In HP153, “PV Grounding and Bonding; Part 2” focused on the National Electrical Code (NEC) requirements associated with equipment grounding conductors (EGCs). This article will further explore system grounding requirements and methods.
NEC Section 690.41, System Grounding, helps set the basic grounding requirements for PV systems. The term “grounding” is used to reference the connection made to earth. In electrical systems, grounding occurs by connecting to a grounding electrode—for example, a long copper rod driven into the earth. Bonding refers to establishing electrical continuity by connecting components together.
Section 690.41 of the NEC requires that one current-carrying conductor (the positive or the negative) be solidly grounded in PV systems greater than 50 volts to limit imposed voltages from outside sources and stabilize the voltage to earth during normal operation. This grounded conductor is similar to AC systems in which, typically, the neutral conductor is bonded to ground in the main distribution panel. This section includes an exception for ungrounded PV systems that comply with Section 690.35. (Ungrounded systems are rising in popularity and will be covered in future articles.)
Section 690.42 establishes where in the circuit the system grounding should be. This NEC section simply states that “the DC circuit grounding connection shall be made at any single point on the photovoltaic output circuit,” to eliminate multiple paths to ground in the circuit. But the informational note in the 2011 NEC and the exception should not be neglected, since they direct placing this grounding connection as close to the PV source as possible to protect from lightning surges. While this may be helpful to limit voltage surges associated with lightning, the equipment connected in the circuit must be considered before establishing a grounding connection at the array.
In a system with ground-fault protection (GFP), that device provides the current-carrying conductor to ground bond. For grid-tied PV systems, the GFP device is located in the inverter; and thus the required grounding connection is already established. When installing battery-based equipment, a GFP device may be included with the equipment or may be added on. When there is a GFP device, the current-carrying conductor to ground bond shall not be duplicated outside of that device. If you were to follow the informational note’s advice and make a second grounding connection in addition to the GFP connection, you would introduce a second path to ground and, inevitably, a ground fault.
Section 690.47 details the requirements for establishing the PV system grounding connection to earth via a DC grounding electrode system. Three installation methods are acceptable for the DC grounding electrode system. The first, 690.47(C)(1), allows the installation of a new, separate DC grounding electrode that is, in turn, bonded to the existing AC grounding electrode. This results in a grounding electrode conductor (GEC) running from the marked location in the inverter to a grounding electrode installed specifically for the inverter. The bonding jumper between the two grounding electrodes is sized to be the larger of either the existing AC GEC or the size of GEC as specified for DC systems in 250.166.