brian john
Senior Member
- Location
- Leesburg, VA
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
-------------------------------------------------------------------------------------------------------------------------------------
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
-------------------------------------------------------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------------------------------------------------------
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/
?-------------------------------------------------------------------------------------------------------------------------------------
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.
?-------------------------------------------------------------------------------------------------------------------------------------
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.
?-------------------------------------------------------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------------------------------------------------------
Megohm = 1,000,000 = one million ohms
Gigohm = 1,000,000,000 = one billion ohms
Terohm = 1,000,000,000,000 = one trillion ohms
-------------------------------------------------------------------------------------------------------------------------------------
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
-------------------------------------------------------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------------------------------------------------------
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/
?-------------------------------------------------------------------------------------------------------------------------------------
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.
?-------------------------------------------------------------------------------------------------------------------------------------
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.
?-------------------------------------------------------------------------------------------------------------------------------------
Last edited by a moderator:
I'd love to hear more on that. Never even heard that discussion before!!!!!!!!!!!
