×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

How accurate are digital meggers ?

How accurate are digital meggers ?

How accurate are digital meggers ?

(OP)
Today, I had the uneasy experience of meggering a generator rotor winding (same temperature, time span of 10 minutes, same voltage of 500 V) with 3 different digital meggers (Two Fluke USA and one Kyoritsu Japan)and getting 3 widely differing values of about 600 megohms, 1500 megohms and 5000 megohms.

Is the case of no two clocks show the same time ? Is it time then to go back to the old & trusty hand-cranked analog meggers ?

Muthu
www.edison.co.in

RE: How accurate are digital meggers ?

(OP)
And to think I sunk a few thousand dollars for these "brand" names... sheesh...

Muthu
www.edison.co.in

RE: How accurate are digital meggers ?

These things should be checked for calibration.  NETA says annual calibration checks.  If you're concerned, you can whip up an impromptu "standard" for a few bucks and an hour's time with a few resistors and hardware purchased off the internet.  It won't be an "official, traceable to NIST or whatever" standard, but it will let you do a reality check on your test equipment.

Check the resistance function at a few points.  Check the output voltage function.

By the way...  make sure the battery is charged before you begin testing, too.

old field guy

RE: How accurate are digital meggers ?

(OP)
Thx OFG.

These meters are less than two years old and come with a 2-year OEM calibration validity.

And yes, their batteries were fully charged too.

I still have my old original motor operated Evershed Vignoles "megger" . Plan to use that as a calibrator and see how these "state-of-the-art" digi meggers fare.

I must admit that my trust in all digital metering is taking a beating today.
 

Muthu
www.edison.co.in

RE: How accurate are digital meggers ?

Edison, are you discharging the insulation between these tests? That could be causing the differences in your readings. It is very common to see a differnce of 10x from your 30 second reading and several minutes later as your cap charging current and absorbsion currents decay from charging and polarizing the insulation in an inductove system.  

RE: How accurate are digital meggers ?

(OP)
Zog

Yes, I discharged between the tests (though it was only 500 V and the winding is pretty much like LV motors - no solid insulation like MV motors for much of polarization to take place). And I repeated the test and the values are in the range I've posted. Well, at least they're consistent in their errors, I guess. :)

Muthu
www.edison.co.in

RE: How accurate are digital meggers ?

Back to the hand crank.....  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources