how does one interpret the results of megohmeter tests officially? i usually connect the black lead to a known good ground reference source to begin with.Then i open the wire or the circuit to be tested.then i crank up the megger.it is my understanding that this test measures the quality of the insulation by introducing a high voltage/low current thru the conductor being tested.usually in my experience it is pass or fail.What i don't get though is how does the megger tell me that a wire is bad when one side of the wire is obviously open?
megger testing
- Thread starter ranger12
- Start date
- Status
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: megger testing
ranger12, There is some information that you have left out in order to get a good answer for megger testing.
1.What voltage is the system that you are testing?
2.It sounds to me that you want to test the insulation of your wiring.
3.I am going to assume that you are testing wiring for a 480vac system.
4.I would set the megger to check a 480vac system with 1000vdc from the megger.
5.Connect one lead of the megger to the conductor, and the other lead to ground, and see what reading you get with the megger.
6.The One-Megohm Rule, I am going to quote out of a book featuring Megger Insulation Testing.It is called a "Stitch in Time".
Quote: Insulation resistance should be approximately one megohm for each 1000 volts of operating voltage with a minimum value of one megohm. For example, a motor rated at 2400 volts should have a minimum insulation resistance of 2.4 megohms.End of Quote.
ranger12, there is a lot to learn about insulation testing that I cannot cover with this short message.
I am a new member, and I do not know the guidelines for giving out telephone numbers, vendors etc. I would be happy to send you a copy of "A Stitch in Time", The Complete Guide to Electrical Testing, featuring MEGGER Insulation Testers. Or you could contact the AVO company, and they would send you a copy.
Regards, bob
ranger12, There is some information that you have left out in order to get a good answer for megger testing.
1.What voltage is the system that you are testing?
2.It sounds to me that you want to test the insulation of your wiring.
3.I am going to assume that you are testing wiring for a 480vac system.
4.I would set the megger to check a 480vac system with 1000vdc from the megger.
5.Connect one lead of the megger to the conductor, and the other lead to ground, and see what reading you get with the megger.
6.The One-Megohm Rule, I am going to quote out of a book featuring Megger Insulation Testing.It is called a "Stitch in Time".
Quote: Insulation resistance should be approximately one megohm for each 1000 volts of operating voltage with a minimum value of one megohm. For example, a motor rated at 2400 volts should have a minimum insulation resistance of 2.4 megohms.End of Quote.
ranger12, there is a lot to learn about insulation testing that I cannot cover with this short message.
I am a new member, and I do not know the guidelines for giving out telephone numbers, vendors etc. I would be happy to send you a copy of "A Stitch in Time", The Complete Guide to Electrical Testing, featuring MEGGER Insulation Testers. Or you could contact the AVO company, and they would send you a copy.
Regards, bob
bonding jumper
Senior Member
G
Guest
Guest
Re: megger testing
friebel you can Private Message (PM) your phone # to ranger12 by clicking on the PM icon
in the ranger12 post.
friebel you can Private Message (PM) your phone # to ranger12 by clicking on the PM icon
Re: megger testing
Thank you for responding Friebel.In this instance i was at amobile home that had a stick framed addition added to it.State Labor and Industries wer'e asked to inspect the addition as no permit had been pulled when it was constructed.They required megger testing of the wiring.It was a 120/240 voltage system.I assumed as far as this correction was concerned that they wanted to assure that the wiring insulation was allright as they never had a chance to inspect it in the first place.I also addressed all the other obvious things like polarity and broken plugs and grounded switches.but really i am trying to learn and understand how the megger works.I did a search on the avo company and came up emptyhanded.If you could give me a site to go to i would appreciate it.I would be happy to buy a book from them if it is worthwhile .Thanks
Thank you for responding Friebel.In this instance i was at amobile home that had a stick framed addition added to it.State Labor and Industries wer'e asked to inspect the addition as no permit had been pulled when it was constructed.They required megger testing of the wiring.It was a 120/240 voltage system.I assumed as far as this correction was concerned that they wanted to assure that the wiring insulation was allright as they never had a chance to inspect it in the first place.I also addressed all the other obvious things like polarity and broken plugs and grounded switches.but really i am trying to learn and understand how the megger works.I did a search on the avo company and came up emptyhanded.If you could give me a site to go to i would appreciate it.I would be happy to buy a book from them if it is worthwhile .Thanks
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: megger testing
Good morning, ranger12,In reponse to your message that you could not find the AVO company on the internet.
Type www.avointl.com I typed it in and boy, I did not realize the information that you can receive. I am always amazed at the info. on the internet.
ranger12 I have the megger books, that AVO has sent to me for my classes. I would be glad to send one to you. You could send me your address on a private message. But, it would be a good idea for you to contact AVO and communicate with them. I live in south Jersey, and I deal with AVO in Valley Forge, Pennsylvannia.
There is so much to learn about Megger Insulation Testing, and how to properly use the Megger.
Good morning, ranger12,In reponse to your message that you could not find the AVO company on the internet.
Type www.avointl.com I typed it in and boy, I did not realize the information that you can receive. I am always amazed at the info. on the internet.
ranger12 I have the megger books, that AVO has sent to me for my classes. I would be glad to send one to you. You could send me your address on a private message. But, it would be a good idea for you to contact AVO and communicate with them. I live in south Jersey, and I deal with AVO in Valley Forge, Pennsylvannia.
There is so much to learn about Megger Insulation Testing, and how to properly use the Megger.
pierre
Senior Member
- Location
- Westchester County, New York
Re: megger testing
Friebel
That is a great link, Thanks
Pierre
Friebel
That is a great link, Thanks
Pierre
brian john
Senior Member
- Location
- Leesburg, VA
Re: megger testing
This has been covered BUT the one megaohm rule IMO is not an acceptable rule/standard. Way too low, Check NETA for test spec.s
http://www.netaworld.org/Category.cfm?CategoryID=3
This has been covered BUT the one megaohm rule IMO is not an acceptable rule/standard. Way too low, Check NETA for test spec.s
http://www.netaworld.org/Category.cfm?CategoryID=3
- Location
- Illinois
- Occupation
- retired electrician
Re: megger testing
Brian,
Why is that too low? On a 480 volt system, a 1,000,000 ohm resistance between the circuit and grounding conductors would result in leakage current of 0.00048 amps or about 1/10 of the current that will casue a GFCI to trip.
Don
Brian,
Why is that too low? On a 480 volt system, a 1,000,000 ohm resistance between the circuit and grounding conductors would result in leakage current of 0.00048 amps or about 1/10 of the current that will casue a GFCI to trip.
Don
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: megger testing
To: brian john, You are correct, the one megohm is too low. I should have explained myself in more detail. If I had a motor that was critical to the operations, and had a one-megohm reading, I would allow the operations to continue to run this motor. But I would start preparations to replace this motor as soon as possible.
It has been my experience that most electricians are not familiar with "what is the minimum reading in megohms" that you can continue to run a motor.
The AVO manual states: one megohm per thousand volts, with a minimum reading of one megohm.
Example: A 2400 volt motor would need a minimum reading of 2.4 megohms.
But a 460 volt motor minimum reading could be no lower than one-megohm.
I have worked on an industrial plant that had about 10,000 motors, from 2300 volts to 575 volts, 460 volts. So in my 45 years on this plant I have done a great deal of testing of motors.
But again brian john, you are correct in your assessment, the one-megohm is to low.
To: brian john, You are correct, the one megohm is too low. I should have explained myself in more detail. If I had a motor that was critical to the operations, and had a one-megohm reading, I would allow the operations to continue to run this motor. But I would start preparations to replace this motor as soon as possible.
It has been my experience that most electricians are not familiar with "what is the minimum reading in megohms" that you can continue to run a motor.
The AVO manual states: one megohm per thousand volts, with a minimum reading of one megohm.
Example: A 2400 volt motor would need a minimum reading of 2.4 megohms.
But a 460 volt motor minimum reading could be no lower than one-megohm.
I have worked on an industrial plant that had about 10,000 motors, from 2300 volts to 575 volts, 460 volts. So in my 45 years on this plant I have done a great deal of testing of motors.
But again brian john, you are correct in your assessment, the one-megohm is to low.
elmarea
Member
- Location
- upstate ny
Re: megger testing
I agree that the 1 meg rule of thumb is widely used.
One additional check that should be done, is to meggar each wire in a cable to one another. In other words for a two conductor cable not only meggar each wire to ground, but meggar each insulated wire to the other. There is always the possibility of a staple shorting the two insulated conductors.
I agree that the 1 meg rule of thumb is widely used.
One additional check that should be done, is to meggar each wire in a cable to one another. In other words for a two conductor cable not only meggar each wire to ground, but meggar each insulated wire to the other. There is always the possibility of a staple shorting the two insulated conductors.
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: megger testing
To: elmarea, A very excellent point that you make.
After testing each conductor to ground, then test each conductor to the other conductors. This subject could go on forever,so I am going to add a little bit more. We keep talking about the one megohm rule, and this should be the minimum reading. Naturally you would want to see your readings up in the infinity range, but when is the point that you need to do something. If you have a preventive maintenance program, and you keep records of your readings, a good sign is when the readings start going down. Meaning moisture, dirt, oil, etc.is starting to get to your system. And again when we say the one megohm rule, we mean one megohm per thousand volts, with a minimum of one megohm period on any system.
Example: A 2400 volt motor, you would want a minimum reading of 2.4 megohms, but you should be looking to take the motor out of service, and clean and dry the windings, maybe even give it another dip in insulating varnish.
Now if it is wiring we are talking about, you will need to replace the wiring.
I need to add this personal comment: I am a new member, but I have been in the electrical field for the past 50 years. I am retired from the Chemical Company that I worked for, but I am teaching at a Technical & Junior College. I am teaching Progrmmable Logic Controllers, Electrical Controls In HVAC/R Systems, and I will start teaching the code in January using Mike's books.
How fortunate you young fellows are to have this network of Mike Holt's. I really enjoy it. By the way, retirement is a word that is not in my vocabulary.
I hope for you all a very blessed and happy Thanksgiving.
To: elmarea, A very excellent point that you make.
After testing each conductor to ground, then test each conductor to the other conductors. This subject could go on forever,so I am going to add a little bit more. We keep talking about the one megohm rule, and this should be the minimum reading. Naturally you would want to see your readings up in the infinity range, but when is the point that you need to do something. If you have a preventive maintenance program, and you keep records of your readings, a good sign is when the readings start going down. Meaning moisture, dirt, oil, etc.is starting to get to your system. And again when we say the one megohm rule, we mean one megohm per thousand volts, with a minimum of one megohm period on any system.
Example: A 2400 volt motor, you would want a minimum reading of 2.4 megohms, but you should be looking to take the motor out of service, and clean and dry the windings, maybe even give it another dip in insulating varnish.
Now if it is wiring we are talking about, you will need to replace the wiring.
I need to add this personal comment: I am a new member, but I have been in the electrical field for the past 50 years. I am retired from the Chemical Company that I worked for, but I am teaching at a Technical & Junior College. I am teaching Progrmmable Logic Controllers, Electrical Controls In HVAC/R Systems, and I will start teaching the code in January using Mike's books.
How fortunate you young fellows are to have this network of Mike Holt's. I really enjoy it. By the way, retirement is a word that is not in my vocabulary.
I hope for you all a very blessed and happy Thanksgiving.
brian john
Senior Member
- Location
- Leesburg, VA
Re: megger testing
Standard test for meggering cables is phase to ground and phase to phase, though some specs call for phase to ground with all other conductors of the circuit grounded.
As for the one megohm rule, I can only tell you from experience that, energizing cables at one megohm resulted in faults with in a short period of time. Not once, not twice but at three times we increased the acceptable level to 5 megohm these also faulted at a later date. I would recommend replacing at one megohm and at 5 megohms we recommend budgeting for replacement tin the near future.
In addition the real advantage of meggering is trending. Stating from day one of installation and monitoring the results on a regular basis.
Standard test for meggering cables is phase to ground and phase to phase, though some specs call for phase to ground with all other conductors of the circuit grounded.
As for the one megohm rule, I can only tell you from experience that, energizing cables at one megohm resulted in faults with in a short period of time. Not once, not twice but at three times we increased the acceptable level to 5 megohm these also faulted at a later date. I would recommend replacing at one megohm and at 5 megohms we recommend budgeting for replacement tin the near future.
In addition the real advantage of meggering is trending. Stating from day one of installation and monitoring the results on a regular basis.
pierre
Senior Member
- Location
- Westchester County, New York
Re: megger testing
I am posing a question for reference:
For feeders, or other wiring, lets say a megger test was performed at the time of installation and all of the results were fine. 10 or 15 years go by, is it time to retest? If so how do you go about retesting when the building is occupied?
Pierre
I am posing a question for reference:
For feeders, or other wiring, lets say a megger test was performed at the time of installation and all of the results were fine. 10 or 15 years go by, is it time to retest? If so how do you go about retesting when the building is occupied?
Pierre
brian john
Senior Member
- Location
- Leesburg, VA
Re: megger testing
The firm I work for offers electrical preventative maintenance (EPM), during scheduled outages we perform inspections, testing, cleaning and servicing of distribution equipment. Generally we have also performed an IR scan prior to the EPM.
In our area EPM is required on all 480 VAC distribution systems above 1000 amps. In addition the local utilities require EPM on any customer owned high voltage distribution equipment.
If bolted pressure switches are in use in your area maintenance is a must, especially on older switches. But as in all work there are proper procedures to follow when servicing these switches.
The firm I work for offers electrical preventative maintenance (EPM), during scheduled outages we perform inspections, testing, cleaning and servicing of distribution equipment. Generally we have also performed an IR scan prior to the EPM.
In our area EPM is required on all 480 VAC distribution systems above 1000 amps. In addition the local utilities require EPM on any customer owned high voltage distribution equipment.
If bolted pressure switches are in use in your area maintenance is a must, especially on older switches. But as in all work there are proper procedures to follow when servicing these switches.
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: megger testing
This message is especially to brian john and pierre.
We are going to beat this megger message to death, but in the end, we will all learn something from it. First, brian john, I am in total agreement with you. One megohm is to low to allow the motor or service to stay in service. What I am saying is: You can prepare for an orderly shutdown to make the necessary repairs to the motor, and/or the motor service.I would not walk away from readings that are this low.
Now to pierre, you asked the question about testing electrical services with a megger, and how should you approach it.
No.1 When you install the electrical wiring, no matter what type it is, you should always record the megger readings on a "Megger Insulation Tester Card" The AVO company that I have mentioned can supply you with this card. It generally is a 5 inch by 8 inch card. You would plot your readings that you get, noting the date, time, etc. How we approach this in the industrial arena was this way. On a scheduled shutdown, we would make known what switches, etc. that meeded to be shut down, so that we could do our meggering. Now when you would notice a downward trend in the megohms, you could start arranging for a orderly shutdown, and make repairs.
With a motor, it might mean a cleaning and overhaul, and possibly a dip in the varnish tank.
In the case of the motor service or whatever service, it would probably mean a new service. brian john brings up a good point about infrared testing. This is an excellent method to detect hot spots in your terminal connections and make necessary repairs before they do some severe damage. I will not go into details on this, because it is another subject, but as brian john points out a very good PM tool.
Have a blessed Thanksgiving men.
This message is especially to brian john and pierre.
We are going to beat this megger message to death, but in the end, we will all learn something from it. First, brian john, I am in total agreement with you. One megohm is to low to allow the motor or service to stay in service. What I am saying is: You can prepare for an orderly shutdown to make the necessary repairs to the motor, and/or the motor service.I would not walk away from readings that are this low.
Now to pierre, you asked the question about testing electrical services with a megger, and how should you approach it.
No.1 When you install the electrical wiring, no matter what type it is, you should always record the megger readings on a "Megger Insulation Tester Card" The AVO company that I have mentioned can supply you with this card. It generally is a 5 inch by 8 inch card. You would plot your readings that you get, noting the date, time, etc. How we approach this in the industrial arena was this way. On a scheduled shutdown, we would make known what switches, etc. that meeded to be shut down, so that we could do our meggering. Now when you would notice a downward trend in the megohms, you could start arranging for a orderly shutdown, and make repairs.
With a motor, it might mean a cleaning and overhaul, and possibly a dip in the varnish tank.
In the case of the motor service or whatever service, it would probably mean a new service. brian john brings up a good point about infrared testing. This is an excellent method to detect hot spots in your terminal connections and make necessary repairs before they do some severe damage. I will not go into details on this, because it is another subject, but as brian john points out a very good PM tool.
Have a blessed Thanksgiving men.
pierre
Senior Member
- Location
- Westchester County, New York
Re: megger testing
Friebel & Brian
Thanks for the answers. I was talking with an engineer today and he said this industry is all about the time and experience, I agree.It is good to be able to talk to men with a lot more experience in particular parts of the industry at a forum such as this. Years ago this was not available, and getting info was a much harder process.
Happy Thanks Giving to you all
Pierre
Friebel & Brian
Thanks for the answers. I was talking with an engineer today and he said this industry is all about the time and experience, I agree.It is good to be able to talk to men with a lot more experience in particular parts of the industry at a forum such as this. Years ago this was not available, and getting info was a much harder process.
Happy Thanks Giving to you all
Pierre
- Status