[RE-wrenches] DC Rating of Disconnects

William Miller wrmiller at charter.net
Fri Nov 21 09:37:05 PST 2008


Peter:

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

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
>peter.parrish at calsolareng.com
>
>----------
>From: Peter Parrish [mailto:peter.parrish at calsolareng.com]
>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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