Manufacturers are rapidly bringing to market lighter weight, more efficient, and space-saving transformerless inverters for use with ungrounded PV power systems. And when these systems are properly installed, their safety is equal or superior to grounded PV systems.
Transformerless inverters—inverters that must be installed with ungrounded PV power systems—are becoming more common in the United States. The differences between ungrounded and grounded systems need to be clearly defined and understood for two important reasons: first, because the National Electrical Code (NEC) sets installation requirements that vary depending upon whether a system is grounded or not; and second, so that the safe functionality of ungrounded systems is not in doubt.
A common misconception about ungrounded PV systems is that they lack all grounding, including equipment grounding—the bonding to ground of exposed metal that could become energized in a fault situation. But all Code-compliant and well-installed PV systems must still have equipment grounding—the bare copper or green-insulated grounding conductors that bond the metal boxes, the metal conduit, the metal rails that hold modules, and the metal frames of the PV modules. This keeps the electrical potential of the metal at zero volts, facilitates the operation of overcurrent protection devices, and helps keep systems and people safe.
A “grounded” PV power source has either the positive or negative DC-carrying conductor connected to ground (called the “DC system ground”). An “ungrounded” PV system has neither the positive nor negative DC current-carrying conductor connected to ground. However, just like the grounded system, it still has equipment grounding. While “ungrounded” is the terminology used in the NEC, many in the solar industry call these arrays “floating,” because of the confusion that can ensue over the variety of PV terminology containing the word “ground.” Floating means that neither the positive nor negative conductors on the DC side of the system have a direct connection to ground.
NEC Article 690.41 mandates system grounding for PV sources greater than 50 volts. However, since 2005, Article 690.35 (listed as an exception to Article 690.41) allows PV power systems above 50 volts to be ungrounded—provided they comply with the requirements discussed below.
In PV arrays, both the positive and negative conductors are current-carrying. In an AC electrical system, such as a 120/240 VAC residential service, both the hot and neutral conductors are current-carrying, but the neutral conductor is the grounded conductor, meaning that at one point (and only one point) in the system, the neutral is bonded to the grounding system (usually at the main disconnect or in the main service panel). This grounding connection creates a potential of zero volts for the neutral conductor relative to ground—the same as all of the metal components in the system that are also connected to ground by equipment grounding.
Potential difference is another way to say voltage, which is analogous to the potential electrical force existing between two points. Remember, it always takes two points to measure this potential. The two “hot” conductors in a 120/240 VAC residential service are the ungrounded conductors, referred to as Line 1 and Line 2. Voltage readings between an ungrounded conductor and either neutral or ground in an AC residential system will result in the same measurement, approximately 120 VAC, since neutral and ground are connected.