Resolving Fire Hazards from the Ground-Fault Detection Blind Spot
Inside this Article
As an MC4 connector sat in a puddle of water on a commercial rooftop in southern New Jersey, a nearly invisible fire risk was developing. Exposed to harsh winters and humid summers in the 6 years since its installation, the seal on this particular connector, shown in Figure 1, had broken down and was now providing a hazardous ground path for the dc current to earth. The traditional fuse-based ground-fault detector interrupters (GFDIs) employed in many central inverters developed specifically for the North American market are not capable of detecting this type of ground fault. This results in a dangerous condition known as the ground-fault detection blind spot, which has caused numerous electrical fires. Bill Brooks reported on this condition in his SolarPro article “The Bakersfield Fire: A Lesson in Ground-Fault Detection” (February/March 2011).
In the past, this MC4 connector fault might have gone undetected. In this case, however, a new ground-fault detection technology called a current sense monitor (CSM) detected the fault and generated an alarm at the command center of a major engineering, procurement and construction (EPC) firm. The EPC firm quickly dispatched a team to the array to perform diagnostics and maintenance. This team found the damaged MC4 connector in a grounded current-carrying conductor by systematically measuring voltages on subsections of the array. After identifying the precise location of the fault via visual inspection, the team replaced the damaged MC4 connector.
The ground path provided by the damaged connector produced only 120 mA of ground-fault current, substantially less than the trip threshold of the 5-amp GFDI fuse in the inverter deployed at this site. In a worst-case scenario, this latent ground-fault condition would have persisted until a second ground fault occurred, shorting the array around the GFDI fuse and causing a fire similar to the incident in Bakersfield, California. Fortunately, the 120 mA fault current was above the 100 mA detection threshold the EPC firm had programmed into the CSM equipment. This enabled the team to remotely identify and quickly remedy the ground-fault condition in the roof-mounted PV array.
Latent Ground-Fault Background
The effects of PV fires can be significant, especially for rooftop PV systems, where PV-initiated fires have the potential to damage not only the array, but also the structure itself, as shown in Figure 2. Historically, the ground-fault detection blind spot has caused many latent ground faults and ultimately resulted in several PV fires in North America. Latent ground faults can either be grounded conductor-to-ground faults or high-impedance ground faults on ungrounded conductors. The initial ground fault is generally not a fire hazard, but will remain latent because the fault current is too low to trip the inverter’s GFDI fuse. In the event that a second ground fault occurs in the array, fault current, which may be very large, will bypass the GFDI device, and the inverter’s ground-fault protection system will not work as intended to prevent a fire.
Industry response. In 2013, the Solar America Board for Codes and Standards (Solar ABCs) analyzed the latent ground-fault problem and concluded that traditional fuse-based GFDIs did not adequately mitigate hazards associated with these types of faults. Sandia National Laboratories further concluded that simply reducing the installed GFDI fuse rating to lower its current detection threshold was not an effective means of closing the blind spot. The reason is that smaller fuse sizes have increased internal impedances, which reduce the fault current and prevent the fuse from blowing when a latent ground fault is present. It is not possible, therefore, to close the ground-fault blind spot with smaller GFDI fuses. Further, retrofitting an inverter with a smaller GFDI fuse would often invalidate its certification listing.
Since latent ground faults were known to cause fires in PV systems, the Solar ABCs stakeholder group hastily identified ground-fault protection alternatives to fuse-based GFDIs. The group largely based these alternatives on European fault-detection techniques that vendors or integrators could implement using readily available products. The alternatives identified included current sense monitors, which measure current flow through the ground bond of dc-grounded systems; isolation monitors, which measure array resistance to earth in temporarily or permanently ungrounded PV systems; and residual current detectors, which measure differential current between the positive and negative conductors.
Industry concerns related to latent ground faults also led directly to changes in the National Electrical Code. As part of the 2014 revision cycle, the Code-making panel revised Section 690.5(A) to explicitly state that the ground-fault protection device or system must be capable of detecting ground faults in “intentionally grounded conductors.” Many inverter manufacturers are now moving to implement one or more of the identified ground-fault protection alternatives for NEC 2014 compliance. However, incorporating these technologies into new products does not protect the PV fleet currently deployed across the country.