I'd like to suggest that Appendix J of John Wiles Suggested Practices
Photovoltaic Power Systems and the NEC be read. It provides good
instruction on how to calculate when an OCPD is needed. It is a function
of the module's Isc rating, the inverter's backfeed current capabilty,
and the module's maximum series sting fuse rating. The rule of thumb is
fusing is usually required above two paralleled sub array circuits, but
with some modules, a greater paralleled number of subarrays can be
installed without fusing. It does make troubleshooting more difficult
and as said, it is inexpensive to spec subarray fusing.
Chris
On 2/21/2011 3:51 PM, Phil Schneider wrote:
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 <http://www.CreativeEnergies.biz>
On Mon, Feb 21, 2011 at 12:52 PM, <[email protected]
<mailto:[email protected]>> 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 <http://www.Foxfire-Energy.com>
NABCEP #092907-44
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