Megger (earthing resistance)

[Yjoshi]

Dear all, I need to purchase a megger to measure the resistance to earth on overhead transmission lines. Eventhough initially will be used for a new project I was interested in buyin a HF megger.

Do anyone have any suggestions on a model appropriate for my needs?

Thanks

YJO

 

[DTR2011]

Megger is both a Company Name (Formerly Biddle/AVO, etc) and a generic term for an insulation resistance test set. This is much like Doble Engineering or Kleenex if you wish. Insulation resistance is a (relatively) High Voltage test that can be used to diagnose the insulation of electrical apparatus.

For testing ground resistance, the Complany (Megger), does make a nice instrument called the DET 2/2. The generic term would be ground resistance or earth resistance test set. This is a low voltage / low current test. There are numerous manufacturers that make such a kit. If this is only a once time test, consider renting the equipment, or hiring a qualified service company to perform the work. You will need to disconnect from the utility ground to perform the "Ground Resistance Test". Plan for an outage and coordination with the utility company.



Any test set should be capable of producing off (mains) frequency test signals and be capable of performing a 3 point (fall of potential) and 4 point (Soil resistivity) test.

After re-reading your initial post, you are measuring the "static /ground wire" on transmission lines. This is something else entirely. DO NOT PERFORM this test with the line, or parallel lines energized. Proper lock out / tag out of the lines is absolutely required and close coordination on both ends is a must.

Are you trying to prove ground wire continuity or the quality of the insulators? How long is the line?

Thanks and hope this helps.

 

[Yjoshi]

Thanks SG, actually I am just measuring at the stub the resistance to ground. When I mentioned that this is a new project I meant that I will be measuring the resistance during the civil works, prior to the stringing of the conductors/shield wire. What I was thinking is that I would like to have the option to use the equipment on existing lines without having to disconnect the shield wire from its clamp (HF)

 

[hdp]

To my knowledge, the two scopes of work you mention will require two different instruments. As mentioned above, a 3 point instrument is typically used for work during the "civil" phase. To perform ground resistance testing after the static is connected (without disconnecting leads) requires a clamp-on device that will not work unless the electrode under test is connected to a multi-grounded system. Check AEMC...