[RE-wrenches] To Megger or not to Megger

Tue Apr 28 12:48:59 PDT 2009

Hi Bill,

I so totally agree with your comment about the price of these being a
non-issue, all things considered. As long as we're getting the features we
need in the sub-$750 range, that is. When it comes down to it, this is
meaningful safety and reliability stuff. No slam to the Bolinas crew, but
the SunEye is a lettle less important than that. Unless, of course, you
shade a certain type of module and the bypass diode fails and the thing
ignites.... Then you could make a case for the SunEye being prudent for
Disqualification of location, which would mean there wouldn't be a need for
the megger on that non-job.

Perhaps you could address a couple of aspects that I have questioned for a
long time.

First of all, is it or is it not true that megger testing an array at 1kV
may result in flaky-bias diodes (my term) if polarity of test leads is
incorrect relative the array and/or the array wiring is grounded/not
grounded during the test? (The root of my policy to NOT megger arrays unless
I have specific procedure provided by the mfr... Me no wanna get in a "you
meggered it so you don't have a warranty" conversation if ya know of what I
speak.)

Which of the mainstream module manufacturers are publishing a procedure for
megger testing their arrays for use by installers?

Are these being published for wet or dry testing? Both?

Which of these manufacturers are supplying this documentation in a publicly
accessible manner? (Asked quite a few sales reps over the years and only 1
ever produced a document... I have been able to get some from engineers in
the factory, but this was generally when I had a big stick to pound on
somebody's desk.) 

Why don't most installers and distributors know this documentation exists or
understand the procedure at this point?

Yeah, it's one of those thorny issues with me. Maybe we can get our heads
together to steamroll a solution to the above. Happy to take the subject up
off-list if you like.

On to today's question regarding resolution.... 

When you are measuring an array, why is the resolution so critical?
What value or value range do you believe constitutes a Pass on a quality Csi
product?
How about Asi / CdTe with crappy glass?
How about Asi with Tefcel?
Wet? Dry?
Does your answer change relative the capacity of the array portion under
test?
Does the answer change with temperature &/or relative humidity?
Does the answer change relative test voltage compared to array VOC?

What I'm getting at is, unless the minimum "Pass" value is a very low number
for the test setup, say 200K - 2M, or has a very narrow "Pass" range, a
super-fine resolution shouldn't be that critical. On the few occasions back
in the day when I actually meggered arrays, I used an analog megger. Top end
range @ 1kV was 250M. My recollection is that these things pulled 15M or
better, but I can't really testify to that... Fuzzy memory on the actual
values. I'd have to dig through some deep stacks of paperwork to find out if
I have any archives of the tests, and I'm not gonna do that for this post.
Had to submit the test results to SMUD (can't find them) and the mfrs.
Nobody ever said, "Hey. Close your other eye and tell me what it says." When
I asked what values would be appropriate, all I ever got was, "Just like you
got here. These are good." Never could nail them down and, frankly, never
got the idea that they were all-to-happy having installers owning meggers.

I'm with you on testing the array and module interconnects for leakage.
Really. But, if the arrays rings out as "faulted" when it's really "OK", why
bother? What are you looking for? If the resistance is that low, maybe we
should be using a "Kohmer" for the arrays and a Megger for the wiring &
equpment bussing. If you're getting such a low resistance, aren't you
picking it up with a "Static" test... Would the Earth Bond Resistance
function of the 1507 & 1503 be sufficient for these situations?
http://us.fluke.com/usen/products/specifications.htm?cs_id=35391(FlukeProduc
ts)
<http://us.fluke.com/usen/products/specifications.htm?cs_id=35391(FlukeProdu
cts)&category=HMA(FlukeProducts> &category=HMA(FlukeProducts)  This is
basically a voltage shot like a megger, but measures resistance in the <20k
range. Resolution very nice there! This tester has a gap between 20k & 100k
(0.1M). 

>From a "reasonable resolution" perspective, I think the gap is more like 20k
to 2M. Anything over 2M, the 0.1M resolution should be fine. If the array is
that close to failing, fail it and tell the mfr to start making some decent
glass. They might understand it better this way:  您吸

Dunno. I think it comes down to figuring out what ranges equal "Pass" for
the application and going from there. Will look forward to what you find in
your side-by-sides later this week. Please do post back. With a basic
description of the glass-type, mounting, wire-management, test results and
findings, if you can.

I'm gonna hang onto my 1520... If anybody wants to see what one looks like
with a 100% Battery Level, let me know... I'll hold it up in front of my
webcam for ya!

No Ground Faults!

Matt Lafferty

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill Brooks
Sent: Tuesday, April 28, 2009 10:40 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] To Megger or not to Megger

Dave,

The issue is definitely resolution. The cool thing about the Fluke 1520 is
that it goes down to 0.001MOhms (1000 Ohms). That is the resolution you are
looking for. Neither of the current Flukes go low enough. That is why I’m
going into the field later this week to see how a 0.1MOhm meter stacks up to
finding faults―I have an array with a pesky fault that should make it fun.

Many arrays will ring out as a faulted array when, in fact, everything is
fine. This is especially true of a-Si arrays with low quality glass. They
are very leaky. I’ll keep looking and report back on a recommendation for
Christmas shopping.

Just to add punctuation to this thread, I always recommend that contractors
megger their arrays, because it has saved my butt several times. Also, with
exterior wiring systems it is even more important. The problem in the early
days is that people would whine about the $600-$1000 price tag. In my
opinion, that argument is gone. With contractors routinely putting down
$1400 for a SunEye, the value of a good megger is similar and costs less.

Bill.

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of David
Palumbo
Sent: Tuesday, April 28, 2009 7:59 AM
To: gilligan06 at gmail.com; 'RE-wrenches'
Subject: Re: [RE-wrenches] To Megger or not to Megger

Matt,

“Muggering”, I think that will catch on. The last post, I think, ol’
Uncle Bill Brooks (4/13/09 8:17 PM) had on this topic raised a concern about
“not having enough resolution in the low impedance area. PV arrays can have
an impedance to ground of a slow as 2kOhms. A resolution of 0.1MOhms will
likely not cut it.” Bill went on to say that he had bought a cheaper meter
to test out “for fun”.  Some of us do enjoy “muggering around”.

So my question is.  Do  the Fluke 1503’s and 1507’s have enough resolution
in the low range? 

Dave

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Matt
Lafferty
Sent: Tuesday, April 28, 2009 9:56 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] To Megger or not to Megger

RE: Suggested replacements for Fluke 1520 and "pricey meters"... Good News
below!

Another Wrench sent me a note off-list last night, asking how much a
"muggering" would cost. Here's what I sent him:

When I got that 1520 it was just over $600. I just checked online and find
that it's a discontinued item... Bummer! 

Fluke recommends the 1587 or the 1507 or the 1503. They also mention the
1577, but it's an ugly stepsister, or maybe a retarded adopted relative, to
the 1587. 

The 1587 is basically a multimeter that also has a <TEST> button to
discharge a high voltage shock into the sample under test... It runs about
$620 from standard distributors.
<http://us.fluke.com/usen/products/Fluke+1587+1577.htm?catalog_name=FlukeUni
tedStates>
http://us.fluke.com/usen/products/Fluke+1587+1577.htm?catalog_name=FlukeUnit
edStates  You can get it for $522 here: 

 <http://www.testequipmentdepot.com/fluke/megohmmeters/1577_87.htm>
http://www.testequipmentdepot.com/fluke/megohmmeters/1577_87.htm  NOTE: It
does not test at 10A in DMM mode which means it wouldn't be able to test
short-circuit current in strings. Don't bother with the 1577! 

The 1507 & 1503 are more robust equipment. You can check them out here:
<http://us.fluke.com/usen/products/features.htm?cs_id=35391(FlukeProducts)&c
ategory=HMA(FlukeProducts>
http://us.fluke.com/usen/products/features.htm?cs_id=35391(FlukeProducts)&ca
tegory=HMA(FlukeProducts)  While they do measure AC & DC Voltages to 600V as
well as some lower ohm & continuity work, they are really more tailored to
being a megger. The 1503 will suffice for most of what I expect you will get
into, but there are some features on the 1507 that might make it worth
considering. Mind you, I haven't looked at at price on either yet, so that
last comment may just be noises coming out my ass. 

The 1507 has a Compare function which basically sets up a Pass/Fail value so
you can quickly run through repeated tests. This would be useful for doing
larger systems where you are testing dozens or hundreds of circuits at a
time. For my purposes, I want my guys to think a little more than "Buzz =
OK" and I want them to write an actual tested value down on a piece of paper
so this is not a big plus to me.

The 1507 also does Polarization Index and Absorption Ratios. These are more
advanced di-electric tests that you are not likely to need in smaller scale
PV. Use of these features would come into play when playing with real
high-voltage gear or transformers. Might also come into play on
super-sensitive equipment. They might also be used to assess older or aging
underground or overhead feeders. 

The 1507 & 1503 both have an Earth Bond Resistance function which is likely
to become more of an issue in the future. This feature lets you test the
resistance between a grounding electrode or a grounding electrode conductor
and actual earth. Another use for this feature would be to set the output to
1kV, connect the alligator jaw to one of  <Insert name of your
least-favorite building official here> extremities and use the probe lead as
a tongue depressor while you push <TEST>. This feature used to be in a
single-purpose tester that ran >$2K. They also both have backlit displays
which comes in real handy out in the field.

The primary features you want are: 500V & 1kV Test Voltage and 2 Gohms
(2,000 Megaohms) or above for the top end of the range. Any of the 3 meet
this spec. 

Well, I just checked prices for the 1503 & 1507 and I'm pleasantly
surprised. The 1503 ranges from about $291-$370. Here's the place I found it
for $291   <http://www.testequipmentdepot.com/fluke/megohmmeters/1503.htm>
http://www.testequipmentdepot.com/fluke/megohmmeters/1503.htm That's marked
down from the regular price of $342.  

The 1507 ranges from about $385 to $500. Here's where I found it for $385...
<http://www.testequipmentdepot.com/fluke/megohmmeters/1507.htm>
http://www.testequipmentdepot.com/fluke/megohmmeters/1507.htm  That's a
special price, for you only, down from $454. 

All things considered if I were in your shoes, I'd go for the 1503. That
give you a solid megger that you can dedicate to that purpose for a
reasonale muggering. If it were me, in Matt's shoes... Oh, what am I saying?
I've already got mine and I just found out I paid too much for it! But I
have Battery Check... Neener neener neener!

Be safe out there!

Matt Lafferty

I hope more hands go up on this topic. Thanks for asking, Keith.

Peace and Palm Trees everybody,

Matt Lafferty

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