New Safety Requirements

The 2011 NEC (National Electric Code) requires Arc-Fault Protection for photovoltaic (PV) solar power systems. This is an important standard to address the fire risks associated with PV solar power systems since arc fault can be a major source of fire.

CyboInverters are designed to have multiple DC input channels, each of which directly connects to a solar panel which has a typical DC output that is less than 50 volts. Therefore, a CyboInverter based PV solar system does not have high-voltage DC and therefore complies with the 2011 NEC Arc-Fault Circuit Protection code.

On the other hand, a central or string inverter based PV solar power system does have high-voltage DC because a series of solar panels are connected together, many of which can have as high as 600 to 800 volts DC. The NEC 2001 Arc-Fault Circuit Protection code (690.11) is an important standard to address this risk factor.


Why Are CyboInverters and Microinverters Intrinsically Safe?

In our opinion, even if the central or string inverters can detect the arc fault and quickly disconnect from the DC source, as long as the solar panels are in the sun, the high voltage DC will still be present.  Therefore, if there is fire, a commercial roof top with a central or string inverter based PV solar power system can still be very dangerous.

Microinverters and CyboInverters that have UL 1741 certification are intrinsically safe for all installations because there is only low voltage DC (i.e., 50V DC) in the solar power system. If there is fire, the fireman will first shutdown the AC power for the building. According to the Anti-islanding requirement by UL 1741 , the CyboInverters and microinverters will shutdown immediately. Then, there will be no AC in the building or on the roof top. In this case, even if the solar panels are still in the sun, there will only be low voltage DC, which is safe.


Reference
     
  The following information is from the NFPA70 NEC 2011 book published by National Fire Protection Association, One Batterymarch Park, Quincy, MA 02169.  It is a highly recommended reference source for anyone who is interested in this topic.

"690.11 Arc-Fault Circuit Protection (Direct Current).

Photovoltaic systems with dc source circuits, dc output circuits, or both, on or penetrating a building operating at a PV system maximum system voltage of 80 volts or greater, shall be protected by a listed (dc) arc-fault circuit interrupter, PV type, or other system components listed to provide equivalent protection. The PV arc-fault protection means shall comply with the following requirements:

 
  (1) The system shall detect and interrupt arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or other system component in the dc PV source and output circuits.

(2) The system shall disable or disconnect one of the following: a. Inverters or charge controllers connected to the fault circuit when the fault is detected b. System components within the arcing circuit.

(3) The system shall require that the disabled or disconnected equipment be manually restarted.

(4) The system shall have an annunciator that provides a visual indication that the circuit interrupter has operated. This indication shall not reset automatically."
 
     
     
     
 
 

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