600 VDC switches [RE-wrenches]

[matthew tritt]

William,

Your questions bring up some very interesting points. I did an install
yesterday using a Sq D 3 pole breaker which has no jumper and no mention
anywhere of a decreased rating. I'm going to check with the distributor and
see if I can get an answer on the need to add a 3rd breaker to the system,
or if these switches are actually rated @ 600 volts per contact.
Also, I regularly used to switch both DC inputs until I was admonished by an
electrician who insisted it was illegal to break the negative lead. I
believe that he said this because of his assumption that the neutral in AC
is grounded and it would be somehow interrupting the ground to break the
neutral on the DC circuit as well. Why not switch both?

Matt
----- Original Message -----
From: "William Miller" <wrmiller at slonet.org>
To: <RE-wrenches at topica.com>; "John Berdner" <jberdner at sma-america.com>
Sent: Tuesday, September 17, 2002 9:42 PM
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
>
>
>
> __________________________________________________________________
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