<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#ffffff" text="#000000">
On 7/1/2011 10:04 AM, Bill Brooks wrote:
<blockquote cite="mid:01c601cc3811$04d7e900$0e87bb00$@com"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
<meta name="Generator" content="Microsoft Word 12 (filtered
medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman","serif";}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-reply;
font-family:"Calibri","sans-serif";
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
<div class="WordSection1">
<p class="MsoNormal"><span style="font-size: 11pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">Nick,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 11pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 11pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">If your voltage is low and there is no ground
fault, either positive and negative are connected mid
string, bringing the voltage down, or a bunch of modules
never got connected in series. In either case, the Voc
difference between the two strings will create a significant
current flow at open circuit. That is because the two
strings are no longer at open circuit, they are connected
together so they have a circuit. You will see significant
current flow in those two strings even though they are not
connect to anything else. This is a function of the IV
curve. The higher voltage string is generating power in the
first quadrant (the one everyone sees on spec sheets), and
the lower voltage string is operating in the 4<sup>th</sup>
quadrant—the quadrant nobody wants to see. This is the
region where you take the IV curve from where it crosses the
voltage access at Voc and proceed into the negative current,
positive voltage region. Therefore the power generated by
the higher voltage string is being absorbed in the lower
voltage string. An IV curve tracer would do a nice job of
showing both of these curves and then the composite curve is
going to look very strange.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 11pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 11pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">Bill.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 11pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);"><o:p> </o:p></span></p>
</div>
</blockquote>
<br>
That's what I was going to say, Bill... <br>
<br>
Only having two strings means that the series fuse rating will not
be exceeded<br>
and those fuses won't blow. So that won't hurt any modules if
that's all it is.<br>
<br>
Maybe there is some junction box that is shorting out more than one
module ? Maybe the<br>
modules that are shorted (if that's what it is), are in one row ?<br>
<br>
boB<br>
<br>
<br>
<br>
<br>
<br>
<br>
<blockquote cite="mid:01c601cc3811$04d7e900$0e87bb00$@com"
type="cite">
<div class="WordSection1">
<div>
<div style="border-right: medium none; border-width: 1pt
medium medium; border-style: solid none none; border-color:
rgb(181, 196, 223) -moz-use-text-color -moz-use-text-color;
padding: 3pt 0in 0in;">
<p class="MsoNormal"><b><span style="font-size: 10pt;
font-family:
"Tahoma","sans-serif";">From:</span></b><span
style="font-size: 10pt; font-family:
"Tahoma","sans-serif";">
<a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On
Behalf Of </b>Nick Vida<br>
<b>Sent:</b> Friday, July 01, 2011 7:03 AM<br>
<b>To:</b> wrenches<br>
<b>Subject:</b> [RE-wrenches] how would you measure a
partial short circuit<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal" style="margin-bottom: 12pt;"><span
style="font-family: "Courier New";">Hi Bob,<br>
<br>
thanks for thinking about my question!<br>
<br>
I landed the fuses and put the fuses back in (not it-
typo) on my sma disconnect, and there was an arc across
the fuse holder. Seems like it could have been closing a
short circuit on the bus once the parallel connection was
made.<br>
<br>
--------<br>
<br>
Hi Bill,<br>
<br>
Although I am not entirely sure there is no ground fault,<br>
I took the conductors off the terminals so they were in
free air, and measured pos to neg, pos to ground, neg to
ground on both strings, and all I saw was the taper
towards 0 from about 20 vdc, and no steady voltage
anywhere but pos to negative. That makes me think the
voltage is in a short circuit somewhere.<br>
I cant yet make any conclusions about the wiring mistake
because I had no access to the pull box where the
different parts of the strings were connected to each
other and to the home run. I do think there might be a
short circuit between strings because when the fuse went
back in there was an arc on the fuse holder as if there
was a short circuit instead of a simple parallel
connection. I guess it might have had to do with the 2
different voltages, but there is voltage 'missing'
somewhere.<br>
<br>
Thanks for thinking about my question.<br>
<br>
------------<br>
<br>
Bob,<br>
<br>
re: inverter capacitors charging, I havent seen that much
of an arc with SMA disconnects, and the switch was in the
off position, so I dont think there was any signal path to
any real electronics. I always pull my fuses and do not
replace them until everything is landed and hot and the
voltages look proper when I install, and this arc was way
out of range of normal.<br>
<br>
re: backfeeding, I just suspected that too while thinking
about your responses, and yes the 2 voltages were
different by about 200 volts.<br>
<br>
Thanks for the thoughts Bob.<br>
<br>
</span><br>
</p>
</div>
</div>
</blockquote>
</body>
</html>