# Megger Readings



## faber307 (Jan 22, 2007)

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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## MDShunk (Jan 7, 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?


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## faber307 (Jan 22, 2007)

PS,
I shrunk these photos easily with: Easy thumbnails


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## faber307 (Jan 22, 2007)

Yes MD
500volts, then 250 just messin' around


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## brian john (Mar 11, 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.


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## MDShunk (Jan 7, 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?


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## wireman3736 (Mar 3, 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


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## MDShunk (Jan 7, 2007)

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.


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## wireman3736 (Mar 3, 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.


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## faber307 (Jan 22, 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.
> 
> ...


 
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.


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## brian john (Mar 11, 2007)

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


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## faber307 (Jan 22, 2007)

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.


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## brian john (Mar 11, 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 (Feb 3, 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!


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## zippie2223 (May 3, 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...


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## MDShunk (Jan 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.


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## faber307 (Jan 22, 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:


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## Pierre Belarge (Feb 3, 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.


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## brian john (Mar 11, 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 (Jan 22, 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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## brian john (Mar 11, 2007)

Pierre:

There may be a good possibility that what you state could happen. Moist soil, low resistivity soil........


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## Pierre Belarge (Feb 3, 2007)

Tab
Sorry myself for taking so long to see this post again.

Yes, there probably are many situations like this in congested city/urban areas. With park areas, kids tend to layin/thrash about in/ play on the ground. With the leakage current, who knows what may happen...I cannot be sure. I would venture to say that the chances for shock can be fairly substantial as the time goes on while this type of situation exists.

I am not sure what to say as far as fixing this. There is a way to go before we can convince some as to the real need to fix it.


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## Dave Borowski (Oct 23, 2014)

Looking for advice --- frequently HVAC techs will use a mega meter to validate winding integrity in a hermetic compressor. The new refrigerants are hydroscopic by nature - I'm wondering has this resulted in false positives of failing windings?


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## Bad Electrician (May 20, 2014)

Dave Borowski said:


> Looking for advice --- frequently HVAC techs will use a mega meter to validate winding integrity in a hermetic compressor. The new refrigerants are hydroscopic by nature - I'm wondering has this resulted in false positives of failing windings?


This is an old thread a new post MIGHT be a better approach.

But funny you should mention this I was looking into this yesterday and did not find any thing on the web YET.


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## bobelectric (Feb 24, 2007)

Oldie but Goodie


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## Wirenuting (Sep 12, 2010)

Dave Borowski said:


> Looking for advice --- frequently HVAC techs will use a mega meter to validate winding integrity in a hermetic compressor. The new refrigerants are hydroscopic by nature - I'm wondering has this resulted in false positives of failing windings?


We haven't had false readings with the newer refrigerants, only bad compressors. 

But we had more problems years ago when the newer blends hit the market. Deep freezers for medical use was the first use that we saw. Those compressors failed quicker.


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## Big John (May 23, 2010)

Dave Borowski said:


> Looking for advice --- frequently HVAC techs will use a mega meter to validate winding integrity in a hermetic compressor. The new refrigerants are hydroscopic by nature - I'm wondering has this resulted in false positives of failing windings?


 That's an interesting argument. 

I deal with a lot of old hygroscopic insulation, and if it sucks up moisture, it's going to pass more current, and if it reaches a critical level of conductivity it that can cascade into destructive failure.

In the case of the compressors the suggestion is that a megger would read through the moisture contamination in the refrigerant and this could lead to inaccurate diagnosis. 

I don't deal with refrigerant much, but here's my thinking:
A) By design I would expect a properly installed refrigeration system to be reasonably moisture free. So, regardless of whether the refrigerant _can_ suck up moisture, it doesn't seem to me that moisture would be the most likely cause of poor insulation resistance.
B) If there is enough moisture in a compressor to actually cause reduced insulation resistance readings, these aren't "false positives." It means that you're reading current flow where normally there should be high-resistance insulation, thus your electrical insulation is contaminated. If the current flow is high enough and your megs are low enough, it is because your electrical insulation is compromised.

Unless someone has a white-paper explaining otherwise, I would say that low insulation resistance values in any compressor are a solid indicator of poor insulation health.


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## Dave Borowski (Oct 23, 2014)

I understand your line of thought. You're correct about a properly charged / installed refrigeration system, but it seems the old mineral oils (non conductive) in some fashion added to the winding values or at least didn't contribute to failed windings. The new R410A compressors all use a hydroscopic oil & actually are inducing moisture concentration at the compressor.

Here's the exact event that had me raise the inquiry. I had a tech condemn a comp via a mega meter. On testing the refrigerant, it had a high moisture content confirmed by the litmus liquid indicator sight glass. 

By replacing both driers w/ new high capacity driers - the sight glass cleared & a subsequent retest w/ a mega meter gave the compressor a clean bill of health. 

So I'm a bit dubious of the combo of mega meter & POE oils.


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## Wirenuting (Sep 12, 2010)

Dave Borowski said:


> I understand your line of thought. You're correct about a properly charged / installed refrigeration system, but it seems the old mineral oils (non conductive) in some fashion added to the winding values or at least didn't contribute to failed windings. The new R410A compressors all use a hydroscopic oil & actually are inducing moisture concentration at the compressor.
> 
> Here's the exact event that had me raise the inquiry. I had a tech condemn a comp via a mega meter. On testing the refrigerant, it had a high moisture content confirmed by the litmus liquid indicator sight glass.
> 
> ...



It sounds like a bad install started the problem. It wasn't evaporated properly the first time. 
But another thing to think about is that a minor leak, will allow just one part of the blend to Excape. This changes the blends composition and isn't something that can be field verified. 
Because we reclaim so many types of units here, we just reclaim a charge and after repairs, charge with new or cleaned refridgent. 
The mixed blends we send out for disposal. 
This seems to extend the life of the units.


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## Dave Borowski (Oct 23, 2014)

Thanks - I'm aware the new refrigerants are a blend, thus liquid charging. And I'd concur if moisture is present a poor install / service / evacuation is at the root of the problem itself.

The problem I'm trying to evaluate as a go forward practice is the dependability of condemning a compressor via a mega meter reading when (in this case) a proper moisture removal brought the compressor back to specs. It would seem, IF we have a failed mega meter reading a mandated moisture content test must be done (on POE oils) prior to condemning a compressor.


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## triden (Jun 13, 2012)

Dave Borowski said:


> I understand your line of thought. You're correct about a properly charged / installed refrigeration system, but it seems the old mineral oils (non conductive) in some fashion added to the winding values or at least didn't contribute to failed windings. The new R410A compressors all use a hydroscopic oil & actually are inducing moisture concentration at the compressor.
> 
> Here's the exact event that had me raise the inquiry. I had a tech condemn a comp via a mega meter. On testing the refrigerant, it had a high moisture content confirmed by the litmus liquid indicator sight glass.
> 
> ...


Just because it doesn't read high resistance now doesn't mean your insulation hasn't been compromised. You need conductive ions to fail a test, and since you recharged the system and removed the water, you removed anything conductive. One would think that even with a concentration of water in the compressor that it should still pass a megger test. I would still condemn that compressor myself as IMO it's only a matter of time before the rest of the insulation fails.

How often do compressors pass/fail tests when using the hydroscopic oils?


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## Big John (May 23, 2010)

Dave Borowski said:


> ...Here's the exact event that had me raise the inquiry. I had a tech condemn a comp via a mega meter. On testing the refrigerant, it had a high moisture content confirmed by the litmus liquid indicator sight glass.
> 
> By replacing both driers w/ new high capacity driers - the sight glass cleared & a subsequent retest w/ a mega meter gave the compressor a clean bill of health....


 Alright, I gotcha: It's the right response for the wrong reasons.

My position was that it's correct to condemn the system with a low insulation resistance value, but I agree you still have to repair the problem. And in your case the "false positive" is not that you don't have poor insulation resistance, but rather _what is causing_ the insulation resistance to be poor? 

We have the same issue with oil-filled transformers. Yes, you can get low insulation resistance values, now is it because of oil contamination or is it because of a physical insulation failure? For us it's just a matter of doing a couple more tests to determine the root cause. 

I think in your case it should be standard that once you get poor meg values, you need to follow up with a refrigerant test to determine moisture content.


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## Dave Borowski (Oct 23, 2014)

*How often do compressors pass/fail tests when using the hydroscopic oils?*

I don't have any clinical data (yet - or maybe never)

The largest part of my concern is misrepresenting the life expectancy of the compressor where and "expert" (by definition: a man from out of town w/ his own tools) comes in after our diagnostic to save the day with lower cost repairs.

Based on the input I've picked up here - the salient element is the presence of moisture in the oil is not a driver of the condition of the winding insulation but rather in a fashion escalated the ID of winding condition. 

At this juncture it seems the smart money is to:

Make the Mega Meter reading as a standard of care
IF a fail & IF R410A w/ the new POE oils - test the refrigerant for moisture.
IF no moisture the compressor is definitely on it's way out
IF moisture is present - educate the consumer; recovery of the refrigerant and or new driers + a moisture indicating sight glass CAN extend the life of the compressor ... but the damage is done.
Make sense?


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## Big John (May 23, 2010)

Dave Borowski said:


> ...At this juncture it seems the smart money is to:
> 
> Make the Mega Meter reading as a standard of care
> IF a fail & IF R410A w/ the new POE oils - test the refrigerant for moisture.
> ...


 It does. The only thing I'd say is #4 may not be accurate. Removing contamination from insulation (in this case moisture) may reverse a downward trend. It seems very possible to me that if the winding insulation had not deteriorated to the point of tracking or flashover, that you could salvage the compressor by recharging and putting in new filter/dryers. 

Seen it many times with motors and transformers that once they are cleaned and dried, the insulation resistance values go right back up into the many-thousands-of-megaohms. The trick is simply catching this before you get permanent physical deterioration of the winding insulation.


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## Dave Borowski (Oct 23, 2014)

Perfect -- I'm going to seek out the manufacturer of the mega meter to have them weigh in as well.

Thanks for responding - Dave B.


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## MDShunk (Jan 7, 2007)

Dave Borowski said:


> Perfect -- I'm going to seek out the manufacturer of the mega meter to have them weigh in as well.
> 
> Thanks for responding - Dave B.


I'm having difficulty wrapping my head around a seemingly technical question from a person who uses technical terms improperly, like hydroscopic instead of hygroscopic and mega meter instead of megohm meter. Nevertheless, I think the audience knows what he really means. 

Certainly, the presence of moisture entrapped in the interstitial spaces of any motor winding will lower the measured IR values. The presence of moisture in hygroscopic hermetic compressor oils in a FUNCTIONAL system are not enough to sufficiently lower IR measurements in an otherwise healthy hermetic compressor motor winding. When water is mixed with the oil to the extent that it artificially lowers the IR values ("false positive") the system also no longer functions. Frozen cap tubes, frozen TXV's, fixed orifices, etc will be the symptom long before you will measure an artificially low IR value from entrapped water. If the system functions on the refrigerant circuit side, the water vapor levels are not enough to affect the IR values. If you get a "bad" IR measurement in a system that still cools, the windings are, in fact, failing.


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