In the past, inverters were commonly referred to as “grounded” because one of the DC current-carrying conductors was physically connected to ground through a fuse. However, this terminology was somewhat misleading. If the fuse blew, typically due to an arc fault, the connection to ground would be lost, leaving the system ungrounded. This revealed that the term “grounded” was not entirely accurate for describing these isolated transformer-based inverters, as they could become ungrounded under certain fault conditions.
Transformerless (TL) inverters, on the other hand, were often labeled as “ungrounded.” In these systems, neither the positive (PV+) nor the negative (PV-) DC conductors were directly connected to ground. Instead, grounding was managed through the inverter’s software and electronic controls during operation. This approach led to the misconception that TL inverters were completely ungrounded. In reality, these non-isolated inverters do establish a grounding connection via the inverter’s internal systems, though not in the traditional physical sense.
To address the confusion and improve clarity, the 2017 National Electrical Code (NEC) introduced the concept of “functionally grounded” inverters. This term encompasses both traditional transformer-based inverters with physical grounding and transformerless inverters that achieve grounding through electronic means. The “functionally grounded” designation more accurately reflects the operational grounding status of these systems, regardless of whether the grounding is achieved through a physical connection or via inverter software and electronics.
In Oregon, most residential PV systems are grounded through traditional methods, typically involving a physical connection to ground. However, with the rise of transformerless inverter technology, there is an increasing adoption of functionally grounded systems. These systems offer enhanced safety features and more flexible design options, aligning with modern safety standards and installation practices.
In Oregon, the top two inverter systems commonly installed for residential PV systems are:
- SolarEdge Inverters: Known for their power optimizers, SolarEdge systems allow for individual panel optimization, improving overall system performance and providing detailed monitoring capabilities. These inverters are popular for their efficiency and flexibility in design, especially in installations with shading issues or complex roof layouts.
- Enphase Microinverters: Enphase microinverters are installed at each solar panel, allowing for independent operation and optimization. This system is particularly advantageous in situations where shading or varying panel orientations are a concern. Enphase systems are also known for their robust monitoring features and ease of scalability.
The NEC (National Electrical Code) includes several codes that are relevant to the installation and grounding of photovoltaic (PV) systems. The key NEC codes at play include:
- NEC 690.41 – System Grounding: This section covers the requirements for grounding PV systems, including the grounding of the DC system and the inverter grounding. It specifies the conditions under which PV systems must be grounded or ungrounded and addresses the use of functionally grounded systems.
- NEC 690.43 – Equipment Grounding: This section outlines the requirements for grounding equipment in PV systems, including the grounding of metallic frames and enclosures. It ensures the safety and integrity of the PV system by providing a path to ground for fault currents.
- NEC 690.45 – Size of Equipment Grounding Conductors: This section specifies the sizing requirements for equipment grounding conductors in PV systems, ensuring that they are adequately sized to handle fault currents without overheating.
- NEC 690.47 – Grounding Electrode System: This section details the requirements for the grounding electrode system, which provides a connection to the earth for the PV system. It covers the use of grounding electrodes, grounding electrode conductors, and interconnection methods.
- NEC 690.48 – Continuity of Grounding Systems: This section ensures that grounding systems in PV installations maintain continuity, which is crucial for safety and proper operation.
- NEC 690.5 – Ground-Fault Protection: This section mandates the use of ground-fault protection devices in PV systems to detect and interrupt ground faults, enhancing system safety.
These codes work together to ensure that PV systems are safely and properly grounded, protecting both the system and the people who may come into contact with it. The introduction of “functionally grounded” systems in the 2017 NEC was a significant update to these standards, reflecting advancements in inverter technology and installation practices.