PV System Ground Faults
Inside this Article
A ground fault not only causes PV system shutdown and energy loss, but it also presents a safety concern that all designers, electricians, installers and service technicians need to be aware of.
According to the Photovoltaic Systems textbook, published by the National Joint Apprenticeship and Training Committee (NJATC) for the electrical industry, a ground fault is "the undesirable condition of current flowing through the grounding conductor." The cause of this undesirable current flow is an unintentional electrical connection between a current-carrying conductor in the PV system and the equipment grounding conductor (EGC).
Ground faults are not only some of the most aggravating and labor intensive situations encountered in PV installations; they can also be downright dangerous. While both the National Electrical Code and UL require that grid-tied photovoltaic inverters shut down in the event of a ground fault, shutting down the inverter does not normally clear the fault. This can create a number of hazards since the normally grounded current-carrying conductor may no longer be at ground potential. In addition, if the grounding system is inadequate, module frames, metal structures or enclosures may be energized and can present a shock hazard to personnel. Electricians, installers or service technicians troubleshooting a ground fault need to proceed as if everything conductive is at lethal potential to ground—until a multi-meter proves otherwise—lest they become the inadvertent ground path for that fault current.
Before delving into ground faults in detail—why they happen, how to prevent them and how to safely find them when they happen—we first consider the evolution of ground-fault protection requirements for PV systems in the NEC.
GROUND-FAULT PROTECTION FOR PV SYSTEMS
NEC Article 690.5 specifies the ground-fault protection requirements for grounded dc photovoltaic arrays. Groundfault protection is also required for ungrounded PV systems; these requirements are detailed in 690.35(C). The specified purpose of a ground-fault protection device (GFPD) as part of a PV power system is to reduce the risk of fire associated with a ground fault. "If the ground fault is a short-circuit," the NJATC’s Photovoltaic Systems notes, "the fault current can be very high. In addition to being a safety hazard, this creates a significant fire hazard as bare metal is heated by the current flow."
Because of this fire hazard, ground-fault protection for PV array circuits was first required for roof-mounted residential PV installations, where the perceived risk of fire is great. The potential for property damage or loss of life when a PV system is installed on the roof of a home, for example, is greater than it is where a PV system is ground-mounted in a field at some distance from a building.
Nevertheless, as the fire hazards associated with PV systems have become increasingly understood and documented, the GFPD requirements outlined in the Code have become more inclusive. Ground-fault protection is not just for residential roofs anymore. The 2008 NEC, for example, requires ground-fault protection for all “grounded dc photovoltaic arrays.” The seminal event that prompted this recent change was a ground fault that melted through the side of metal conduit on a medium sized commercial system.
The NEC lists two exceptions to this general prescription. The first exception to the general GFPD requirement is for ground- or pole-mounted PV arrays that are isolated from any buildings and limited to one or two parallel source circuits. This exception might apply to a small water pumping system, for example. The second exception is granted to PV arrays installed at “other than dwelling units,” provided that the EGC is sized per Article 690.45. If groundfault protection is not provided, 690.45(B) requires sizing the EGC ampacity for “at least two (2) times the temperature and conduit fill corrected circuit conductor ampacity.” As explained in the NEC 2008 Handbook, this ensures that ground-fault currents can safely circulate continuously in the EGC until the fault is corrected. Both these exceptions are somewhat moot for grid-tied PV systems since UL 1741 now requires all grid-tied inverters to have built-in groundfault protection.
As described by John Wiles, program manager for the Institute for the Energy and the Environment, in the February/ March 2008 Home Power article "Ground-Fault Protection Is Expanding," Article 690.5 was added to the NEC in 1987. One of Wiles’ first projects in the PV industry was to develop “prototype hardware” to meet the new Code requirement. "The basic concept (of the original GFPD prototype)," writes Wiles, "was to insert a 0.5- or 1-amp circuit breaker in the dc system-bonding conductor." This small circuit breaker senses any current between the grounded current-carrying conductor and the grounding system, tripping if current exceeds the 0.5 A or 1 A rating. By mechanically linking this smaller breaker to larger capacity breakers that are installed inline with the ungrounded current-carrying array conductors, it is possible to detect, interrupt and indicate that a ground fault has occurred, and to disconnect the fault.