Insulation Testing

[yellow77]

When doing an insulation test with a 1000v. megger I normally connect the ground lead to the grounded conduit and the other to the conductor. What would be the reason to connect the leads to parallel conductors in the same conduit on a new installation.

 

[LLSolutions]

That would only check for the where both conductors' integrity was lost to each other or both to the conduit, It can happen, So why not check it. In a non metallic conduit having to go from conductor to conductor would be a no brainer. I check from conductor to conductor, and conductor to grounded.

 

[yellow77]

Wouldn't the 1000v. test from phase to groung detect a problem or have an inconsistant result in at leat one conductors insulation in a metallic conduit? Just wondering.

 

[brian john]

Industry Standards and specifications REQUIRE, phase to phase, phase to ground, neutral to ground, and phase to neutral.

 

[winnie]

Air is a pretty good insulator; look at all of those multi-kv lines surrounded by nothing but air.

You can have a line-line fault with no corresponding line-conduit fault.

-Jon

 

[brian john]

In addition if this is acceptance test it gives you a base line for future test.

 

[yellow77]

If all wires were clear on a basic continuity test, but
say one still had damage to the insulation. With it not touching the conduit nor another conductor would it be detected in the megger test from condiut to conductor?

 

[ohmhead]

If all wires were clear on a basic continuity test, but
say one still had damage to the insulation. With it not touching the conduit nor another conductor would it be detected in the megger test from condiut to conductor?

Well No it would not ! and Yes it might! but it would depend on the distance between ground and the damaged or pin hole cut in the insulation most likely it would pass the 1000 volt megger test .
Our job is installing new wire and cable everyday and testing each from phase to phase, phase to neutral, phase to ground, neutral to ground most electricians think you need to get a full reading on a megger meaning the highest reading that the megger has to give .
This is a misconception and i know this from testing wire and cables for many years .Air is a insulator but at that certain higher voltage it becomes a conductor thats called tracking over insulators or creepage , most new conductors megg low will pass after conductors dry up or have a load attached for a day or two they actually read higher . take care best to yas

 

[yellow77]

Thanks for the clarification.

 

[ohmhead]

Well we test meaning megg switchgear, buss duck ,cables and all conductors feeders that we install we only do new work this takes weeks to months to test .
Each megg test we do has a different reading you rarely get the same reading even on parralled run conductors its off slightly by hundreds of megg ohms .
Switch gear is around 25 megg to 100 megg do to air or exposed buss .
Conductors or feeders are around 350 megg to 800 megg ohms or max out on megg ohm meter infinity meaning highest you can get on your meter .

But when you pull new feeders in underground conduits you get water in the jacket or soap during the pulling process if you cut the ends it clears or if needed heat gun the ends of each conductor it clears ,or put a load on them conductors and it will disapear the next time you test .
Same with switchgear & transformers put a heater in that room next day it will give ya a higher reading . drying up water in jacket ,or mositure in gear or equipment like buss duck keep it dry . Megging is a learning process it takes hundreds of conductors to figure it out i can tell you ive seen people pull out a perfectly good cable because they cant read a megger correctly .
Most companys like SQ-D give you the megg reading required on new gear and there buss duck. best to yas

 

[zog]

Yellow 77. Download "A stitch in time" from www.megger.com You ned to register but it is a free download. You also ned to know what standards you are testing to, there are several and there are differences, if there is no spec given refer to table 100.1 on ANSI.NETA ATS/MTS.

 

[brian john]

If all wires were clear on a basic continuity test

For this type of testing/work continuity test are worthless.

 

[richxtlc]

Megger testing is a trending process, a single reading has little value other then to tell you what is happening now. For example, when is a 20 Megohm reading a problem, when the last reading was 50 megs. You would only know this if you kept a record of the readings, including temperature and humidity. The above reading would tell you the equipment is ok, but the trend is deteriorating and may be cause for checking the equipment at a more frequent interval. Also, if you don't do the test for the required full minute, the reading may have no real value as other factors enter into the reading, such as capacitive charging current, and absorption current. These will be important on large machinery.
Cable should be meggered one wire at a time with the others shorted and grounded. This insures the intergity of the wire being tested and the insulation between the other conductors.
If testing is done on equipment containing oil then the reading has to be adjusted for temperature. If your megger has a guard terminal, leakage caused water moisture entering the end of the cable can be eliminated by proper use of the guard terminal.