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Peter:<br><br>
I am resending, the first version inexplicably deleted some
text:<br><br>
An arc is created in the process of disconnecting any current under
load. DC circuits have an enhanced ability to sustain that
arc. PV circuits exacerbate this problem as a short or load is
disconnected because they can sustain a short but the voltage rises
drastically as a circuit is disconnected.<br><br>
This all justifies the added expense of a DC rated disconnect. We
use the Square D HU361 without hesitation. There are few
alternatives available and none with any significant cost savings.
In selecting a disconnect, a design-build contractor must use the highest
Voc possible. Make sure you are entering the 690.7 table with the record
low temperature for the location.<br><br>
If we used a disconnect without the correct rating we would open
ourselves to unacceptable liability exposure and, worse yet, would not
experience that pleasant feeling one gets when you know you've done the
job right.<br><br>
William Miller<br><br>
<br>
At 09:11 AM 11/21/2008, you wrote:<br>
<blockquote type=cite class=cite cite><font face="arial"> <br>
<br>
</font><br>
<font face="arial" size=2 color="#000080">Peter T. Parrish, Ph.D.,
President<br>
California Solar Engineering, Inc.<br>
820 Cynthia Ave., Los Angeles, CA 90065<br>
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885<br>
CA Lic. 854779, NABCEP Cert. 031806-26<br>
peter.parrish@calsolareng.com</font><font face="arial" size=2 color="#0000FF">
<br>
<hr>
</font><font face="tahoma" size=2><b>From:</b> Peter Parrish
[<a href="mailto:peter.parrish@calsolareng.com" eudora="autourl">mailto:peter.parrish@calsolareng.com</a>]
<br>
<b>Sent:</b> Friday, November 21, 2008 9:10 AM<br>
<b>To:</b> 'RE-wrenches'<br>
<b>Cc:</b> 'Peter Parrish'<br>
<b>Subject:</b> DC Rating of Disconnects<br>
</font><font face="arial"> <br>
I am looking into using the PV Powered PVP2000 inverter for some small,
low voltage designs. The string Voc will be around 200 Vdc, worst
case.<br>
<br>
Since the PVP2000 doesn’t come with a DC disconnect, I am wondered what
other wrenches who install the PVP2000 have been doing about this. I hate
to spend $150 on a 600 V AC/DC disco such as the SD HU361RB to provide
the means of disconnection. I believe that the 240 V AC discos such as
the SD DU221RB have no DC rating.<br>
<br>
From an academic point of view, I wonder why the smaller discos are not
dual rated, since we all know that the real-time voltage difference in a
240 V AC system can be 41% greater than 240 V.<br>
<br>
In any event, any Code-compliant solutions would be greatly
appreciated.<br>
<br>
- Peter<br>
</font><br>
<font face="arial" size=2 color="#000080">Peter T. Parrish,
President</font></blockquote></body>
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