PV System Grounding on metal outbuilding

[mongobird]

The application is a ~30kW system root mounted on a steel building. The question is how to ground.

There will be four strings, and each one will feed an inverter in a different building. The string voltage will be about 600Vdc. The steel outbuilding has two connections to the main building: the first is an underground conduit with 100A 240Vac sub-feed to that building. The second is an underground conduit with 4 DC string loops, plus a ground conductor.

The 240 Vac subpanel will be grounded with a single rod.

Here is where my concern begins. The ground for the PV system also gets tied to that single grounding rod. Of course, the PV system being on a steel roof on a building with steel siding, means that in theory the entire building sits at the potential of the grounding rod, which has an implied 20 ohm impedance to the earth. The exact impedance is not really important, but the fact that there is a non-zero impedance is.

Consider that there could be a fault in the DC conduit, and that this could mean that PV potential of +/- 600Vdc could return via the ground to the steel building, it is possible that the building could be energized. Furthermore, this has a potential should the grounding rod fail in some way, of backfeeding HV DC into the main building. The setting is a farm, and the mainbuilding is a cluster of connected buildings including a residence.

I consider this risk to be significant, and would like to know how it might be mitigated. We have consulted with the code inspector, and confirmed his grounding expectations. Explaining our concern, he has acknowledged it. While he did not offer an alternative, he appears to be open to a rational solution. Any ideas or similar experiences.

[Mike90250]

Some Charge controllers, like the Midnight Classic, have Arc Fault & DC Ground Fault detection. But I've not those features and I don't know if that is what you are looking for

[mongobird]

DC fault detection would be nice, but since the steel building and the ground could be at different potentials, the issue is how to reduce the risk of that happening is the question. Touching the knob to enter the building could be someone's last act.

[solarix]

Ground rods are cheap. Most AHJs now require two of them (6' apart) to ensure low impedance path to ground. Do this for the array on every building. Overkill in grounding is a good thing.

[bob-n]

solarix's advice is very good.

I believe that your strings will be isolated by the inverter. No string wire will be exposed. Likewise, each panel has isolation. The metal frame is electrically insulated from the wires.

Conventional good ground wiring has a ground wire going from the array to the ground rod and another connection from the building shell to the ground rod. The array will also connect to the building shell by the metal mounting hardware. That is a redundant ground.

If all of that is true, you have a system that can accept any one fault and still operate safely. You have two paths from any piece of exposed metal to earth. If any one path is damaged, there is a backup. Everything at 600V is isolated from exposed metal. If that isolation fails in any one place, that point will be grounded. There will not be high current in the exposed metal or ground wire because there is only one fault.

solarix's advice of having two ground rods is also very good. That gives one more level of fault tolerance.

We hope that the odds of one fault are low. If true, the odds of two faults are extremely low, perhaps low enough that you don't need to worry about it. It depends on your odds of fault and your risk tolerance. Your actions will be guided by the specific risks of your installation. We're familiar with smaller residential installations involving one building. Common practices above mitigate those risks. Your install may be more complex.

It might be helpful to brainstorm faults. We call this failure mode effects analysis or FMEA.

For example: What if the building connection to earth corrodes away? If the severity of that fault is high and the likelihood is high, then perhaps you will want a second ground wire from building to earth. Or you may want to investigate ground connections that are less succeptible to corrosion, such as dielectric grease coated connections or welded connections.

Common ground rods are copper-coated steel. The copper will eventually corrode away. If that is a likely and severe risk, consider a buried solid copper ground plate or another grounding technique with more durability.

Is it possible that routine maintenance will damage a ground wire? I don't know your buildings or expected foot traffic. If this is an important risk, put the ground wire in secure conduit.

I hope this helps.