Megger test

[RHJohnson]

[QUOTE usually any cable under 100megaohms is no good. theres also a rule of thumb of one megaohm per 1000 volts of rated voltage for equipment. [/QUOTE]

I did start-up on new industrial construction: On new LV cables 100 megohms was a general requirement, because the cable was new. And exceptions were often accepted. In place "old" wiring would often megger much, much lower than 100 megs.

 

[zog]

Doing Insulation Resistance testing on cables is one of the most misunderstood tests in the industry, even if it seems simple.

1st off, this 1Meg for 1000V rating is nonsense, it comes from an old motor spec for MOTOR windings made before 1970, and is misused and misapplied all of the time, it used to be a rule of thumb found in some ANSI/IEEE standards but vanished about 10 years ago (Check the publication date on a stich in time)

Second, what values are acceptable for cables will depend on the type of insulation, the humidty, the tempature of the cable under test (Must be temp corrected to 20 degrees C to have any meaning as a value) and most importantly the length of the cable. Megohms/Ft at 20 degrees C is as good as you can get for a spec for a specific cable type.

This is all based on the assumption we are talking about LV cables, MV cables is a whole different issue, a IR (Or Megger as it keeps being called in here) test on a MV cable is useless on its own.

 

[electricalperson]

Doing Insulation Resistance testing on cables is one of the most misunderstood tests in the industry, even if it seems simple.

1st off, this 1Meg for 1000V rating is nonsense, it comes from an old motor spec for MOTOR windings made before 1970, and is misused and misapplied all of the time, it used to be a rule of thumb found in some ANSI/IEEE standards but vanished about 10 years ago (Check the publication date on a stich in time)

Second, what values are acceptable for cables will depend on the type of insulation, the humidty, the tempature of the cable under test (Must be temp corrected to 20 degrees C to have any meaning as a value) and most importantly the length of the cable. Megohms/Ft at 20 degrees C is as good as you can get for a spec for a specific cable type.

This is all based on the assumption we are talking about LV cables, MV cables is a whole different issue, a IR (Or Megger as it keeps being called in here) test on a MV cable is useless on its own.

i remember reading something about that in the book by paul gill. insulation testing requires a little skill and knowledge. im sure most people who do this on a daily basis have years of experience and training. i mainly use my megger to help me find short circuits and to do go/no go tests. a stitch in time is the beginning of the wonderful world of insulation resistance testing.

 

[zog]

the wonderful world of insulation resistance testing.

Ha, thats funny. Glad you read the Paul Gill book, thats a good start, you already know more than 99% of the megger owners out there.