Does my megger work?

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mark32

Senior Member
Location
Currently in NJ
For those who frequent this forum you'll recall I just did a megger test on some heat trace cables. All the cables passed but I wanted to play with this megger some more as I've owned it for three years and this is the first time I'm using it. So, I grabbed some twisted 18/2 bell wire and tested it at 1000v's, it passed with over 1g's. I then stripped an inch of each conductor and put them as close to each other as I could without them touching. I still got over 1g @ 1000v's. Is this a valid result? How would I actually test this megger to see if it actually works?
 

fmtjfw

Senior Member
For those who frequent this forum you'll recall I just did a megger test on some heat trace cables. All the cables passed but I wanted to play with this megger some more as I've owned it for three years and this is the first time I'm using it. So, I grabbed some twisted 18/2 bell wire and tested it at 1000v's, it passed with over 1g's. I then stripped an inch of each conductor and put them as close to each other as I could without them touching. I still got over 1g @ 1000v's. Is this a valid result? How would I actually test this megger to see if it actually works?

Do you have a voltmeter capable of reading the megger's output voltage?
If so, test it that way.

If not, think about using some megohm resisters in series as a voltage divider across a portion of which you connect you meter (at the grounded end) and across the whole string connect the megger. Also check your meter's impedance and take that into account.

AND WEAR YOUR RUBBER GLOVES and OTHER PROTECTIVE GEAR [that you always wear when you are using your megger, of course]
 

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
two ways--

put a megohm resistor between the leads-crank it-It should read I megohm--

or my favorite:ashamed:

have your apprentice grab the two leads and then crank away--:jawdrop::rant:If he "runs about and screams and shouts" it works:happysad:

Seriously though--DO NOT do the apprentice thing--as stated above use the proper PPE when doing any work!
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
For those who frequent this forum you'll recall I just did a megger test on some heat trace cables. All the cables passed but I wanted to play with this megger some more as I've owned it for three years and this is the first time I'm using it. So, I grabbed some twisted 18/2 bell wire and tested it at 1000v's, it passed with over 1g's. I then stripped an inch of each conductor and put them as close to each other as I could without them touching. I still got over 1g @ 1000v's. Is this a valid result? How would I actually test this megger to see if it actually works?
An open circuit is still open regardless of insulation providing the separation or air space.

Calibration Is good but it matters zilch if you dont know what you are looking at. Grab a paper and pencil. Draw a line on the paper. Connect a lead to each end of the line. Use a lower voltage at first. Make the line heavier. Erase part of it. Cut the paper in half. Increase the applied voltage. Play some more with it. Check every motor you work on.
 

controlled

Senior Member
An open circuit is still open regardless of insulation providing the separation or air space.

Calibration Is good but it matters zilch if you dont know what you are looking at. Grab a paper and pencil. Draw a line on the paper. Connect a lead to each end of the line. Use a lower voltage at first. Make the line heavier. Erase part of it. Cut the paper in half. Increase the applied voltage. Play some more with it. Check every motor you work on.

Very helpful
 

Fulthrotl

~Autocorrect is My Worst Enema.~
two ways--

put a megohm resistor between the leads-crank it-It should read I megohm--

or my favorite:ashamed:

have your apprentice grab the two leads and then crank away--:jawdrop::rant:If he "runs about and screams and shouts" it works:happysad:

Seriously though--DO NOT do the apprentice thing--as stated above use the proper PPE when doing any work!

but, but... but... fluke says the "light the muppet" test is valid, but ONLY at 5 kv.......
 

Fulthrotl

~Autocorrect is My Worst Enema.~
For those who frequent this forum you'll recall I just did a megger test on some heat trace cables. All the cables passed but I wanted to play with this megger some more as I've owned it for three years and this is the first time I'm using it. So, I grabbed some twisted 18/2 bell wire and tested it at 1000v's, it passed with over 1g's. I then stripped an inch of each conductor and put them as close to each other as I could without them touching. I still got over 1g @ 1000v's. Is this a valid result? How would I actually test this megger to see if it actually works?

find yourself a set of feeders or something a couple hundred feet long, and test them at 1KV, and there
will be enough dielectric that it'll take a while to charge up the insulation, and you can watch the voltage
and resistance climb up....
 

mark32

Senior Member
Location
Currently in NJ
I megged a very old circular saw today along with an extension cord and a work light and they all pegged my megger at >1000m so, as ken9876 mentioned, I filled a bucket with water and I stuck the t-stat cable into it. Finally the meter's indicator went the other way, but it went ALL the way to 0m. I removed the cable from the bucket and then, for the first time, I got a reading other than max or min, it read like .14m at first but it varied greatly as I held the test button down. So I'm guessing it works, I may get my hands on a resistor like others said just to see what reading I get. Thanks again fellows.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
... So, I grabbed some twisted 18/2 bell wire and tested it at 1000v's, it passed with over 1g's. I then stripped an inch of each conductor and put them as close to each other as I could without them touching. I still got over 1g @ 1000v's. Is this a valid result? How would I actually test this megger to see if it actually works?
That is one of the things that many users of insulation resistance testers fail to understand. The voltage is not high enough to jump a gap. You can pull conductors with chunks of insulation into a metal conduit and the conductor will pass the test unless there is something like wire pulling lube or other contaminate bridging the gap between the actual conductor and the metal conduit.

One online source shows the arc distance in air for a 1,000 volts DC to be between 0.005 and 0.006". That is a very small gap. This source also used a sharp point discharge which decreases the gap as compared to the rounded surfaces of the wire placed next to each other.
 

mark32

Senior Member
Location
Currently in NJ
That is one of the things that many users of insulation resistance testers fail to understand. The voltage is not high enough to jump a gap. You can pull conductors with chunks of insulation into a metal conduit and the conductor will pass the test unless there is something like wire pulling lube or other contaminate bridging the gap between the actual conductor and the metal conduit.

One online source shows the arc distance in air for a 1,000 volts DC to be between 0.005 and 0.006". That is a very small gap. This source also used a sharp point discharge which decreases the gap as compared to the rounded surfaces of the wire placed next to each other.

You bring up a good point Don. Most of these "Megger" threads have to do with how to meg something, but what isn't discussed is what exactly is taking place when you hit the test button. By me stripping the insulation off the bell wire, I thought the meter would pick up that uninsulated spot by way of "Leakage" but it did not. (I never thought it would actually arc) I use the term leakage but I don't know what I'm talking about. A megger is also called an insulation resistance tester, hence why I figured it would pick up that missing insulation, but like you said, and now that I see, if conductors are damaged during a pull, they could remain like that for a very long time as long as the damaged areas are at least .006in apart. Unless of course there is some contaminate present. So, how does a megger actually take measurements, in other words, how does it sense damaged areas?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
... So, how does a megger actually take measurements, in other words, how does it sense damaged areas?
I guess my point is that it really doesn't do that directly. It injects voltage and measures the current flow. If there is no current flow the insulation resistance is infinite. Where there is current flow it simply uses ohms law to show the resistance.

Don't get me wrong, it is a great troubleshooting tool, but it may not detect things like missing insulation. As long as there is no current path that the injected voltage can flow on the tool will tell you that there is no problem where there very well be one. There was a thread here about conductors that were damaged when they were installed and the contractor used a "megger" test to show that there were no problems. The customer repeated the test with water in the raceway and the conductors failed. When the conductors were removed, there were chunks of missing insulation that were caused by poor installation practices.
 

zog

Senior Member
Location
Charlotte, NC
I guess my point is that it really doesn't do that directly. It injects voltage and measures the current flow. If there is no current flow the insulation resistance is infinite. Where there is current flow it simply uses ohms law to show the resistance.

.
I wrote an article for a magazine many years ago called "You can't measure resistance" that was the premise of the whole article.
 

mark32

Senior Member
Location
Currently in NJ
That statement is so far off kilter that I will not even attempt to correct it! Current is current, except when counting displacement current.

Hi Gold, if there was direct contact one could just do a continuity test to find such a fault. So how does the one lead sense this leakage current from the other if there is no physical contact, that's what I want to know.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
For a very long line there will be capacitive current as the insulation dielectric polarizes, effectively charging a capacitor. This current falls toward zero over time and has no effect once the meter reading has stabilized.
As far as a stable Megger reading goes, it represents real current flowing from wire to wire or wire to ground through or around damaged insulation.
If all you have is an air gap and no surface contaminants or conductive liquids, the Megger will not be able to detect it.
Applying a voltage high enough to arc across an air gap would run the risk of damaging good insulation too.
All a Megger is is a sensitive ohmmeter that applies a much higher voltage than a regular meter.
 
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