grounding electrode for PV remote structure

[electrofelon]

Two part question here. First is a basic yes or no, second is NEC reasoning/science/philosophy. This is one of those things that I am surprised that I have never come across nor though of. Say I have a building with a PV array on the roof. The building has no electrical system other than the PV source and output circuits. The PV output circuit is routed to a remote structure that has utility service and a GTI. The question is if a GES is required at the array building. In 2014 we have 690.47(D) which is quite clear, however prior to 2014, was there any requirement for a GES at the array building? 250.32 jumps to mind immediately, but of course that is for buildings supplied by a feeder. Was there no requirement or am I missing it? Now we wouldnt need a GES if a building was supplied by a single branch circuit, so is there a scientific reason why there is no GES requirement for that or a building with an array on it, but there is once a feeder is involved?

 

[jaggedben]

690.47(D) was also in the 2008 code. It was taken out of the 2011 and there still seems to be some dispute over whether that was really the CMP's intent. I don't believe that the requirement existed before the 2008 NEC (the section is shown as a revision in that edition). FWIW, the first draft of the 2017 would remove the requirement again, and instead permit array electrodes and proscribe some rules for them.

As far as science/philosophy goes, the rest of this is just my opinion... so watch out.

My take is that it's a balance between two bad things:
a) potential shock hazard danger if there's a fault on the system and no electrode. This could conceivably be an AC fault at the other building.
vs.
b) if there's an electrode at the array, a nearby lightning strike could damage the conductors and anything they run through or over, especially if bonding conductors between electrodes are too small.

With that said, I really don't see the shock hazard danger as being as significant if you only have DC running to the building. Granted there could be serious shock hazards between parts of the installation in case of a fault, but unlike with an AC grounded system I don't see adding an electrode as helping with any of that. I suppose some of the problems with AC systems could also manifest on the grounded parts of your PV installation, but if you're not running a grounded AC conductor out there my impression is that those potential problems are much fewer. So I think the justification for an electrode in your case is less than with an AC branch circuit.

Ergo, as far as the NEC is concerned, I don't think an electrode would be required if not for 690.47(D). Or if that section is ignored.

This thread discussed the subject more generally, albeit with no firm conclusions...

 

[pv_n00b]

As you said there is 250.32 that covers buildings that have feeders or branch circuits run to them. If the PV AC connection could be considered a single branch circuit then the exception would block the grounding electrode requirement. I would think most AHJs would require an electrode anyway since there is more wiring required for the PV system even if it is connected with a single branch circuit, and then there is 690.47(D) which can make it a moot point.

The exception probably relates to the lower available fault current in a branch circuit verses a feeder and the probability of a lower voltage rise between a faulted circuit and ground that someone could get across at the building. That's a guess though.

 

[electrofelon]

690.47(D) was also in the 2008 code. It was taken out of the 2011 and there still seems to be some dispute over whether that was really the CMP's intent. I don't believe that the requirement existed before the 2008 NEC (the section is shown as a revision in that edition). FWIW, the first draft of the 2017 would remove the requirement again, and instead permit array electrodes and proscribe some rules for them.

As far as science/philosophy goes, the rest of this is just my opinion... so watch out.

My take is that it's a balance between two bad things:
a) potential shock hazard danger if there's a fault on the system and no electrode. This could conceivably be an AC fault at the other building.
vs.
b) if there's an electrode at the array, a nearby lightning strike could damage the conductors and anything they run through or over, especially if bonding conductors between electrodes are too small.

With that said, I really don't see the shock hazard danger as being as significant if you only have DC running to the building. Granted there could be serious shock hazards between parts of the installation in case of a fault, but unlike with an AC grounded system I don't see adding an electrode as helping with any of that. I suppose some of the problems with AC systems could also manifest on the grounded parts of your PV installation, but if you're not running a grounded AC conductor out there my impression is that those potential problems are much fewer. So I think the justification for an electrode in your case is less than with an AC branch circuit.

Ergo, as far as the NEC is concerned, I don't think an electrode would be required if not for 690.47(D). Or if that section is ignored.

This thread discussed the subject more generally, albeit with no firm conclusions...

Thats a good link thanks, Ill read through it when I have a few spare minutes. Perhaps this situation irrationally jumps out at me as being odd. I am so used to in my mind thinking that every structure with 'lectricity needs a GEC. I was aware of the branch circuit exception but I have only utilized that a few times in my career for a light in a small tool shed. Also wasnt aware that 690.47(D) was in 2008. I never had a 2011 and the highlighted text in the 2014 indicates that it is "new"