuse in the event of an ungrounded conductor fault to ground actually helps to drive the fire event.

--vtMaps

niel

Re: Latest interpretation of NEC rules for PV system grounding

i skimmed it myself and i agree he is starting to listen to us some as the drawings no longer seem to have the ground wire going from the pvs and into the dwelling or did i just misinterpret the drawing without reading into it?

inetdog

Re: Latest interpretation of NEC rules for PV system grounding

niel wrote: »

i skimmed it myself and i agree he is starting to listen to us some as the drawings no longer seem to have the ground wire going from the pvs and into the dwelling or did i just misinterpret the drawing without reading into it?

I particularly welcomed his comment that actual fire studies have shown that a Ground Detection fuse blowing in an ungrounded system will actually increase the hazard caused by the panel string fault! How many on this and other forums have been arguing that for quite a while!

Desert Rat

Re: Latest interpretation of NEC rules for PV system grounding

Niel touched on something that I'm confused about. I have seen many other installation guides that recommended tying the PV ground electrode to the other ground electrode(s). Here in the desert it would seem to me that a buried bare ground conductor between the two ground electrodes would also greatly improve contact with earth. But the drawings in the article show no such conductor between the ground electrodes. What's the consensus on this?

SolaRevolution

Re: Latest interpretation of NEC rules for PV system grounding

Desert Rat wrote: »

Niel touched on something that I'm confused about. I have seen many other installation guides that recommended tying the PV ground electrode to the other ground electrode(s). Here in the desert it would seem to me that a buried bare ground conductor between the two ground electrodes would also greatly improve contact with earth. But the drawings in the article show no such conductor between the ground electrodes. What's the consensus on this?

That's only for the "Supplementary Grounding Electrode".
The AC and DC Grounding Electrodes must be bonded together with a conductor sized for the larger of the Grounding Electrode Conductors required by either system.

I guess it all depends on what you call it?:roll:

The difference is that the Supplemetary GE is not a bonding point for any current carrying conductors where as the DC and AC Grounding Electrodes are connected directly to the bonding points of seperately derived electrical systems. This means that if there is a fault in one system which makes contact with the other system they are solidly bonded and will therefore allow current paths to properly operate overcurrent protection devices.

It is hard to see any "electrical" difference between the supplementary GE and the steel poles for a ground mounted rack. The difference is that a manufactured ground rod is NEC approved for the purpose where as a steel pipe incased in concrete is not. (unless it is in at least 8 feet of contact with the concrete and earth without any moisture barrier.)


Interesting note: In San Francisco the AHJ does not allow exterior exposed copper due to problems with copper theft.


-Alex

inetdog

Re: Latest interpretation of NEC rules for PV system grounding

Desert Rat wrote: »

Niel touched on something that I'm confused about. I have seen many other installation guides that recommended tying the PV ground electrode to the other ground electrode(s). Here in the desert it would seem to me that a buried bare ground conductor between the two ground electrodes would also greatly improve contact with earth. But the drawings in the article show no such conductor between the ground electrodes. What's the consensus on this?

Maybe not in the desert, but in some soils there will be a problem with buried bare copper corroding. A similar thing would happen to a bare copper ground rod, but those are assumed to be thick enough to last awhile despite the corrosion. (Or else are steel or zinc coated??)

niel

Re: Latest interpretation of NEC rules for PV system grounding

oh yes, i do condone tying the pv ground rod with the other ground rod(s) underground. the pv frame is not required to connect to the other ground rods though and sometimes the soil or distances involved prevent that connection. i do advise large wires to help offset any possible corrosion with #6 as a minimum.

the one i was referring to was the required ground wire to run along side the pv + and - back to the building and then daisy chain until it gets to the utility mains ground point. i objected because this needlessly introduced a path that could harm life and property with the lightning or emp being forced in the vicinity of you and your equipment before reaching ground.