[RE-wrenches] To Megger or not to Megger

Tue Apr 28 03:24:25 PDT 2009

Just went to check the Fluke 1520 price. Looks like it is discontinued.
They do have a couple suggested replacements.

Just for reference,
Bob

On Mon, Apr 27, 2009 at 11:08 PM, Matt Lafferty <gilligan06 at gmail.com>wrote:

>  Hi Keith,
>
> Both hands up here. From day 1 in PV.
>
> Every single field-installed current carrying conductor to the point of
> termination. AC & DC. BEFORE energizing them! One megger lead on the
> conductor, the other on the raceway or ground-wire. Megger @ 1kV. Must be
> greater than 250 Megohms to pass. Record the values on your commissioning
> sheet.
>
> I only meggered the actual arrays on projects where it was a requirement
> and I only did that following specific procedures provided by the module
> manufacturer. No procedure from the mfr = No array megger.
>
> I believe it's a critical thing to do as part of the commissioning. But
> then again, I don't like ground faults or fires.
>
> I'll never forget a couple of head scratchers that the megger found.... One
> was a perfectly fine, 6' long piece of Orange #12 XHHW-2 that would only
> pull 40 Megohms installed in EMT. Pulled it out and replaced it. Inspected
> it for any visible flaws. None found. Not a scratch or scuff. Used it as a
> training tool for my guys. Hung it on the wall over my desk as a handy
> reminder. If anybody wants a picture...
>
> The other was interesting, too. The very first PV system I ever installed. Was
> shooting the DC circuits from the inverter all the way to the connector on
> the output jumper cables at the modules. Megger showed dead short on one
> wire. Fluke 79 on ohms showed clear. Hit it with the megger again. Dead
> short. I'm known for being anal about conduit and wire installation, so
> this was quite unexpected, not to mention a little embarrassing. This
> happened to be a Solarex Millennia Integra system with an Omnion inverter
> and the array happened to be scattered all over this roof. For those of you
> who who have dealt with that situation, you'll understand what a
> cluster-*$#! that is... Especially being my very first PV system and
> all... I wasn't anywhere near confident that I had a clue by that point. All
> ready to see my first install fire up and this happens.
>
> It was my first install and I had 3 people getting paid prevailing wages to
> learn... I was learning too, but I wasn't making anywhere near prevailing
> wages, let me tell ya. Well, it was early January, late in the day and
> foggy as foggy gets, but we went back onto the roof anyway. Yeah... Overtime
> for everybody but me. We opened up the condulets and J-boxes for
> inspection. Pulled all the wires up out of the J-boxes. Pulled the endcaps
> and ground clips off *(unique components of the Integra product)*.
> Couldn't find anything suspicious. Shot it with the megger again, and it was
> all clear. But no sign of anything that could have been shorted. We put
> everything back together and shot it again. Dead short! Aiyeeeee!
>
> Call it a day. Go home. I couldn't get back to that site until about the
> same time the next day. Foggy again. Only took 1 helper with me this time.
> We completely repeated the entire exercise, with exactly the same results.
> By now, I was pretty sure this solar stuff wasn't my calling. I've always
> been regarded as a gifted trouble-shooter and all I could say at this point
> was that I didn't know _____! All signs pointed to it being associated with
> one of the J-boxes or the endcaps or the ground clips, but there was NOT a
> thing we could see. Oh, well. I would have to come back another day. When
> I went back, I took the same helper and made sure I had all day if necessary
> to fix the problem. No matter what it turned out to be. We planned to
> take it apart 1 piece at a time and re-shoot it with the megger after each
> step. I left my helper on the ground with the megger while I took it apart
> on the roof, 1 screw at a time. Again.
>
> Well, lo and behold, he hollers up that "It's clear" when I pulled the
> cover off one of the 2-gang bell boxes. Hadn't moved a wire. Just took the
> cover off. Let me tell ya, I stuck my nose and four eyes in that box real
> close, but didn't touch any wires. The box was a tight fit with those big,
> sealant-filled blue wire nuts that came with the Solarex Kit, but I still
> couldn't see anything wierd.
>
> "Shoot it again."
>
> "All clear, boss."
>
> I put the cover back on and installed all the screws. "Shoot it again."
>
> "Dead short." We had a talk later about shouting things like "Dead short"
> on the jobsite, but I won't get into all that right now.
>
> I took out one of the cover screws. "Shoot it again." Same answer from the
> ground.
>
> I took out a second screw. "Shoot it again." This time the reply was
> different. I took out the third screw and we repeated the process. Still
> clear. And the fourth. Still clear. WTF? I lifted the cover for what seemed
> like the hundredth time. I studied the nested wires, all neatly and
> systematically put into what I had thought would be their final resting
> place... Days before and time and time again at this point.
>
> I began gently lifting the wires out so they all stood up like dandelions
> in a spring lawn. I inspected them for the nth time. Looking for a stray
> strand outside its protective hat. Nope. Overtightened wire nut with the
> spring or a conductor coming out the tip? Nope. But what's this little,
> nearly imperceptible dent in the side of this wire nut? As I looked closer,
> I could see that it was round, flat bottomed and about the same diameter as
> a 6-32 screw. Hmmmm. I rolled the wires back into their resting position.
> Uh, huh. This particular wire nut's natural home was directly under the
> cover screw which had cleared the fault when removed and the dent lined up
> perfectly. The screw could just barely be long enough to pierce the plastic
> and contact the wire-spring.
>
> I replaced the wire nut, repositioned the conductors to make sure we
> wouldn't have a repeat, and buttoned up the J-box. I put the wirenut in my
> pocket for later reference. One more megger shot proved the system to be all
> clear. We could now energize the DC and do our Voc & Isc testing prior to
> startup. Thank goodness the sun was shining. I wasn't happy about this whole
> ordeal, but it was a good thing we caught it with the megger before we put
> the power to it.
>
> That wire nut became the first exhibit in what became my collection of
> training materials for the "what can go wrong and why we do these
> procedures" trainings. Verification that cover and mounting screws cannot
> come in contact with energized equipment was officially put in the
> checklist. *All *cover & mounting screws in *all* types of enclosures.
> There would be no exceptions. There would be no excuses. I actually had to
> write one guy up for a violation of this at one point. Good thing we caught
> that one with the megger before putting the power to it.
>
> I would posit that at least one of the recent PV fires would *absolutely
> have been prevented* if the installer followed the procedure described
> above. Too bad most don't. Kudos to those of you who do. It's only a
> matter of time until more and more of these AVOIDABLE problems surface.
>
> If you don't have a megger, get one. If you have one, use it! The Fluke
> 1520 is a nice unit. I recommend it over analog models. I cut my teeth with
> analog gear, but really like my 1520. A lot! Rugged instrument that gives
> you an actual number to write on your commissioning sheet. In addition to
> Megohms, it also does VAC, Lo Ohms, Continuity, has a display backlight,
> Lock and Zero functions and my favorite.... Battery Check!  Reads out an
> actual % value for your battery condition. Nice! Uses 4 "C" cells.
>
> I hope more hands go up on this topic. Thanks for asking, Keith.
>
> Peace and Palm Trees everybody,
>
> Matt Lafferty
>
>  ------------------------------
> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Keith Cronin
> *Sent:* Monday, April 27, 2009 2:37 PM
> *To:* RE-Wrenches
> *Subject:* [RE-wrenches] To Megger or not to Megger
>
>   Hi
>
> I was wondering, by a show of hands, how many of you megger every project?
>
> Do you have a cut off- like if it is "x" sized system, you will or decide
> to opt out of performing this task?
>
>
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