[RE-wrenches] DC Rating of Disconnects

[ASAP POWER! 2]

In 2008, we have had to replace our standard Sq. D HU361RB used since our
beginnings, with the Sq. D H361RB (U stands for unfused).   Was there some
sort of code change NEC or CBC maybe that made most jurisdictions in So Cal
suddenly start requiring fused DC disconnects?  Typically, we fill the fused
Sq. D. box with 15A 5" fuses.
 
Peter D.

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of William
Miller
Sent: Friday, November 21, 2008 9:43 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] DC Rating of Disconnects


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:



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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 doesnt 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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