SOLAR TODAY - April 2011

NEC and Fire Safety in PV Systems

The new code will require arc-fault circuit interrupters in high-voltage DC systems.

By JAY HOLMAN

Jay Holman (jholman @ energy-insights.com) is research manager for renewable energy strategies at IDC ( idc.com). He holds an MBA from the Sloan School of Management, a master’s EE/CS from MIT’s School of Engineering, and a bachelor’s in physics from Colgate University.

The 2011 U.S. National Electric Code includes increased safety requirements for rooftop PV systems, and the industry is actively working on technologies that will reduce the risk of fire in these systems. Chief among these is the DC arc-fault circuit interrupter (AFCI), which will begin to see widespread adoption later this year.

Understanding the Risks

In recent years only a handful of fires have been attributed to rooftop PV systems. The low incidence rate for PV fires might suggest that there is little need for concern, but recent significant changes in the number and nature of the PV systems being installed have changed the fire-risk equation for the worse.

For the first few decades the PV industry was in existence, the vast majority of PV systems were small and off-grid, often used to charge batteries that then supplied power to a home, building or remote piece of equipment. The low voltage levels ( 12, 24, or 48 VDC) of these systems, as well as the small number of electrical connections in the systems, minimized their fire-related risks.

The situation changed in the late 1990s, when the industry installed a growing number of grid-tied PV installations operating at voltages up to 600 VDC. These grid-tied systems also have a large number of connections, each of which presents an opportunity for a mistake that can lead to an arc-fault. Arc-faults, in which electricity arcs across an undesired gap in an electrical circuit, pose the greatest fire risk for rooftop PV systems. The industry’s long experience with low-voltage systems can’t serve as a predictor of the safety of complex high-voltage systems. And because the expected operating life of a PV system is more than 20 years, the true fire incidence rate for high-voltage PV systems may not become apparent for another 10 to 20 years, when the large number of systems installed over the past decade start to reach their end of life.

first line of defense is proper installation, with competent post-installation inspection. Regular ongoing system inspections can ensure continuing safe operation.

When arc faults do occur in PV systems, it is possible to detect and extinguish them before they start a fire using a device called a DC PV arc-fault circuit interrupter. The 2011 U.S. National Electric Code (NEC) includes a new requirement (690.11) for the use of these devices in rooftop PV systems operating at DC voltages over 80 V.

Despite this new requirement in the NEC, it will be some time before DC PV AFCIs see widespread use. Many states and jurisdictions do not adopt the NEC as soon as it is released, delaying adoption for months or years. There is not yet a UL standard against which to certify DC PV AFCIs, which means that no devices certified to the new NEC requirement are available on the market.

There is not yet a UL standard to certify DC PV AFCIs.

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Reducing Fire Risk in Solar PV Systems

The risk of arc faults in PV systems can be reduced in a number of ways. PV system design also influences arc-fault risk level: In general, low-power systems pose less risk than high-power systems, low-voltage systems pose less risk than high-voltage systems, and AC systems pose less risk than DC systems. However, PV system design cannot reduce the arc-fault risk to zero, so proper installation and maintenance is critical for any PV system. The

That could change soon, as UL could publish an Outline of Investigation for the relevant standard, UL 1699B, as early as the first quarter of 2011. The Outline defines two types of DC PV AFCIs: Type 1 devices address series arc faults only, and Type 2 devices address both series and parallel arc faults. As soon as the Outline is published, manufacturers will be able to certify their products to it. Stand-alone device manufacturers such as Eaton, and DC Optimizer manufacturers like Tigo Energy, SolarEdge and National Semiconductor, as well as some inverter manufacturers, are already working on solutions and could have listed products available soon.

Although the NEC only requires the use of Type 1 devices, some vendors are going beyond that requirement by developing Type 2 devices. The use of safety as a selling feature is a promising development that should promote faster adoption of DC PV AFCIs than the industry would see if safety was driven solely by compliance with the NEC.

Meanwhile, project developers and PV system owners should become familiar with the new standard. Hopefully this will lead not just to widespread deployment of DC PV AFCIs, but also to improved installation and system maintenance practices that prevent arc faults from occurring in the first place.