Grounding With Auxillary Electrode

[esquared]

Here is what I have: A large (800 kW) array on the roof with ballasted racking. The module frames and rails are bonded together and a #6 GEC is taking it down off the roof to a new auxillary DC GE. This ground will not be connected to the combiner boxes or the DC EGC. [690.43] [250.54] The Aux. GE will also not be connected to the existing GES because it is on the other side of the building.

The DC EGC then originates inside the combiner boxes (it is not electrically continual with the frames or rails) and is taken down to the inverter, where eventually it will share the same bonding point as the AC EGC, and DC GEC. The AC GE is not accessible and therefore I will be sharing the function of the AC EGC with the DC GEC. [690.47(C)]

If there were a fault in a PV Source Circuit to the frame/rail would there be a hazard? Wouldn't the fuse simply blow in the combiner because of the intentionally grounded circuit conductor?

I believe this is a code compliant method, what doesn't sit right with me is that there is no grounding between frames/rails and the combiner boxes.

 

[BretHeilig]

To simply answer your question, if your rails/modules are not electrically continuous with the point where your grounded conductor is bonded to your EGC (typically inside the inverter), there certainly will be a hazard if there is an ungrounded-to-ground fault in the rails; fault current will not flow (since the fault circuit will not be complete), and no OCPD will be triggered.

 

[jaggedben]

The Aux. GE[C] will also not be connected to the existing GES because it is on the other side of the building.

I'm pretty sure this is not an acceptable reason to not bond the two GES systems together. Looking at the 2011 code, you have to bond them somewhere. There are three different options of how to do it but it has to be done.

In my opinion the code requires everything on the DC side of the system, including (but not limited to) the panel frames, racking, combiner boxes and DC raceways, to be ultimately bonded to a single DC GEC. I do not understand why you are not doing this. You have a choice of where and how the DC GEC is bonded to the AC side of the system, but I don't see how bonding part of the DC side to a separate DC GES and another part to the AC GES is allowed.

As Bret said, it seems to be a dangerous setup.

...no OCPD will be triggered.

...and no GFDI will be triggered. Maybe you are not using GFDI as allowed in 690.5, but if you are using it, this is another reason you can't do it the way you're planning.

Wouldn't the fuse simply blow in the combiner because of the intentionally grounded circuit conductor?

I don't think so. And not even if you do bond all the DC parts together. PV source circuit OCPDs are sized to protect the source circuit from a fault to other source circuits or output circuits possibly resulting in fault current higher than the max current of the source circuit. They are intentionally sized larger than the max current the source circuit is expected to produce, so that they system can operate at maximum power and do its job. They thus do not typically blow in case of a ground fault that only affects a single source circuit.

 

[Marvin_Hamon]

The module frames and mounting have to be bonded together and to the rest of the equipment through an EGC (250.4(A)). So you need to run the array ECG to the DC combiner ground and then on to the inverter. The aux grounding electrode connected to the PV array is just gravy and in a roof mounted system really only helps with lightening protection. That is why the requirement was removed from the 2011 NEC. The aux grounding electrode does not have to be connected to the premises grounding electrode system(250.54).