Grounding provides a path for fault current or lightning surges to flow through to protect people and equipment from electric shock hazards.
Equipment Grounding Conductor (EGC)
PV modules mounted to metal racks are effectively grounded when the module frames are secured to and in electrical contact with the rack, and the rack is grounded. However, since the integrity of the electrical contact between the module frames and mounting structure cannot always be assured, individual module frames are connected together with equipment grounding conductors (EGC). This can be accomplished with a few continuous runs of bare conductor that are secured to each module with a special connector, or with many short bonding jumpers between adjacent modules.
Some exceptions apply to mechanical WEEBS or similar devices that bite into the modules frame and the raking rails and they are UL listed as grounding devices. In this case, equipment grounding conductors are not required.
When ground-fault protection is used, PV circuit equipment grounding conductors are sized in accordance with Article 250, which establishes the minimum size for equipment grounding conductors based on the overcurrent protection rating in the circuit. NEC table 250.122 is used for sizing. For example, if the PV output circuit overcurrent protection device is 60 A, then a 10 AWG equipment grounding conductor is required. When the sizes of ungrounded conductors are increased above what is required (such as to decrease voltage drop), the equipment grounding conductors must be increased proportionally.
Grounding Electrode Conductor (GEC)
A grounding electrode is a conductor rod, plate, or wire buried in the ground to provide a low-resistance connection to the earth. NEC 690.47 establishes requirements for grounding electrodes used for PV systems. Most PV systems involve both AC and DC systems, and they are considered two separate systems according to NEC article 690 since the DC grounded conductor is not directly connected to the AC grounded conductor.
Some AHJ may require an interpretation of some NEC editions to include an array grounding in addition to the DC grounding requirements. It is a grounding electrode that is buried in the ground as well.
For more information about the grounding and lightning, you can refer to Chapter 11 in the textbook.
Ground fault protection
Ground Fault is the undesirable situation where the current flows through grounding conductors. Ground Fault Protection is mentioned in NEC 690.5 and is not required for ground mounted arrays. It is required for PV systems mounted on roofs of dwellings. The ground-fault circuit breaker trips when current between the grounded and grounding conductors exceeds its rating and forces the other circuit breaker to open the ungrounded conductor. For low-voltage PV systems, a pair of circuit breakers can be used to provide array ground-fault protection. For interactive PV systems, ground-fault protection is usually built into the inverter, which includes a serviceable fuse. In order to fulfill the requirement to open the ungrounded conductor, the inverter is designed to immediately shut down if the fuse is opened.
A ground-fault circuit interrupter (GFCI) is a device that opens the ungrounded and grounded conductors when a ground fault exceeds a certain amount, typically 4 mA to 6 mA. It does this by sensing a difference between the current flowing out through the ungrounded conductor and returning through the grounded conductor.
A GFCI device is used in AC branch circuits to protect persons from electrical shock hazards. GFCI protection is often included in receptacles and is required in wet environments, such as bathrooms, with a greater potential for ground faults. Article 210, “Branch Circuits,” provides details and required locations for GFCI devices.
Because PV arrays are mounted on elevated buildings and structures such as rooftops and poles, many PV systems are protected from potential lightning that can cause severe damage. Lightning protection system requirements are covered briefly in Article 250 and more extensively in NFPA 780 (Standard for the Installation of Lightning Protection Systems). Lightning protection is especially important in places where the probability is high, such as the southeastern United States.
Lightning protection systems consist of a low-impedance network of air terminals (lightning rods) connected to a special grounding electrode system and not connected to the DC or array electrode conductors.