VAST DIFERENCE IN DIGIATL VS ANALOG METERS 2

[EEAOC]

I have had vast difference in reading on insulation- resistance for MI Cable, the digital meter [by YOKOGAWA -24060 MEAS] shows about 6.0 Meg Ohms and an analog [MEGGER by BIDDLE] meter shows about 7 Gigs for the MI cable. The reading with both meter were taken more than twice and appears to give readings. Can anyone explain why such a vast difference for the same feeder? The full-scale setting for analog meter was correct.

EEAOC

 

[busbar]

Compare the two meters reading 1 megohm, then a string totaling ~100 megohms of 5% fixed resistors.

Might the Guard terminal been used on one meter and not the other?

 

[edison123]

Are the polarities of both meggers same ? Some have +ve grounded and some have -ve grounded.

 

[NorthSlope]

MI cable is notoriously difficult to measure. Temperature makes a difference as does relative humidity. How far apart were the readings taken? I agree with the comment on polarity. Standards say rhat the item inder test (a conductor in your case) should be Negative and the reference electrode (the jacket?) positive. There's a lot of capacitance (the dry chemical used as an insulator) involved in any run of usual length. This can sometimes confound an electronic tester - as opposed to a hand crank or motor driven generator.

 

[bgisborn]

The 7 Gigohms makes no sense. Remember a megger has to have sufficient voltage to break down surface conditions at the point your making the connections. That's one of the reasons for the High Pot test. There is an inherent danger in high potting. Insulation can be compromised. In this regard, the digital meter is safer. Are you sure you read 7x10^9 and not 7x10^6?