Megger Readings

20946 Views 35 Replies 12 Participants Last post by MDShunk, Oct 28, 2014

faber307

Discussion Starter · May 5, 2007

4

Hey guys,

I was called to look into some bad underground at our city park.
They have a 200amp panel in a pavillion that feeds underground to a bath house and another smaller pavillion.
The city man said when they turned on the breakers they heard a loud buzzing or hammering sound, meter was spinning erratic then it all stopped. No power to bath house or pavillion 2.

He said he measured voltage at the breakers and had 240 and 120 to ground, which I verified.

Then he said they were digging right after this happened and the ground was pretty wet, one side of the hole was noticeably warmer than the other.

Why they were digging right in the spot of the fault I don't know. This is just what he told me.

So I started digging and found the feeds. all were severed in half.

So I returned today with buttsplices and heat shrink tubing. spliced all then megger tested.

NONE of the underground lines read over 3Megs, they were reading between 1 and 3.

I remember from reading John's posts that he looks for 100Megs. I set my megger alarm for readings less than 5M.

So what's up? immenent failure? I turned them on anyway and measured voltage and amped. All Ok and functional when I left.

I'd really like to get a handle on megger readings besides the obvious dead short.

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1 - 20 of 36 Posts

MDShunk

· #2 · May 5, 2007

I would call any reading less than 20 megohms no good. With 1 to 3 megs, I think it's fair to say that the insulation is compromised in one or several other places. Not so uncommon with direct-bury, in my experience. They'll get to revisit this problem down the road. Maybe not this week or next, but perhaps in a few years.

Just curious, did you megger them at 500 volts?

faber307

Discussion Starter · #3 · May 5, 2007

PS,
I shrunk these photos easily with: Easy thumbnails

faber307

Discussion Starter · #4 · May 5, 2007

Yes MD
500volts, then 250 just messin' around

brian john

· #5 · May 5, 2007

240/120 VAC, may hold a while at those readings but that cable is on a downward spiral.

Depending on the age of the cables it may be a localized problem, but you have to find where local is.

But with the both conductors reading low (did you check the neutral)
May be time to budget replacement.

MDShunk

· #6 · May 5, 2007

Tab Faber said:
Yes MD
500volts, then 250 just messin' around

Curious to know how your 250V readings compared to the 500V readings? Half? Some logarithm of the 500 volt readings?

wireman3736

· #7 · May 5, 2007

Off topic a little but I was Just curious, I've been saying I'm going to buy a megger but haven't done so yet, What should I look for and what kinda price range are we in.

Thanks

MDShunk

· #8 · May 5, 2007 (Edited)

wireman3736 said:
Off topic a little but I was Just curious, I've been saying I'm going to buy a megger but haven't done so yet, What should I look for and what kinda price range are we in. Thanks

Depends on what you want to do with it. Just for day-to-day troubleshooting, I posted about the Supco M-500, and Brian John posted about the Extech 403360. They're both around 150 bucks. You can spend many, many hundreds (even thousands) if you want to. It sounds like Tab's megger is a lot more full-featured, so he probably dropped quite a few shekels on it.

wireman3736

· #9 · May 5, 2007

I'll check out the two you mentioned, I'm looking for something basic that won't cost me an arm and leg, I'm not looking to get into anything too complex, I just want to be able to check the integrity of underground cables and maybe trouble shooting some motor problems, nothing over 480 volts.

faber307

Discussion Starter · #10 · May 6, 2007

brian john said:
240/120 VAC, may hold a while at those readings but that cable is on a downward spiral.

Depending on the age of the cables it may be a localized problem, but you have to find where local is.

But with the both conductors reading low (did you check the neutral)
May be time to budget replacement.

Didn't check neutrals, the remote pavillion panel neutral lug was incredibly tight. I didn't want to make another problem on saturday.
The neutrals are bonded at the sub panels.

I had all conductors free at the source. but of course the neutrals read 0.

remote panels are main lug and all breakers were in off position.

I guess the local problem could be anywhere along the 300' stretch of Underground.

brian john

· #11 · May 6, 2007

A desent basic megger that offers a 250 VDC-1000 VDC analog or digital, I prefer a digital with an analog scale.

faber307

Discussion Starter · #12 · May 6, 2007 (Edited)

MDShunk said:
Curious to know how your 250V readings compared to the 500V readings? Half? Some logarithm of the 500 volt readings?

I'll be honest, I didn't document the readings.
But there was negligible difference honestly.
And that was really the only reason I checked them at 250volts.
Due to the prior megger posts.
Looking for consistant difference between the different voltages applied.

Next run in I'm going to jot all info down.

brian john

· #13 · May 6, 2007

One advantage of this situation (buried conductors) is restoring powering with these low megger readings is relatively safe.

We normally put the customer on notice of the dangers involved, the chance of a failure sometime in the future, and have them accept liability.

Quite a few years ago in an apartment complex with direct burial cables at 480 VAC (each building had a step down transformer) these feeders had been ran and backfilled with junk. We had a call for loss of power, meggered the feeder found the fault repaired and re-meggered to find very low readings (less that 5 megohms, I do remember the exact values). We told the customer the situation and they accepted liability for re-energizing of the feeder, with a promise of a PO for a replacement in the mail. About 5-6 months later on Christmas Eve we got the call the whole complex was down. Found the fault 27 feet down, all the conductors for each building were in one ditch. The main electric room was on a hill side and the conductors were run prior to back filling.

This was a multi day repair, management had to put tenants up in a motel, we had to find a back hoe operator, and technicians wiling to work OT in the week between holidays

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Pierre Belarge

· #14 · May 7, 2007

With the readings you obtained, and the wetness of the earth, would you not be afraid that maybe a child crawling in that park may get shocked or worse from current in the earth...especially as those conductor age even more?

BTW:it looks like you did a nice job on the repairs!

zippie2223

· #15 · May 7, 2007

If the cable were severed and the ground was wet, water in the cables could give you a low meg reading. I've seen a lot of flood damage lately. If you put low amps on the cable over night sometime it will inprove your meg reading. If not replace the cable...

MDShunk

· #16 · May 7, 2007

zippie2223 said:
If the cable were severed and the ground was wet, water in the cables could give you a low meg reading. I've seen a lot of flood damage lately. If you put low amps on the cable over night sometime it will inprove your meg reading. If not replace the cable...

Putting low amps on the cable will improve the megohm reading ONLY because it's drying out the soil, and decreasing the soil's conductivity. Wet or dry, if the insulation is compromised, the megohm reading will be low. The low reading has absolutely nothing whatsoever to do with water; it's due to compromised conductor insulation. You can submerse and saturate good cable, as well as the conductor inside, and you'll get good megohm readings. It's all about the insulation, baby.

faber307

Discussion Starter · #17 · May 8, 2007

Pierre Belarge said:
With the readings you obtained, and the wetness of the earth, would you not be afraid that maybe a child crawling in that park may get shocked or worse from current in the earth...especially as those conductor age even more?

BTW:it looks like you did a nice job on the repairs!

You'd think if the voltage were actually leaking to earth, the voltage readings would be off, or there would be some load on the energized conductors.

None of that was observed.:blink:

Pierre Belarge

· #18 · May 8, 2007

Tab
I believe that you could have leakage current to earth without being able to read a load on your meter. It would not take much leakage current to pose a problem.

brian john

· #19 · May 9, 2007

Anytime you meg a conductor to ground, and you get a reading less than infinity (actually max reading for the meter) the reading reflects leakage. There is no infinity there is just not enough resolution to read the leakage.

Your meter may have a full scale reading of 2000 megohms, you get a full scale reading of infinity, someone else meggers has a full scale reading of 10,000 megohms, he reads 2,100 megohms. Some of our meggers have readings in the Tera range.

mega-million-megohm
giga-billion-gigohm
tera-trillion-teraohm

As for drying out conductors this is usually a short term fix, if there is enough water to result in low readings the conductors are on the road to scrap dollars.

Drying does work for busway and transformers in some circumstances, typically when you megger and the meter fluctuates this is an indicator of moisture, the fluctuations are due to short term drying.

We have dried large dry type transformers, but it was a special circumstance, and it took several days. Involved dehumidifiers, heat and low voltage watching current and regular megging to verify improvements.

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faber307

Discussion Starter · #20 · May 14, 2007

Pierre Belarge said:
Tab
I believe that you could have leakage current to earth without being able to read a load on your meter. It would not take much leakage current to pose a problem.

Sorry I didn't reply sooner, Spring tends to take me away from this gadget.
What problems could arise? and don't you think there are many situations just like this in every city?

Just curious as to how you would handle it.

Thanks Pierre

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