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A Stand Alone System is a NEC definition of an Off-Grid Battery System like you would use for a cabin, tiny house, storage shed, detached garage, etc. where there is no commercial AC or any off premise power source. As such NEC has minimum requirements for Bonding & Grounding.
Bonding & Grounding as it pertains to NEC Article 250 is poorly written, uses the wrong terms and when compounded make it awfully confusing. To start Ground is a misused term with no meaning or function. NEC Article 100 defines Ground as: THE EARTH or DIRT. There is only one Ground Wire on any system called the Ground Electrode Conductor aka GEC. It is the only wire or cable connected to DIRT. Everything else is a Bonding Jumper or Conductor. Get your noodle wrapped around that, and you are half way there to understanding. There are 3 primary reasons we Bond and Ground:
1 To provide a 0-Volt Reference Point. So when you touch something conductive, it is at 0-Volts Touch Potential. You do not get electrocuted if something goes wrong. That job is accomplished by the GEC and GEC only.
2. All other conductors, except GEC, provide a low impedance planned fault path back to the source of power which is not DIRT or GROUND. In a Stand Alone System there are two sources of power, the panels and batteries. Dirt or Ground has anything to do with it. In fact it is forbidden to use dirt as a conductor.
3. To protect you from outside high voltage sources like the utility and lightning. That is the GEC job and something that is called an auxiliary Ground which I will get into.
So it will help if you understand what function each wire serves. OK below you see a Full Blown Off-Grid Battery System drawing showing two Optional Methods of Bonding the Solar Panel Frames. System shown is a Grounded System. What makes it a Grounded System is each power source has a Circuit Conductor Bonded to the GEC. Note battery negative term post and the Solar Panels Negative Return Buss are connected to green wires. A Grounded System is used to facilitate simple and cost effective Over Current Protection Devices like fuse and breakers to protect Circuit Conductors, the conductors carrying normal load current. It is part of the Planned Path to clear faults. Dirt has nothing to do with it.
I used 3 parallel strings to demonstrate what Grounding and Bonding is required because circuit breakers are used to protect the power wiring from the panels to controller. If you use 2 or fewer strings, then the fuses and or circuit breakers are not required. In that application only the panel frames are required to be Bonded to nearest compliant Dirt.
OK this is where it comes together. Make note I am showing 2 optional methods of Bonding the Solar Panel Frames. Both methods are compliant, but work completely differently, and one method is clearly better than the other. In a nutshell NEC 250.134 allows you to treat the Panel Frames like equipment or Air Strike Terminals. Just about every contractor will treat the Panel Frames like Equipment meaning they will run an Equipment Bonding Jumper with the panel circuit conductors. It is cheap and easy to implement. The other method allowed is to treat the panel frames like Air Strike Terminals.
So what you say? Well NEC requires any conductive objects that contain electrical wiring of any kind to be Bonded so in the event you touch it will not receive an electrical shock. Either method will accomplish that. Both methods will operate the OCPD.
If you do what most contractors do by using an Equipment Bonding Conductor and treat the frames as Equipment, you have invited Lighting and High Voltage right into your home wiring and equipment. Ironically NEC permits this because they do not concern themselves with Lightning Protection and is out of scope of the NEC. What the NEC does do for you is gives you the customer an option. You can do it the NEC way, or NFPA-780 Standard For Installing Lightning Protection Systems way. Your choice.
So now look again and things should be clearer. Option one is the green dotted line to the right of the panels, and option two is the green dotted lines to the left of the panels. If you use option one you are treating the panels as equipment frames. The green conductor is ran with the solar panel wiring, and the Bond provides a Planned Fault Patch back to the Panel Negative Return Bus, and provides touch potential protection. The green wire will be the same size as the Solar Panel largest conductor typically a 12 or 10 AWG. If lightning were to strike a panel, it will take a path right through you house to find its way to DIRT.
Option two uses a Lightning Down Conductor as defined by NFPA-780 going straight to DIRT. See the difference? If lightning strikes a panel, it is routed around your home going straight to DIRT. As shown I show it on its own Ground Rod, and that rod is Bonded to all electrodes as required. I showed it this way to remind you Lightning Down Conductors are to be kept as straight and short as possible. If the nearest point down from the roof is above where you have the Solar wiring equipment installed can use the same Rod(s). What you do not want to do is span across the roof to go down. Whatever panels is closest to the edge is where you want to go down.
NEC is real simple when it comes to Bonding and Grounding. Draw this picture in your mind. Bond everything below ground together, Bond everything above ground together, and then Bond the two together at one point only and call it the GEC as the GEC and Lightning Down Conductor are the only two Grounds in a system. Everything else is just a Bonding Jumper or Conductor.
EDIT:
As an edit note it was brought to my attention I did not discuss the Green Wire between the Charge Controller G terminal and the solar panels Negative Return Bus. I touched on it but most users are not going to have 3 or more parallel strings. Most will only be 1 or 2 strings, and thus no Negative Return Buss or Fuses required. In NEC terms this would be called a Main Bonding Jumper and its sole purpose in life is to make the fusses or circuit breakers operate correctly and quickly. It is a Planned Fault Path so in the event one of the Solar Panels hot wire touches a raceway, beaks loose and falls into a chassis or frame will provide a planned path back to the source which is the Solar Panels Negative Return Buss. Although not code compliant, if you left out the other Green conductor between the "CC G Term" and Intersystem Bonding Bar, the fuses and breakers would work and do their job just fine. The only purpose of the Intersystem bond is to establish the 0-Volt reference point. If you only use 1 or 2 strings, there will be no Green Wire from the CC to Negative Return Bus or fusses. You are still required to bond the frames and raceways. but no requirement to run a Grounded System so you can Float the panels circuit wires.
What trips up even some electricians is an Off-Grid Battery Systems is made up from two Separately Derived Systems. One power source is the panels, and the other is the battery. To make it work requires a Charge Controller made to work in premises wiring systems where the chassis is not bonded to any Battery Polarity. This type of Controller and Inverter has no Galvanic connection between Input and Output. In other words no direct electrical connection between Input and Output. They are Isolated from each other, and each system has its own Bonding. This is what you see below. A Bonding Jumper for the Controller/Panels so facilitate fuse and breaker operation on the panel circuits. You see it repeated two more times for the Battery circuits and one for the Inverter. The 3rd Green Ground wire for the AC circuits is part of the AC wiring not shown, but those will be from the Inverter Line Feeders.
Bonding & Grounding as it pertains to NEC Article 250 is poorly written, uses the wrong terms and when compounded make it awfully confusing. To start Ground is a misused term with no meaning or function. NEC Article 100 defines Ground as: THE EARTH or DIRT. There is only one Ground Wire on any system called the Ground Electrode Conductor aka GEC. It is the only wire or cable connected to DIRT. Everything else is a Bonding Jumper or Conductor. Get your noodle wrapped around that, and you are half way there to understanding. There are 3 primary reasons we Bond and Ground:
1 To provide a 0-Volt Reference Point. So when you touch something conductive, it is at 0-Volts Touch Potential. You do not get electrocuted if something goes wrong. That job is accomplished by the GEC and GEC only.
2. All other conductors, except GEC, provide a low impedance planned fault path back to the source of power which is not DIRT or GROUND. In a Stand Alone System there are two sources of power, the panels and batteries. Dirt or Ground has anything to do with it. In fact it is forbidden to use dirt as a conductor.
3. To protect you from outside high voltage sources like the utility and lightning. That is the GEC job and something that is called an auxiliary Ground which I will get into.
So it will help if you understand what function each wire serves. OK below you see a Full Blown Off-Grid Battery System drawing showing two Optional Methods of Bonding the Solar Panel Frames. System shown is a Grounded System. What makes it a Grounded System is each power source has a Circuit Conductor Bonded to the GEC. Note battery negative term post and the Solar Panels Negative Return Buss are connected to green wires. A Grounded System is used to facilitate simple and cost effective Over Current Protection Devices like fuse and breakers to protect Circuit Conductors, the conductors carrying normal load current. It is part of the Planned Path to clear faults. Dirt has nothing to do with it.
I used 3 parallel strings to demonstrate what Grounding and Bonding is required because circuit breakers are used to protect the power wiring from the panels to controller. If you use 2 or fewer strings, then the fuses and or circuit breakers are not required. In that application only the panel frames are required to be Bonded to nearest compliant Dirt.
OK this is where it comes together. Make note I am showing 2 optional methods of Bonding the Solar Panel Frames. Both methods are compliant, but work completely differently, and one method is clearly better than the other. In a nutshell NEC 250.134 allows you to treat the Panel Frames like equipment or Air Strike Terminals. Just about every contractor will treat the Panel Frames like Equipment meaning they will run an Equipment Bonding Jumper with the panel circuit conductors. It is cheap and easy to implement. The other method allowed is to treat the panel frames like Air Strike Terminals.
So what you say? Well NEC requires any conductive objects that contain electrical wiring of any kind to be Bonded so in the event you touch it will not receive an electrical shock. Either method will accomplish that. Both methods will operate the OCPD.
If you do what most contractors do by using an Equipment Bonding Conductor and treat the frames as Equipment, you have invited Lighting and High Voltage right into your home wiring and equipment. Ironically NEC permits this because they do not concern themselves with Lightning Protection and is out of scope of the NEC. What the NEC does do for you is gives you the customer an option. You can do it the NEC way, or NFPA-780 Standard For Installing Lightning Protection Systems way. Your choice.
So now look again and things should be clearer. Option one is the green dotted line to the right of the panels, and option two is the green dotted lines to the left of the panels. If you use option one you are treating the panels as equipment frames. The green conductor is ran with the solar panel wiring, and the Bond provides a Planned Fault Patch back to the Panel Negative Return Bus, and provides touch potential protection. The green wire will be the same size as the Solar Panel largest conductor typically a 12 or 10 AWG. If lightning were to strike a panel, it will take a path right through you house to find its way to DIRT.
Option two uses a Lightning Down Conductor as defined by NFPA-780 going straight to DIRT. See the difference? If lightning strikes a panel, it is routed around your home going straight to DIRT. As shown I show it on its own Ground Rod, and that rod is Bonded to all electrodes as required. I showed it this way to remind you Lightning Down Conductors are to be kept as straight and short as possible. If the nearest point down from the roof is above where you have the Solar wiring equipment installed can use the same Rod(s). What you do not want to do is span across the roof to go down. Whatever panels is closest to the edge is where you want to go down.
NEC is real simple when it comes to Bonding and Grounding. Draw this picture in your mind. Bond everything below ground together, Bond everything above ground together, and then Bond the two together at one point only and call it the GEC as the GEC and Lightning Down Conductor are the only two Grounds in a system. Everything else is just a Bonding Jumper or Conductor.
EDIT:
As an edit note it was brought to my attention I did not discuss the Green Wire between the Charge Controller G terminal and the solar panels Negative Return Bus. I touched on it but most users are not going to have 3 or more parallel strings. Most will only be 1 or 2 strings, and thus no Negative Return Buss or Fuses required. In NEC terms this would be called a Main Bonding Jumper and its sole purpose in life is to make the fusses or circuit breakers operate correctly and quickly. It is a Planned Fault Path so in the event one of the Solar Panels hot wire touches a raceway, beaks loose and falls into a chassis or frame will provide a planned path back to the source which is the Solar Panels Negative Return Buss. Although not code compliant, if you left out the other Green conductor between the "CC G Term" and Intersystem Bonding Bar, the fuses and breakers would work and do their job just fine. The only purpose of the Intersystem bond is to establish the 0-Volt reference point. If you only use 1 or 2 strings, there will be no Green Wire from the CC to Negative Return Bus or fusses. You are still required to bond the frames and raceways. but no requirement to run a Grounded System so you can Float the panels circuit wires.
What trips up even some electricians is an Off-Grid Battery Systems is made up from two Separately Derived Systems. One power source is the panels, and the other is the battery. To make it work requires a Charge Controller made to work in premises wiring systems where the chassis is not bonded to any Battery Polarity. This type of Controller and Inverter has no Galvanic connection between Input and Output. In other words no direct electrical connection between Input and Output. They are Isolated from each other, and each system has its own Bonding. This is what you see below. A Bonding Jumper for the Controller/Panels so facilitate fuse and breaker operation on the panel circuits. You see it repeated two more times for the Battery circuits and one for the Inverter. The 3rd Green Ground wire for the AC circuits is part of the AC wiring not shown, but those will be from the Inverter Line Feeders.
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