Peter:
I am resending, the first version inexplicably deleted some text:
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.
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.
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.
William Miller
At 09:11 AM 11/21/2008, you wrote:
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
CA Lic. 854779, NABCEP Cert. 031806-26
[EMAIL PROTECTED]
----------
From: Peter Parrish [mailto:[EMAIL PROTECTED]
Sent: Friday, November 21, 2008 9:10 AM
To: 'RE-wrenches'
Cc: 'Peter Parrish'
Subject: DC Rating of Disconnects
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.
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.
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.
In any event, any Code-compliant solutions would be greatly appreciated.
- Peter
Peter T. Parrish, President
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