NEC Article 690.5 requires DC ground-fault protection (GFP) for nearly every type of PV array installation, primarily for reducing fire hazards. GFP is built into all new grid-tied inverters, including transformerless inverters for ungrounded systems. Some older grid-tied inverters may not have integrated ground-fault protection, but GFP has been an NEC requirement since the 1984 Code cycle.
Ground faults happen when a current-carrying conductor makes inadvertent contact with an equipment grounding conductor, or any piece of metal that is grounded. This can occur through damaged conductor insulation or with improper installation. This is a potentially dangerous situation, and inverters are intended to detect these faults. For grounded arrays, the GFP device in the inverter is the point of connection between the grounded conductor and ground—the DC system ground—which is established through a fuse in the inverter. Transformer-based grid-tied inverters use the fuse or breaker as both the point of system ground on the DC side and as the GFP device. Smaller inverters (less than 10 kW) generally use a low-amperage fuse (typically 1 A); larger inverters use larger current devices (up to 5 A). This GFP device is meant to protect against fires—not against shock, like household GFCI receptacles—because the threshold for harmful shocks is much lower, around 10 milliamps.
The GFP device is designed to detect current on the grounding system (i.e., a ground fault), which will blow the fuse and open the circuit. To understand this process, consider a negative-grounded system, wherein the negative conductor is connected to all of the grounded parts of an array (through the GFP fuse). If there is a fault (for example, from a positive wire pinched against a mounting rail), the current will try to flow from the fault on the grounded rail, through the fuse, and back through the negative conductor to complete the circuit, because that is a path of lower resistance than through the inverter electronics. Once the fuse’s current rating is exceeded, the fuse will blow, opening the system ground (and thus stopping any short-circuit arc between the positive wire and the rail). The inverter will turn on a light to indicate a ground fault, and will be triggered to simultaneously open the ungrounded circuit conductors and cease supplying output power, effectively shutting off the system. Although the source of the fault is not fixed, nor its location known, the system is shut down to await repair.
With a transformerless inverter, since there is no isolation, a DC fault could affect the AC side of the system. In this case, it is just as critical (if not more so) to provide protection. Transformerless inverters have an entirely different type of ground-fault detection. Before the inverter starts operating, resistance to ground from the DC current-carrying conductors is checked. If there is low resistance to ground (an indication of a ground fault), the inverter will not start operating. Once operational, a differential current sensor continually monitors the current on both the positive and negative conductors. If there is no fault, the current should be identical. If a fault occurs, such that some portion of the current flows through a grounding conductor or grounded metal, the inverter detects the current difference, and thus the fault, shuts off, and indicates a ground fault. The fault threshold is much lower than a fused device, usually 300 milliamps, which is one reason ungrounded arrays can be safer than grounded systems.
A second reason the differential current-sensing GFP devices in transformerless inverters are considered safer than fused GFP devices is that they are equally good at detecting faults on both positive and negative conductors. A negative-grounded array may have a ground fault from negative to ground that never causes enough current to flow through the GFP fuse to trip the device. It’s only when another, possibly catastrophic, positive-to-ground-fault occurs that the GFP fuse will blow. At this point, it may be too late to stop a fire from happening at the point of the initial or secondary fault.