Meggar readings

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bamiller

Member
This is not really a code issue but, What is an unexceptable Meggar reading. I have a run that has 4000Mohms Phase to ground and 800Mohms phase to N but phase to phase one is .75Mohms. PLEASE HELP before we energize.
 

charlie tuna

Senior Member
Location
Florida
Re: Meggar readings

miller,
consider using a ten amp fixture fuse or low amperage rated fuse with proper voltage ratings with no loads connected--this will apply rated voltage to the circuit and tell you if the circuit is clear!
 

bamiller

Member
Re: Meggar readings

4 - 500MCM I know the conductors are clear, no continutiy with meter. I heard 1Meg ohm per 1000V, so .5Meg Ohm for 480V - approximately. Does anyone agree with that?
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Meggar readings

If the runs are long I would tend to think you are just seeing capacitive leakage coupling between conductors.
 

kiloamp7

Senior Member
Re: Meggar readings

Believe the old rule-of-thumb is 1 megohm per KV of the circuit voltage that you will be using, with a 1 megohm minimum.
I think the rule simply implies that it is safe to energize.

Be sure that any potential coils, metering, little bitty chumpy things, etc., etc. are not connected to the conductors that you are megging. For the 480V. application, I assume you are using a 500VDC meggar.
 

brian john

Senior Member
Location
Leesburg, VA
Re: Meggar readings

1st DO NOT ENERGIZE.

If this is a 480/277 or 208/120 or 240/120 system and the conductors are 600 VAC rated, this is new cable and you are meggering at 1000 VDC and your test duration is at a minimum one minute than NETA specs are 100 megohms. As a rule of thumb we like to see a minimum of 50 megohms on any equipment, have lived with 5 megohms knowing the equipment was slated for replacement in tne near future. And have energized direct bury cable at 1 megohms the cable faulted within a week.

Of course you could be frying the phase monitor in a switchboard, I hope assume the cable is not connected to any loads at either end, phase monitor, TVSS, or control transformers or POTs

[ June 02, 2005, 07:57 PM: Message edited by: brian john ]
 

tx2step

Senior Member
Re: Meggar readings

If you don't have someone available that already knows how to use a megger, what is the best way to learn how to use one properly and to interpret the results correctly?

Is there a simple source for this type of self-education?

Is a 1,000 VDC megger a lot better than a 500VDC megger?
 

friebel

Senior Member
Location
Pennsville, N.J.
Re: Meggar readings

Megger, and how to use and interpret the readings.
I have always dealt with the Biddle Megger Company and now the Company name is AVO.
The person that I deal with when I have a question is, Jeff Jowett of the AVO Company. Jeff is a Applications Engineer with the AVO company.

{Moderator's Note: Edited to remove Jeff's personal contact information. If you wish to contact Jeff, please send a Private Message to friebel, and ask for the contact phone numbers and email address.}

If you contact Jeff, I know that he will send you two booklets. One is called "A Stitch in Time", it is a Complete Guide to Electrical Insulation Testing Featuring Megger Insulation Testers.

The other booklet is "Getting Down To Earth". Believe me, if you read and understand all of the information that is in the two books, you will be well prepared to handle any situation with Megger Testing.

Now to answer the question about what is an acceptable reading. Naturally, Infinity would be the best and ultimate reading that you would like to see, but what is the acceptable low reading.
In some of the previous threads the "One-Megohm-Reading" was mentioned.
Let me give you an example, if you had a 2300-volt motor, and you were testing the motor windings with a 1000 volt megger, the minimum reading that would be acceptable would be 2.3 Megohms. That is where this "One Megohm per 1000 volts" comes in.
If you were checking a 460 volt motor, you would use the 1000 volt scale, and the minimum reading that is acceptable is One-Meg-Ohm.
But, when your reading is down this low, it is time for you to start thinking about changing the motor and putting a motor in that will be reading Infinity.

[ June 09, 2005, 10:10 AM: Message edited by: charlie b ]
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Meggar readings

friebel If you want to stay friends with Jeff you should edit your post and remove his contact info.

His email may get spammed if it is posted here.

What you can do is ask folks to PM you asking for the info and you can PM them back. :)

[ June 08, 2005, 06:03 AM: Message edited by: iwire ]
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Meggar readings

Originally posted by iwire: friebel If you want to stay friends with Jeff you should edit your post and remove his contact info.
I've taken care of it. Thanks for pointing this out, Bob.
 

rcwilson

Senior Member
Location
Redmond, WA
Re: Meggar readings

I would be concerned that only one phase-phase reading is low. If the wires are isolated, like all the other posts have mentioned, you've got a cable problem. It might hold at 480 V for the first minute or week, but the problem will show itself later.

If the wires are connected to equipment, a bad phase-phase reading is usually a control power transformer, meter, relay or VT still on the system. Ihave also found dirt and junk shorting leads inside a main breaker give readings like that. Isolate equipment and wiring until you identify where it is.

Do not energize until you find the problem.

Bob Wilson PE, (& former Certified NETA Test Technician)
 
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