NEC 2014 has not yet been widely implemented, but inverter manufacturers are already working on meeting requirements for arc-fault protection and rapid shut-down.
Arc Fault Circuit Protection (DC)
NEC 2011 established arc-fault protection requirements for DC PV circuits, but only for systems with a maximum voltage more than or equal to 80 VDC and with DC circuits on or entering a building. But the phrase “on or penetrating a building” in the previous edition of the code has been left out of NEC 2014 section 690.11, which expands the requirements to nearly all solar projects except the largest utility-scale systems.
The intention is to help protect people and property from damage due to fires, which the code now recognizes can occur on ground-mounted arrays as well as on the roof. Arc-fault protection can help prevent these risks.
“This change has already started to shift the design strategy of many integrators looking to comply with the code,” says Claude Colp, applications engineer at Solectria Renewables, which manufacturers string and central solar inverters.
Many string inverter manufacturers already integrate arc-fault protection. But Colp says inverters larger than 50 kW will likely not include this feature, and instead arc-fault detection will be met through combiner boxes, which — similar to microinverters and power optimizers — will detect arc-faults on a string level, adjacent to the array and allow designers to meet current codes using central inverters.
“The new arc-fault requirement is important to take seriously as we move towards greater solar penetration,” Colp says.
Rapid Shutdown On Buildings
Also, to improve fire and electrical safety, NEC 2014 requires that PV systems on buildings control the conductors (wires). Conductors have to be able to be “de-energized” (or powered down) at any given time, which was not included in the previous version of the code. Experienced firefighters and other professionals expect voltage to be present close to the PV array, but don’t always realize that conductors away from the roof may have live voltage, such as when string inverters are placed on the wall or near the ground, more than 10 feet away from the array.
“In the event of a fire, first responders want to ensure that power and hazardous voltages can be shut down quickly and completely,” says Walter Marusak, product manager at SolarMax USA. “PV modules create a voltage whenever there is light. First responders may cut or come in contact with conductors as they battle a fire, and they want to be sure that there is no danger of shock or electrocution.”
The idea is to be able to shut down current flow and remove any hazardous voltage in wires that run more than 10 feet from the array, or more than 5 feet within a building. These wires are limited to maxing out at 30V with less than and 240 volt-amps current within 10 seconds of shutdown. Marusak says that although this is primarily a PV system requirement, inverters do play a role.
“If an inverter is stopped, the conductors are still live, since the DC cables are supplied from the array and the AC cables are supplied from the grid,” he says. The AC conductor should be opened by opening the PV system disconnect. “If the inverter is located more than 10 feet from the array, a disconnecting switch or contactor must be added to the DC conductors also within 10 feet. This contactor might have the big red button for rapid shut-down. When both AC and DC conductors are opened, the inverter shuts down automatically. But to remove voltage from AC and DC conductors, both actions are required: Opening the PV system disconnect, and actuating the rapid shut-down contactor.”
However, Marusak says the requirement waives the need for the DC contactor if the inverter is located within 10 feet of the array.
“As a result, for commercial rooftops, system installers and designers want to avoid the cost of the DC contactor systems and install the inverters near the array on the roof,” he says. “Commercial rooftops are exposed to full sun, and potentially higher ambient temperatures. Installers expect the inverters to stand up to this environment and want inverters that can be installed on flat roofs.”
“Manufacturers would prefer to see inverters installed in shaded, more protected areas, with designs intended for wall mounting,” Marusak adds. “There is a lot of discussion about how inverter manufacturers will meet these new demands.” SPW
For a more thorough analysis of NEC 2014, see this Engineering Developments feature.