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 @
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
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
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.
14 April 2011 SOLAR TODA Y solartoday.org
Copyright © 2011 by the American Solar Energy Society Inc. All rights reserved.