Nitrogen Purging of Conductors

[mdshunk]

What is a perfect megger reading?

I don't know, what would you like to see? :grin: Ideally, I'd like ot see full scale on a new conductor, wouldn't you?

Don't go getting all technical on me now. It's late on a Saturday. It's more helpful sometimes to talk in generalities and get more specific only when required.

 

[zog]

I don't know, what would you like to see? :grin: Ideally, I'd like ot see full scale on a new conductor, wouldn't you?

Don't go getting all technical on me now. It's late on a Saturday. It's more helpful sometimes to talk in generalities and get more specific only when required.

Just wondering what your idea of perfect was. "Full scale" depends on what type of megohmeter you are using, there are types of equipment I work on that full scale on some cheapo hand held 1000V meggers full scale is not high enough to be acceptable.

NETA specs (And IEEE) require at least 100M @1000VDC for 60 seconds for 600V rated cable.

 

[mdshunk]

Just wondering what your idea of perfect was.

I guess the exact reading was less important in the context of the post I was making. Sometimes the minutia can be a barrier to effective overall communication. Your point is certainly right, but isn't really the road I was going down. We're talking about building wire here and not the niche stuff you work on.

 

[zog]

I guess the exact reading was less important in the context of the post I was making. Sometimes the minutia can be a barrier to effective overall communication. Your point is certainly right, but isn't really the road I was going down. We're talking about building wire here and not the niche stuff you work on.

600V building wire still needs to be >100M, no?

 

[drbond24]

Are you just nuts, or a hard head in general?

I plead the fifth.

....a good megger reading does not mean that the cable is good.

If it tests bad, it is bad. If it tests good, it could still be bad...? Is there not a better way to test? How useful is a test result that you can't trust?

....if you have no water or other conductive contaminates around the conductor and you megger it, it can test good even with physical bare spots.

How do you megger dry cable in a dry conduit?

 

[don_resqcapt19]

If it tests bad, it is bad. If it tests good, it could still be bad...? Is there not a better way to test? How useful is a test result that you can't trust?

There are other methods of testing that can detect these types of problems.

How do you megger dry cable in a dry conduit?

The same way you megger any conductor. You connect one lead to the conductor itself and the other to the raceway if it is metallic or to the EGC if it isn't. You also meg phase to phase as well as phase to ground. With nonmetallic conduit and an insulated EGC, it would be very easy to have a "perfect" megger reading and a bad cable.

 

[DWZipse]

D. W. Zipse, P.E., IEEE Life Fellow

D. W. Zipse, P.E., IEEE Life Fellow


Contractor left 15 kV cable coiled up in a manhole without any seal on the end of the cable. Length left in manhole long enough to go 25 feet in conduit and up pole. Manhole filled with water in Nov. In March ready to finish installation. Lifted end of cable out of manhole and water ran out of MV cable like a water pipe. Dry end was slightly up hill and inside building.

Applied dry N2 gas under pressure to dry end. Placed vacuum pump at manhole ? wet end. Checked moisture content of air coming from cable at manhole daily. After maybe a month declared cable dry. High voltage dc tested cable at 80 % of factory test voltage ? do not remember exactly but probably at 64 kV dc.

Cable passed and was terminated and placed in service. The cable was still in service 15 years later when electrical system re-vamped.