[RE-wrenches] 1.56 ISC Minimum OCP is STUPID! (Was: ground fault troubleshooting)

[William Miller]

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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