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<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>Hi Drake,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>Disclaimer #1: i do not recommend megger-testing an array (or
module or string) unless you have an approved procedure from the
module manufacturer for the installed configuration. </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>Disclaimer #2: i am not the foremost authority on the list for
this topic... Mr. Brooks? (Note: Bill's post just came in, but i want to share
this anyway)</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>i have, however, conducted these tests, as part of the
commissioning requirements, on about a dozen systems which were installed under
R&D grants. Back in the day, pretty much all systems bigger than 4 or 5
kilowatts were required to be megger tested, if they were "on the radar" in any
way. This was a function of the fact that public dollars were being spent in the
HOPE that PV could some day become a mainstream technology. And the fact that PV
modules were so expensive. The combination of these factors meant that, in any
meaningful sense, only projects that received subsidies were built, and grant
requirements were in play. One of the conditions was that these tests be
performed. This is a logical requirement in many ways. </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>Each system, that i've done this on, was different from the
others. </FONT></SPAN><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>In each case, the procedure and analysis methodology was different.
Sufficiently different, that i learned early on to NOT MAKE UP MY OWN PROCEDURE.
Manufacturer procedures varied in lead placement, array sections to be tested,
</FONT></SPAN><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>surface wetting techniques, test voltages, temperature compensation,
and Pass/Fail values. i've made up lots of procedures over the years, but i'm
not touching this one. I don't understand it well enough.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>The basic theme and theory was the same, pretty much across
the board. But the actual methods... The steps... They were different. The very
limited information i was able to extract from manufacturers' engineers varied.
Some warned about reverse-biasing diodes and others didn't think it was a
concern. Some insisted that test-lead arrangement was super-critical and others
didn't. Some wanted to test at voltages lower than the system voltage and others
said 1kV was fine on a 500V circuit. Some wanted to connect one pole to ground.
Most didn't.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>Applying test voltages to, and measuring the
resistance of, an installed, illuminated source circuit or array, is very
different than applying test voltages to, and measuring resistance of, a
single, unilluminated and shorted module. Myriad additonal factors and
considerations must be accounted for. </FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN><SPAN class=734281618-17112010><FONT
color=#0000ff size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>My observation is that at least half of the
manufacturers' engineers were only guessing, and crossing their
fingers that everything would be fine after performing megger testing on a
source circuit or array. Another observation is that at least half of these
engineers had never contemplated megger-testing an array as a completed
system... Only as a single module within the hi-pot test regime for
manufacturing and listing purposes. As a group, they all were resistant to
provide a documented procedure and acceptable test values. None, as in Z-E-R-O,
were able to predict actual test results with any reasonable degree
of accuracy. As in, off by megohms in many cases. </FONT></SPAN><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial>Even though the
subjects under test were designed, and largely manufactured, by their companies.
Even though these guys were responsible for the actual system design, in most
cases. </FONT></SPAN><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>Even though my experience leads me to believe these guys are
at least half full of crap, i do believe this test is valid and has merit. i
believe that all arrays should be tested for their dielectric resistance during
the commissioning process, in fact.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial>If we are going to megger our field-installed conductors...
And we absolutely should be... It's a simple step. </FONT></SPAN><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial>In order to do
this, as an industry, we just need a better understanding of the characteristics
and test procedures. </FONT></SPAN><SPAN class=734281618-17112010><FONT
color=#0000ff size=2 face=Arial>My requests that manufacturers publish a
procedure for each module they make, with acceptable test results in common
circuit configurations, have resulted in squat. Going forward, i hope somebody
forces them to do it, 'cause they ain't likely to bother
otherwise.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010>Under controlled conditions,
such as those found during module manufacturing, predicting a test result within
a reasonable degree of accuracy has a much higher chance for success
than a prediction under uncontrolled conditions in a multi-subject
combination. I get that. Nevertheless, we are deploying these products into an
installed configuration. There is shipping to the distributor. Shipping to
the installer. Shipping to the jobsite. Unpacking and handling. Getting it to
the roof. Bolting it down. Wiring it up. All before it actually gets to start
its productive life.</SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#000000
size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010>It is reasonable to be able to
expect and require that these tests be performed during the commissioning
process. After all, when would you, as an installer carrying the warranty,
rather find out if something has a higher chance for failure during its
lifetime? At the time of installation when you can still get your hands around
the neck of somebody or five years later when the sales-slug you know works
somewhere else?</SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN></FONT></SPAN></FONT></SPAN> </DIV></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>The fundamental purpose of megger testing an array is to measure the
dielectric resistance between conductive circuit elements (positive and
negative) and conductive non-circuit elements (module frames & racking). The
higher the resistance (megohm) value, the greater the dielectric
resistance. Busbars and cables and switches and receptacles... They all
need dielectric resistance in order to work. The entire theory of useable
electricity depends on dielectric resistance. </FONT></SPAN></FONT></SPAN><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial>Without
dielectric resistance, you have a short
circuit.</FONT></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Each model of module is a little different in its construction. As a
consequence, the absolute values for each are different.
</FONT></SPAN></FONT></SPAN><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>This may or may not mean there is a different minimum acceptable
value for a given class of module. Resistance is resistance, after
all.</FONT></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010>One key function of any
test is to compare the results of the test to expected results... More
accurately, to compare the test results to an acceptable threshold value. In our
case, absent some other authority, that value has to come from the manufacturer.
Furthermore, this threshold value, which i'll refer to as the minimum acceptable
resistance, changes downward as the surface area of the array being tested
increases. This phonomena is consistent with wires, too.... The larger and
longer the conductor, the lower the theoretical minimum acceptable insulation
resistance is.</SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010></SPAN></FONT></SPAN><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010></SPAN></FONT></SPAN><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial>Test lead
placement is a fundamental piece of the puzzle. But it's only part of the
puzzle. </FONT></SPAN></FONT></SPAN><SPAN class=734281618-17112010><FONT
color=#0000ff size=2 face=Arial>You need to know several things that are
<U>specific to your application</U>... For example, the procedure for testing a
single module may differ greatly from testing a string of 10 in series. Testing
two paralell strings of 10 may have a different procedure... In any case, the
expected values for each setup will be different. Based on my experience, the
key items you need to know, from the manufacturer and for your specific
application, are:</FONT></SPAN></DIV>
<OL dir=ltr>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Any pre-megger tests to be performed</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Test lead placement (May be different for pos to ground than neg to
ground)</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Circuit conductor configuration</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Test voltage</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Time on test</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Acceptable irradiance during test</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Number of times to repeat the test</FONT></SPAN></DIV>
<LI><SPAN class=734281618-17112010><FONT color=#0000ff size=2 face=Arial>Wet
or Dry</FONT></SPAN>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>If Wet, what solution to use</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Temperature compensation</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Irradiance compensation</FONT></SPAN></DIV>
<LI>
<DIV align=left><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial>Minimum acceptable resistance for each test (May be different
for pos to ground than neg to
ground)</FONT></SPAN></FONT></SPAN></DIV></LI></OL>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010>If you are testing at the
inverter, you will need to account for the resistance of the insulation
of your wiring. You will also need to be sure that each module frame has a
good connection to the grounding conductor you are using as a
reference.</SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010>In my experience, i have seen
differences in which test lead connects to which circuit conductor. Some had
different Pass/Fail values for positive and negative and some did not.
</SPAN></FONT></SPAN><SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial><SPAN class=734281618-17112010>Some included temperature compensation
and others didn't. Some were done wet and others done dry. Test times varied.
Some said a single test was fine and others said three passing tests were
required before the system passed. </SPAN></FONT></SPAN><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010>And they all had different minimum acceptable
resistance values. Most systems passed the tests. Some didn't on the first try
and required troubleshooting and module replacements. One never did pass and had
to be removed. One thing i have NEVER SEEN is two identical test results on the
same circuit.</SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010>As you push for a procedure,
expect them to request that you sign an NDA before they give it to you. You can
decide how you choose to respond to that. For what it's worth, i'm somewhat
confident that they will work with you on this. In large part, this
is because it's Sanyo. They are a grown-up company who really believes in
quality products and customer relationships. Can't say that about many
others. <SPAN class=734281618-17112010><FONT color=#0000ff size=2
face=Arial><SPAN class=734281618-17112010>If you run into resistance, get them
on the phone, cough, then say "Solar
Ark".</SPAN></FONT></SPAN></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010>i certainly wish you the best of experiences with this
and hope you find the problem(s) one way or the
other.</SPAN></FONT></SPAN></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010>Pray for Public Array Megger
Procedures!</SPAN></FONT></SPAN></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010></SPAN></FONT></SPAN></SPAN></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial><SPAN class=734281618-17112010><SPAN
class=734281618-17112010><FONT color=#0000ff size=2 face=Arial><SPAN
class=734281618-17112010>Solar
Janitor</SPAN></FONT></SPAN></SPAN></FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=734281618-17112010><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <B>On Behalf Of
</B>Drake<BR><B>Sent:</B> Wednesday, November 17, 2010 9:24 AM<BR><B>To:</B>
RE-wrenches<BR><B>Subject:</B> Re: [RE-wrenches] Megger for array
testing<BR></FONT><BR></DIV>
<DIV></DIV>Thinking more about the Megger test of an array, is seems impossible,
despite that fact I've been told it could be done. If I put the tester
positive to the 500 V DC negative, that would put two 500 V sources in series
for 1000 VDC. If I put the positive to positive, the resulting voltage to
ground would be zero. What is the trick?<BR><BR><BR><BR>At 10:36 AM
11/17/2010, you wrote:<BR>
<BLOCKQUOTE class=cite cite="" type="cite">Hello Wrenches,<BR><BR>Looking
through the archives on using a Megger for testing modules, it appears
that it is best to keep voltages to 500VDC. I read all that I could
find. <BR><BR>Talking to a Sanyo representative, I was told that keeping test
voltage under 600 would not void the warranty or be an issue with UL. He
said they have tested Sanyo modules up to 1000 VDC with no problems.<BR><BR>As
a final test on a system, I want to Megger test the array from the
terminations at the inverter. The approach I'd plan to use is:
<UL>
<LI>Clamp the Fluke 1587 negative to the enclosuer ground.
<LI>Put the positive test lead to the negative conductor from the array and
test
<LI>Put the positive test lead to the positive conductor from the array and
test </LI></UL>All tests would be done at 500 V. The modules are
illuminated, but there is no significant reading from either positive or
negative terminal to ground.
<UL>
<LI>The conductors would be removed from their terminals at the inverter and
be in free air.
<LI>The conductors would be continuous through the string of 10 Sanyo 210
modules. </LI></UL>My questions are:
<UL>
<LI>Does anyone see a problem in this approach?
<LI>Since the array often has 500 VDC open circuit readings and shows no
indication of a fault whatever, would this test accomplish anything more
than the standard voltage to ground tests I've already done?
<LI>Is there any danger to diodes or other PV components? </LI></UL>Thanks in
advance.<BR><BR>Drake <BR></BLOCKQUOTE></BODY></HTML>