[RE-wrenches] NEC 690.9 and Two String Arrays

[Phil Schneider]

Dan,

My understanding has been that unless the inverter spec lists a "0 A
backfeed current" then it is conceivable that fault current from the grid
(up to the inverter OCPD) could feed a fault on the DC side, so DC fusing
would be necessary (Fronius IG Plus spec sheet, for example, states this).
SMA specs state the maximum backfeed current to be equal to the maximum OCPD
for a given inverter size.  However, I've asked SMA tech support about this
directly, and they assured me that it isn't possible for current from the AC
side to reach a fault on the DC side.  This is not very clear to me, but the
cost of fusing two strings is generally quite low, so when I doubt I spec
fusing.  Curious to hear if others think this is too conservative.

P.

Phil Schneider
Creative Energies
p 307.332.3410
c 307.438.1195
f 888.456.6625
www.CreativeEnergies.biz



On Mon, Feb 21, 2011 at 12:52 PM, <dan at foxfire-energy.com> wrote:

> Hey Everybody.. I got the impression that 690.9 ex. allows up to two
> parallel strings to be connected (thru a disconnect) to a grid interactive
> inverter (which does provide Ground Fault Protection).. Do I still need to
> protect the Array from possible inverter back feed? This particular array
> consists of two pole mounted arrays out in the boonies in the middle of a
> field. The Inverter is mounted on one of the poles...  Let the egg toss
> begin... db
>
>
> Dan Brown
> Foxfire Energy Corp.
> Renewable Energy Systems
> (802)-483-2564
> www.Foxfire-Energy.com
> NABCEP #092907-44
>
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