Ground-Mounted Racking Considerations: Page 5 of 7
Inside this Article
Electrical equipment pad. As is the case with typical ground-mounted systems, centralizing the location of the inverters offers cost and performance benefits. The optimal location is typically somewhere along the access road near the center of the array. The shoulder of the access road serves as a means of bringing in the medium- or high-voltage circuit to the distribution transformer from the point of common coupling. Since the road is subject to both vehicular traffic and differential settlement, system designers generally call for a reinforced concrete duct bank to protect these circuits.
Concrete equipment pads for landfill applications may have to be quite large, given the extreme weight of the electrical equipment and the relatively low allowable bearing pressure. A typical 1 MW liquid-filled medium-voltage transformer usually weighs more than 1,000 pounds. In many cases, it makes sense to install a single reinforced concrete equipment pad that supports all of the equipment, rather than multiple smaller concrete pads, as the larger pad will save preparation and installation time. Using a single pad also protects the conductors against damage due to differential settlement, since the single pad will move uniformly as a single unit.
System grounding. Opting for a single equipment pad also provides a means for system grounding. Given the shallow depth of the landfill cover material, it is obviously not possible to install an 8- or 10-foot ground rod as a grounding electrode. Instead, a copper ground ring with supplemental copper grounding plates typically offers adequate system grounding. The thickness of the concrete pad, plus the depth of the fill material beneath it, provide the burial depth required for the ground ring.
Part 3: Bonding and Grounding
By Marvin Hamon, PE
To provide a safe PV system, designers and installers need to understand the fundamentals of bonding and grounding. Whereas proper bonding ensures that a ground-mounted PV array is free from stray shock hazards for the life of the system, proper grounding prevents dangerous voltage differentials between the PV array and the ground on which installers or service technicians stand. Since people often confuse these terms and practices, here I define each individually, consider its intent, and review hardware options and installation methods.
Per the definition in Article 100 of the National Electrical Code, equipment is bonded when it is “connected to establish electrical continuity and conductivity.” Employing an equipment-grounding conductor (EGC) is one way to make this low-impedance connection. Another way is to make a mechanical connection by, for instance, bolting two pieces of conductive material together.
Intent. As described in NEC Section 250.4, bonding establishes a low-impedance connection between conductive equipment and materials to conduct fault current safely and minimize potential voltage differences between conductive components. By providing a low-impedance path for line-to-line or line-to-ground faults, the designer ensures that fault currents quickly rise to a level that activates an overcurrent or ground-fault protection device.
Hardware. In the 1990s and early 2000s, installers used thread-forming screws, UL-listed grounding lugs and bare copper EGCs to connect all the conductive parts of a ground-mounted PV system. While straightforward and easy to verify during inspection, this bonding method is also relatively expensive. It requires a lot of lugs and copper, and it is time-consuming to implement in the field. In other words, this approach is not well suited to deploying PV systems at low prices and high volumes.
About a decade ago, companies such as Wiley Electronics (now Burndy) introduced specialized bonding clips for PV applications to address this pain point. These products have sharp or serrated surfaces that bond the exposed metal module frames and other electrical equipment to the metal mounting structure during installation. Though these new bonding products allowed quick and easy installation, some AHJs and industry stakeholders were skeptical that they could effectively replace conventional hardware and copper EGCs. These (often single-use) components also make system inspection challenging since they are invisible once installed.
UL convened a Standards Technical Panel to develop the UL 2703 product safety standard—which covers PV module-mounting systems, clamping devices and ground lugs—in part to address these concerns about effective bonding. Mounting systems listed to this standard include bonding components designed to establish electrical conductivity between PV modules and other electrical equipment connected to metal racks. Further, a Nationally Recognized Testing Laboratory has certified the efficacy of these bonding connections.