When installing a solar Photo-voltaic system (PV), it is extremely important all the equipment is grounded correctly. Failure to ground the entire system to include all the individual pieces, can be devastating, especially in an area that experiences lightning on a regular basis. Even if you seldom have electrical storms, all it takes is one lightning strike or a single lose wire and all the equipment can be destroyed. Worse yet, it can start a fire and cause even more damage to your home.
Electricity follows the path of least resistance, and while it’s almost impossible to know its exact path, we can take reasonable steps to try and direct the electricity someplace safe when a surge occurs. The way we do this is with an earth grounding system.
Every electrical outlet, light socket, electrical device, gas line, copper pipe and service panel in your house is all tied together by a bare copper wire. Somewhere, this bare copper wire is either connected to a copper pipe which is literally buried in the earth, or it may be connected to a piece of re-bar in your home’s concrete foundation. This is where stray electric current is directed in the event of a lightning strike or short circuit.
The same thing needs to be done to your solar PV system by connecting all the equipment together to include the solar panels, PV mounts, combiner box, inverter, charge controller and any other device that makes up your solar PV system. Here’s where some people make a major mistake: they don’t tie the grounding system of the solar panels to the grounding system of the house. Everything needs to be tied together to the same grounding system. This is called bonding.
To help ensure the electric current can find its way into the earth, you want a good conductive earth ground so the path of least resistance is “obvious” for the lightning strike, faulty wiring or a short-circuit. It is actually preferable to have several copper pipes buried in the ground on a single bonded earth grounded system, provided all the equipment and grounding pipes are securely bonded together. This lowers the electrical potential that can build on the grounding wire should a lightning strike occur. Therefore, one should drive multiple pipes deep into the ground, surround them with rock salt, and moisten the area.
Each grounding pipe should be around 8 feet deep, but no less than 6 feet. The soil tends to stay moist the deeper you go, due to reduced evaporation. This increases conductivity between the grounding pipe and the earth. For the outside grounding wire, use thick enough bare copper wire to handle large electrical loads, such as lightning. I’d recommend nothing less than 6 AWG for all outside equipment. When grounding inside, you may want to use green shielded wiring as it might be required by code. Check with your local permit agency.
The theory gets long and complicated, and I’ll leave other websites to explain that in more detail, but in short, if you have two independent grounding systems, you create an electrical potential differential. If lightning strikes one system and not the other, or if lightning strikes the ground nearby, an electrical differential between the two systems will be created. Electric charge will always try to equalize itself with neutral ground. For example, let’s say there is a common earth ground for all the outlets in your home (as there should be), and your inverter is plugged into one of these outlets. You also have two thick copper wires, negative and positive, going from your inverter, to your solar panel array outside that also has a ground, but is not bonded with the house grounding system. If lightning were to strike the solar panels outside, the electric current may go into the panel grounding system if you are lucky, OR it may ride along the positive and negative wires going back into the inverter inside your house. Once there, it could arc to the house wiring and use its grounding system instead. The arcing is what could destroy your equipment or worse yet, start a house fire. It doesn’t take a direct hit from a lightning strike to induce voltage on the wires, either.
If the grounding systems of the house and the array are tied into together (bonded), there is a much greater probability that the electric current from the lightning strike would stay on the common grounding system and not arc to something else.
Despite my explanation, grounding a solar PV system is kind of a voodoo science. There are probably many ways to do it and hundreds of opinions. My system is permitted and up to code per my city’s requirements. Each city and county may be slightly different and have their own rules for grounding systems.