2011 NEC 690.11 AFCI for PV on DC

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beanland

Senior Member
Location
Vancouver, WA
I work on the design of utility scale PV systems. I was browsing my 2011 NEC and noticed 690.11 that requires arc fault circuit interrupters for DC circuits in PV systems. I had never heard of anything like this existing. In a Google search, I see that Eaton admits that no such product exists but they are diligently developing something. Here is a case of "the cart before the horse."

In PV systems, the correct way to "interrupt" an arc fault is to short the output of the PV panel; this eliminates the source of energy and quenches the arc. PV panels are current limited, the short circuit current is about 5% more than the operating current. The NEC requires wire sized for 125% of the short circut current so wiring can be continuously shorted with no ill effect.

So, the 2011 NEC requires interrupting the circuit, which generally means opening it, but what you want to do is short it.

Has anyone seen any product development announcements for DC AFCI for PV systems?
 

don_resqcapt19

Moderator
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Location
Illinois
Occupation
retired electrician
There is not even a product standard that manufacturers can build to at this time. This code requirement is intended to trigger development of both the standard and the device itself.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
There is not even a product standard that manufacturers can build to at this time. This code requirement is intended to trigger development of both the standard and the device itself.

My gut reaction is that such an intention is beyond the scope of the code. I also am not comfortable with any code requirement that simply can't be met and I have a bigger problem with code makers that make such codes.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Where is the verbiage stating that a requirement is conditional upon existing technology?
90.4 ...This Code may require new products, constructions, or materials that may not yet be available at the time the Code is adopted. In such event, the authority having jurisdiction may permit the use of the products, constructions, or materials
that comply with the most recent previous edition of this Code adopted by the jurisdiction.
 

Klampo

New member
AFCI working like GFCI

AFCI working like GFCI

I think this is something that will be handled in the inverters much like GFCI is today.
 

PaulWDent

Member
DC Arc fault breakers

DC Arc fault breakers

Not only did the NEC get the cart before the horse, but have you tried drawing a block diagram to see where in the heck you would place an arc fault breaker?
For example, take SMA interactive inverters with the PV combiner built into the inverter, so that the wiring for the strings comes all the way from the array to the inverter. Are we supposed to have an arc fault breaker in EACH string wire? That would be ridiculous! And, does an arc fault breaker protect the upstream, circuit, the downstream circuit, or both? If you read 210.12 (B) exception 1 it seems to believe that an arc fault breaker protects only the downstream circuit. In which case - would the arc fault breakers have to be on the roof in a PV system???

The requirment for arc fault breakers in AC house wiring, now extended by the manufacturers' lobbyists against opposition by many electricians to substantially every room in the house, has a getout: The interpretation of 210.12(B) exception no. 1 is that, if the circuit is entirely in metallic conduit to all outlets, you do not need an arc fault breaker. Since 690 already requires some of the DC wiring to be in metal conduit, we should have the same exception spelled out in 690, or have that clause stricken in its entirety, as there are a whole lot of things that have not yet been thought out properly about arc fault breakers, like what happens when your neighbor has an arc-welder.
 
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