GEC for free-standing solar car-port structures

[MOSESNBKLYN]

We have a unique installation in Suffolk County, NY. We are working with a developer as the engineer on solar PV carports; the buildings/ mounting structures consist of many (20-30) steel columns, installed 6ft below grade with concrete footings, each consisting of (2) C-channels #12GA steel 10" x 3.5" back-to-back. These represent a building structure, and could be a significantly low resistance ground system except the steel columns and beams are bolted together and the soil is very high resistance (118-560 ohms-m), and they do not comply with the 10ft depth required by 2008 NEC 250.52.A.2. - we don't have any rebar in this case.

I designed a simple #2/0 ground ring (~200LF buried below frost depth) to connect to one column of each structure/building, and using ETAP calculated the resistance to be ~14ohms. These installations are in public parking lots and we feel the <25ohm standard would be safest. Per IEEE std 142 "The 25 ohm value noted in the NEC [1] applies to the maximum resistance for a single made electrode. If a higher resistance is obtained for a single electrode, a second (paralleled) electrode is required. This should not be interpreted to mean that 25ohms is a satisfactory level for a grounding system. In contrast, the Canadian Electrical Code (CEC), CSA C22.1-1990 [2] uses a criterion of maximum station ground rise of 5000 volts (or less) under maximum ground fault conditions and step/touch voltages to be shown values stipulated in the CEC"....in other words IEEE suggests a step/touch grounding study - which I performed successfully.

Has anyone come across these carport structures, and what was typically required?

This gets even more complicated as we consider the availability of faulted DC voltage/current from the PV modules. The inverters have a GFPD that opens all DC feeder circuits within 10 cycles; but at the faulted array, the entire combiner box (~180A, 600V) could be feeding a fault. This issue has the PV developer considering DC breakers vs fuses but thats another issue.

 

[petersonra]

What is this 10 feet depth requirement?

Why can't the steel structure serve as your GE?

 

[hurk27]

We have a unique installation in Suffolk County, NY. We are working with a developer as the engineer on solar PV carports; the buildings/ mounting structures consist of many (20-30) steel columns, installed 6ft below grade with concrete footings, each consisting of (2) C-channels #12GA steel 10" x 3.5" back-to-back.

I know of no code that would not allow a metal structure that is bolted together to not serve as being bonded, even pool structures that are screwed together are accepted as bonded when a bond wire is attached to a single point.

These represent a building structure, and could be a significantly low resistance ground system except the steel columns and beams are bolted together and the soil is very high resistance (118-560 ohms-m), and they do not comply with the 10ft depth required by 2008 NEC 250.52.A.2. - we don't have any rebar in this case.

The combined resistance of the columns in 6' of Earth will probably provide a much lower impedance that a single or double rod will ever provide, also the 10' should be 8' in contact with the soil.

I designed a simple #2/0 ground ring (~200LF buried below frost depth) to connect to one column of each structure/building, and using ETAP calculated the resistance to be ~14ohms. These installations are in public parking lots and we feel the <25ohm standard would be safest. Per IEEE std 142 "The 25 ohm value noted in the NEC [1] applies to the maximum resistance for a single made electrode. If a higher resistance is obtained for a single electrode, a second (paralleled) electrode is required. This should not be interpreted to mean that 25ohms is a satisfactory level for a grounding system. In contrast, the Canadian Electrical Code (CEC), CSA C22.1-1990 [2] uses a criterion of maximum station ground rise of 5000 volts (or less) under maximum ground fault conditions and step/touch voltages to be shown values stipulated in the CEC"....in other words IEEE suggests a step/touch grounding study - which I performed successfully.

As far as the NEC is concerned, you drive a rod and if it doesn't meet the 25 ohm's you drive a second and go home, there is no requirement that after the second rod to meet the 25 ohm's.

And as far as touch potential goes, grounding electrodes do not, and I repeat "Do not" protect against touch potential, and they never will, only if the soil has a low enough impedance that might open the OCPD could it stop any shock hazard, but this is something that could never be guaranteed and could never be depended upon to provide for safety, this is why grounding electrodes are not installed for these purposes, this is old myths that have been around for a long time and have been proved false many time over, some people just don't want to let go of these myths. Click on this link and read this thread and the links in it and you can understand also why this can't happen: http://forums.mikeholt.com/showthread.php/116358-Earth-Shells-Experiment-time-to-eat-crow

Has anyone come across these carport structures, and what was typically required?

This gets even more complicated as we consider the availability of faulted DC voltage/current from the PV modules. The inverters have a GFPD that opens all DC feeder circuits within 10 cycles; but at the faulted array, the entire combiner box (~180A, 600V) could be feeding a fault. This issue has the PV developer considering DC breakers vs fuses but thats another issue.

Last but the most important thing is, if these are self powered PV carports, and no utility power is run to them, then there is no fault path to Earth what so ever, current only wants to return to source not Earth, any short to the structure will only result in current flowing what ever paths it takes through the structure to return to the source of that current, if the inverters and or DC supply are all part of the structure then the current will never try to leave the structure, no different then a portable generator not touching earth, there would be no shock hazard to earth as there is no return path through earth to the generator.

Now if there are receptacles that a person could plug in a extention cord to then there could be a shock hazard, as the supply does have a grounding electrode system, so there would be a pathway back to the building through Earth, but a GFCI would help in that case.

 

[MOSESNBKLYN]

Wayne - thanks for the detailed response. Yes, this is a grid connected system, the switchyard containing transformer, inverters, and an outdoor switchgear are located remote from the carports, except for one which is very close. Unfortunately, upon review of the structures the steel is only 2ft deep and the concrete footings are deeper.

I have since interpreted 250.32 to require ground electrodes at each building/combiner box.

The min. size of module/array grounding conductor appears to be #6awg per 690.47(D) and 250.166(B), but I am specifying a #2/0 for the ground ring [different for 250.166(E)] mostly for durability. I don't have the exact text of NEC2011 but this is sufficient for most instances of faults at the strings.

The original design used the 1#6awg for both module and equipment grounding, this appears to be a violation of 2008 NEC; how is this treated in 2011?

 

[don_resqcapt19]

What is this 10 feet depth requirement?

Why can't the steel structure serve as your GE?

See 250.52(A)(2)