Kent:
Your comments below are right on the mark and well said.
William Miller
At 09:43 PM 9/7/2010, you wrote:
Matt,
There is a word on the KLKD time delay chart you referenced that is hiding
a very important fact. Those are "average" curves. To illustrate what
that means take a look at the time delay curve for the
<http://www.cbi-electric.com/show_pictures.php?title=Product%20Curves&path=/curves_products/6>CBI
breaker commonly used for battery charging systems. There is a gray area
that extends from 105% to 130% of rating. Over that range, the breaker
may or may not trip. While not shown on the KLKD fuse time delay chart
that you referenced, a very similar gray zone exists. These fuses and
breakers are simply not precise enough devices to distinguish between Imp
and Isc at STC in the best of conditions.
if you did manage to select a fuse that would blow at Isc but not at Imp,
it still wouldn't blow during a short if the irradiance was 800 W/sq
m. An arcing fault at 800 W/sq m can still burn a big hole. It simply
isn't possible to provide good fault protection for PV systems with
fuses. What's needed is a ground fault protector/arc fault protector that
is located in the combiner box. Until that exists, the feeder between the
combiner and inverter needs to be considered an unfused conductor.
Kent Osterberg
Blue Mountain Solar
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