Megger

[brian john]

I put this together this afternoon and have some pics to add, I really was not sure were to post. This is a first draft and I welcome corrections, additions questions.

Questions often arise regarding the use of Meggers I have posted some information, that hopefully is useful. Some of the information noted below is based upon personal experience; all users should familiarize themselves with the correct test criteria and test equipment prior to performing any work. Testing is NEVER completed on energized equipment. If you are not sure about your results HIRE a company that has experience in the test you are performing or obtain help in determining the results prior to possibly damaging yourself or the distribution system you are working on. Lastly NEVER rely on a continuity tester or ohm meter to check any distribution system for shorts/faults. This is a good way to damage the distribution equipment, hurt yourself or worse and of course you could end up on the wrong end of a lawsuit.
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From Wikipedia:
A megger (or sometimes meggar) is often used as an alternate term for an insulation tester - a circuit tester which puts a very high voltage at a very low current across two conductors to make sure that they are properly insulated. The word is short for megohm-meter.
It is in fact the registered trade mark of Megger Group Ltd who have manufactured insulation testers since 1889.
Older types of insulation testers have a small built-in generator turned by the handle. Modern types work with batteries and circuitry to generate the voltage required.
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Megohm = 1,000,000 = one million ohms
Gigohm = 1,000,000,000 = one billion ohms
Terohm = 1,000,000,000,000 = one trillion ohms
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Today meggers typically come is 3 basic styles.
1. Analog and can be line voltage, battery, hand crank or a combination of two
2. Digital and can be line voltage or battery operated or both.
3. Digital with an “analog” scale these are also line voltage or battery operated or both.
Typical DC voltages available are 50. 100, 250, 500, 1,000, 2,500, 5,000 and 10,000 or some combination of these.
A decent megger for the ‘average electrician” has 250, 500 and 1,000 DC voltage settings, though I really like a megger that has a 50 or 100 VDC test voltage (in addition to 250, 500 and 1,000 VDC settings) as it allows me to pre-test a circuit with a certain sense of I am not going to fry something sensitive. I also prefer a digital analog style meter.
There are additional megger types for special test, but I have not included these as they are outside the scope of what the average electrician should have in his “tool case”
See attached photos
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Meggers can be utilized to test all distribution equipment with a few considerations.
1. Length of conductors will affect the readings and you need to take this into account that there is a time to charge the conductors (typical times are one minute).
2. Long conductor runs will “hold a charge” always ground the conductors after testing.
3. Readings that fluctuate often indicate moisture and/or water. Leaving the megger connected can in some cases “dry out the problem” Though if moisture or water is an issue obviously this warrants further investigation.
4. You have to be careful that all loads are disconnected, meters, relays; control transformers can skew readings and/or show as a fault. Nothing worse than telling a customer they have a dead short when in fact it is a tenant sub meter.
5. Meggers can be utilized on
Transformers - Primary to secondary, primary to ground and secondary to ground (assuming any grounded conductor has the connection to ground isolated).
Motors - line to ground (with transformers and motors phase to phase connections should read as a dead short) There are additional test used on motors and transformers this megger test mentioned here are basic go no go field test.
Circuit breakers disconnects, motor starters - with the load disconnected, pole to pole line, pole to pole load, line to load (CB open).
Feeders to include busway - Phase to phase, phase to neutral phase and phase and neutral to ground.
Switchboards, panel boards, MCC’s ECT - Phase to phase, phase to neutral phase and phase and neutral to ground.
Branch circuits – Phase to Phase, phase to ground and the often overlooked, neutral/grounded conductor to ground.
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There are test standards and specifications available for all megger test to be performed, these include NETA, NEMA.
http://www.netaworld.org/
http://www.nema.org/
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There are also some basic “rules of thumb”, for go no go testing also commonly referred to as spot reading and generally the most common test most electricians will perform..
If the distribution equipment in question has gotten wet, experienced a fault that results in carbon deposits IT IS IMPERATIVE a company that specializes in this type of problem be contacted. In the way to many cases (one was too many) of injury and death I have been involved in, water and previous faults were the number one and number two causes of injury/death.
This test applies to conductors that are existing. Obviously we would like a full scale reading (infinity) which depending on the megger can be 1000 megohms, 2000 megohms, 4000 megohms or some meggers have higher megohm readings. One thing to remember about infinity FULL SCALE readings, Infinity on you meter can be beat by another meter this all depends on the full scale of the megger in use and the test voltage.
It is possible to damage equipment with a megger if you apply too high a test voltage, so it is imperative you know what you are testing and what is connected to what you are testing.
Typically I like to see high readings 50 megohms or higher, additionally if the readings phase to ground are A-phase 1000 megohms, B-phase 75 megohms, C-phase 1000 megohms, there may be something happening with the B-phase and while an acceptable reading this may warrant investigation.
I hesitate to say that there are cases where I have had reading below 50 megohms where the distribution equipment was re-energized with mitigating circumstances. BUT in all cases the readings were above 5 megohms and the customer was made aware of the situation and signed off on the re-energizing. A good example of this would be underground feeders and/or branch circuits.
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So you have a megger, you have read the instruction manual, you read AECM and Megger links below now you are ready to test something.
Set the megger to the lowest voltage scale and insert the test leads. Put you gloves on (rubber and leathers), hold the test leads apart and push the test button, the meter should go to full scale (air in most cases is a good insulator), that is an excellent reading. Next touch the leads together and push the test button this is a dead short the megger should read “0”. Now get some paper and a no. 2 pencil and scribble lines two large patches for your test clamps and numerous interconnecting lines. Using different voltage settings hit the test button, the meter may give an infinity reading or some reading in between, or it paper and lines may arc and spark till the paper either catches fire or clears the “FAULT”. With different paper thickness varying the scribbles and changing the test voltage you should be able to get an idea of your meggers reading.
Another test that for some reason always amazes some electricians is to connect your multimeter to the test leads (assuming you multimeter can handle the applied voltages) of the megger and push the test button to check the test voltage. Normally the megger output is a few volts above the published test voltage, i.e. 1000 VDC on the megger results in 1052 VDC on the multimeter.
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[brian john]

FIRST POST WAS TOO LONG.......


What megger should you buy?

In my opinion there are 3 basic meggers I would consider

All should have variable testvoltages up to 1000 VDC, be digital with analog scale, 2000 megohms or more full scale readings. And just my experience buy a Fluke, AVO Biddle Megger, or AEMC brand. Additionally I would recommend calibration of you megger, multimerter and amp clamp at least every few years (we cal all our equipment yearly). Figure on spending between 300.00-700.00 dollars. Obviously the better meters with options to include multiple test voltages are going to cost you... Get a good case and hopefully treat it like you do your multimeter (which is like you life depends on it, because it does). At a minimum select an insulation resistance tester, with 250, 500, 1,000 VDC ranges a tester with 100 VDC would be included in any megger I purchase.
Fluke and AEMC do not mention the Trade name Megger [I think] they list these testers as insulation resistance testers Megohmmeters.
AEMC
http://www.aemc.com/techinfo/Charts/...ison_Chart.pdf
Megger
http://www.biddlemegger.com/cgi-bin/...nk--bi2820.htm
Fluke
http://us.fluke.com/usen/products/categoryinsul.htm
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The best way to familiarize yourself is to test everything and document your results (for many reasons) and test often. Play with the tester. If the results seem questionable, question yourself, your test procedures, check the megger, leads apart then leads shorted.
Additional reading:

AEMC
http://www.aemc.com/techinfo/techwor...ech_megohm.pdf

Megger
http://www.omnicontrols.com/articles...ch_in_Time.pdf

Fluke
http://support.fluke.com/find-sales/...15_ENG_C_W.PDF

Additional reading
http://www.hipot.com/faqs/insulation.shtml
http://ecmweb.com/mag/electric_testi...ave/index.html
http://www.sefelec.com/en/insulation...tance-test.php
http://bg.ecmweb.com/ar/electric_ins...ting/index.htm

Search the web…….

[brian john]

The paper test (about the 5th time posted here) but it is sooo much fun to set fires.

Double click on the image below.

[charlie k.]

Good post Brian, the megger is required tool for commissioning and testing gear, equipment and systems. Even though we have all our gear, motors and large cables tested by an independant test co. I still like to check again before we energize. It is important to document your readings, it may come in handy down the road.

Charlie

[zog]

Originally Posted by brian john

ICircuit breakers disconnects, motor starters - with the load disconnected, pole to pole line, pole to pole load, line to load (CB open).
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Well done Brian, I was thinking we needed a good FAQ on this topic. I have to point out that on breakers/starters, it should be Phase to Phase (Breaker closed), Phase to Ground (Breaker closed) , line to load (Breaker open)

I have some good stuff on the 3 currents measured, I will post and you can add if you like.

[brian john]

Please add and post corrections as necessary.

Brian John, I would think you will soon be hearing from Charlie Eldridge.

"I know we are all electrical professionals; however, everyone who reads these posts is not necessarily an electrical professional. I would hope that no one would be stupid enough to actually follow your advice. Assume for a moment that there was enough impedance in the circuit to delay the overcurrent device for the circuit from opening; have you caused someone to start a fire? OK, assume the available fault current was high enough to do as you have suggested; has the practical joker been aware that maybe there shouldn't be anything around that would catch fire?

I know I am coming down with both feet but this is something for either an accident or a test bench, not to be done on purpose to someone with limited electrical knowledge.
__________________
Charlie Eldridge, Indianapolis, Utility Power Guy"

[76nemo]

Here's some more application notes for anyone else interested.


http://us.fluke.com/usen/support/app...(FlukeProducts)


Enjoy!

[zog]

Originally Posted by Lxnxjxhx

Brian John, I would think you will soon be hearing from Charlie Eldridge.

"I know we are all electrical professionals; however, everyone who reads these posts is not necessarily an electrical professional. I would hope that no one would be stupid enough to actually follow your advice. Assume for a moment that there was enough impedance in the circuit to delay the overcurrent device for the circuit from opening; have you caused someone to start a fire? OK, assume the available fault current was high enough to do as you have suggested; has the practical joker been aware that maybe there shouldn't be anything around that would catch fire?

I know I am coming down with both feet but this is something for either an accident or a test bench, not to be done on purpose to someone with limited electrical knowledge.
__________________
Charlie Eldridge, Indianapolis, Utility Power Guy"

Huh? What are you talking about, he is talking about a test bench. And why are you using Charlies signature?

[mivey]

Originally Posted by brian john

The paper test (about the 5th time posted here) but it is sooo much fun to set fires.

Double click on the image below.

That is a painfully slow scroll. I gave up waiting for the fire.