Article 250, “Grounding and Bonding,” is a very important Code article that receives a lot of attention in PV installations. If you ever get a group of solar installers together, or electricians for that matter, and merely bring up grounding, you are likely to have a lively discussion ensue. Grounding and bonding is a broad and difficult topic to be covered in a brief article—my attempt will be to break the topic down into small, manageable sections that will make the overall topic more user-friendly over multiple Code Corners.
I encourage readers to have their Code books handy and jump from my explanations to the specific Code language. Additional resources include Soares Book on Grounding and Bonding and Mike Holt’s Understanding NEC Requirements for Solar Photovoltaic Installations.
The first section of Article 250 outlines its scope. The article as a whole covers the general requirements for grounding and bonding, as well as six specific requirements listed in 250.1. Table 250.3 lists other Code articles and specific sections for installation requirements that may modify the 250 requirements, with Article 690 being one of many. This means that you need to be aware of not just the rules as defined in 250 but also how 690 may override some of those requirements for PV installations.
Section 250.4 covers the general requirements for grounding and bonding and helps give some insight into the purpose for the requirements. In this section, both grounded and ungrounded requirements are outlined. Our focus will be on grounded systems. Ungrounded PV systems are becoming more common and they will be detailed in future articles.
Systems are grounded to “limit the voltage imposed by lightning, line surges, and unintentional contact with higher-voltage lines.” Another major purpose of the grounding and bonding requirements is to establish an effective ground-fault current path. The installed conductors create a low-impedance circuit that allows the overcurrent protection or ground-fault detector to safely operate.
I’ve heard it said that if you understand grounding, then you haven’t been taught properly. While the statement is good for a chuckle, most of the confusion can be cleared up by understanding a few definitions.
Article 100 contains definitions for many terms in the Code. Several definitions specific to bonding and grounding are included in both Articles 100 and 250. The “PV System Grounding” diagram helps illustrate these definitions.
• Ground—The Earth. I have to say, I love the simplicity of this definition. While it may seem obvious to some, the ground definition is the basis for all other definitions surrounding grounding, so clarifying it in the beginning is important.
• Ground fault—An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally noncurrent-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or the earth.
A ground fault occurs when current is flowing through the metallic components of an electrical installation instead of being contained within the conductors as intended. In PV systems, this can happen for a number of reasons, commonly as a result of conductor insulation being abraded until the copper conductors come into contact with a metallic component, such as the racking system.
In PV systems, ground faults have received a fair amount of attention lately as some fires associated with PV installations have been traced back to ground faults. Nearly every grid-tied inverter is shipped standard with built-in ground-fault protection to reduce the possibility of such faults.