Grounding Compendium for PV Systems

Why is PV system grounding so confusing and the subject so contentious? Perhaps because no concise yet detailed collection of information related to the topic has existed—until now, that is.

PV system grounding encompasses issues ranging from equipment grounding strategies, including bonding modules and grounding racking support structures, to system grounding considerations, including grounding electrode system options. Grounding PV systems correctly and effectively is difficult—and the topic is frequently contentious—because there is no one prescription for either the design process or the methods and materials. The difficulty of grounding PV systems also stems from the interactions of dissimilar metals used for racking structures, module frames and grounding devices. In addition, PV systems are frequently installed in harsh environments, creating situations in which traditional equipment and methods for bonding may not be adequate. Furthermore, PV systems often cover very large physical areas, interconnect to new or existing services, and include transformers of various voltages and configurations, all of which add complexity to designing and installing the grounding system.

Part of the reason why grounding PV systems correctly and effectively is difficult is that applicable UL standards—including UL 1703 (Flat-Plate PV Modules and Panels), UL 2703 (Rack Mounting Systems and Clamping Devices for Flat-Plate Photovoltaic Modules and Panels) and UL 467 (Grounding and Bonding Equipment)—contain requirements that are difficult to harmonize with one another and with the National Electrical Code. Most of the Code requirements related to grounding and bonding are found in Article 250, which describes methods and materials for grounding electrical systems. However, solar professionals also need to understand requirements found in Part V of Article 690 that relate specifically to PV system grounding, as these supplement or modify Article 250 requirements.

In this article, we take a practical approach to the NEC  requirements as applied to grounding PV systems: bonding modules, sizing and specifying equipment-grounding conductors, installing ac and dc grounding electrode conductors and systems, and so forth. In addition, we consider the implications of new methods for mounting modules, in particular racking solutions listed to UL 2703, and present best practices related to the design and installation of grounding systems.

Essential Terminology

While NEC Article 100 defines many of the following terms, here we explain them and put them in context.

Grounded, grounding. Since the NEC defines ground as “the earth,” these words can mean connected, or connecting, to the earth. More often they mean connected, or connecting, to a conductive device that is connected to the earth. The electric potential of the earth is assumed to be zero. 

Solidly grounded. This describes a direct connection to ground, one that does not include any additional impedance or resistance devices.

Bonded, bonding. Components and devices are considered to be bonded when they are connected in a manner that establishes electrical continuity and conductivity. A bonding conductor or bonding jumper is used to establish and maintain a bond.

System grounding. This refers to the practice of intentionally bonding one current-carrying conductor to ground. In ac systems, the grounded conductor is also known as the neutral conductor. DC systems can be negatively or positively grounded—based on the polarity of the grounded conductor—or ungrounded. The dc system grounding connection is accomplished through the main bonding jumper or, in the case of separately derived systems, via the system bonding jumper. System grounding on the dc side of a PV system generally occurs via a ground-fault protection circuit that is internal to a listed inverter, as shown in Figure 1.

Equipment grounding. This refers to the practice of bonding normally noncurrent-carrying metal equipment—like the module frames in Figure 1—to establish an electrically continuous path.

Grounding electrode. This is a conductive component through which an electrical system’s connection to ground is established. NEC Section 250.52(A) describes allowable grounding electrodes.

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