600 VDC switches [RE-wrenches]

Wed Sep 18 13:12:57 PDT 2002

William:

I agree completely that it is safer to switch both.
Unfortunately the NEC does not agree and 690 specifically prohibits you
from having a switch in the grounded conductor.

My suggestion - remove the GFDI fuse when ever you open the dc switch
and before you work on / install the array.  This meets Code and allows
you to float the negative.  With the fuse removed the Sunny Boy will
pull the array to a center grounded configuration via a high
impedance(10 Meg Ohm) voltage divider.  Don't let this lull you into a
false sense of security though.  High voltage PV systems are like guns -
Either one can kill you and there ain't no such thing as an unloaded
one!

If you have any questions, or if I can be of any further assistance,
please do not hesitate to contact me.

Best Regards,

John Berdner

SMA America, Inc.
20830 Red Dog Road
Grass Valley, CA  95945
530.273.4595 (voice)
530.274.7271 (fax) 

-----Original Message-----
From: William Miller [mailto:wrmiller at slonet.org] 
Sent: Tuesday, September 17, 2002 9:43 PM
To: RE-wrenches at topica.com; John Berdner
Subject: 600 VDC switches [RE-wrenches]

Colleagues:

Those of you who have installed the Sunny Boy inverters have used the
600
VDC rated switches, such as the Siemens HNF261.  It is interesting how
the
manufacturers achieve the 600 volt rating:  They use a 3 pole disconnect
and put two of the poles in series.   This way, the contacts break two
times at the same moment.

A few months ago I was unable to obtain a 2 pole 600 VDC disconnect in
short order, so at the same price I was able to purchase a three pole
unit.
 Lo and behold, the series jumper was missing and the cover showed a
rating
of only 250VDC.  I since learned that because only three pole devices
are
commonly manufactured, if you need to use all three poles separately,
you
can not occupy two of them in series, therefore you can not achieve the
600
volt rating.

Should we switch the positive lead?  I submit that there may be a safety
advantage to switching the negative.  The Sunny Boy takes the negative
to
ground.  If the negative is left connected, the array still has
potential
to ground.  If the negative is switched, the array is floating,
therefore
no (or less) shock hazard.  We know this because my helper got zapped
today.  We are still discussing the ramifications so as to avoid this in
the future.

Here's another idea:  Why not switch both leads?  Siemens has a circuit
breaker that does the same trick, and the Siemens catalog contains a
drawing that shows one side of DC going through the two series poles and
the other side going through the one pole.  Is this how I should be
wiring
the DC disconnects for Sunny Boys?

(BTW, the drawing on the cut sheet for the SB1800 shows only the
positive
switched through one pole.)

Thanks for considering these nit picky, nerdy questions.  I believe it
is
in considering these details that safety is achieved.

William Miller

__________________________________________________________________
William Miller
Miller Power and Communications
PO Box 50, Santa Margarita, CA 93453
Voice :805-438-5600		Fax: 805-438-4607	VMail:
805-546-4875
email: wrmiller at slonet.org
http://millerpowerandcomm.com
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