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sjmgd997 (Electrical)
25 Aug 10 10:22
I have been testing 480V cable and I obtained some of the following results:
- 22 giga-ohms

I wanted to know if this seems like a "realistic" value to obtain. Note that these values were obtained at about 80 degrees F and a distance of 150 feet and #2 AWG conductors.

As an example, I believe you can use a 500V or 1000V megger for 480V equipment. Does the value of the insulation resistance you measure change depending on the test voltage you use?

I would think that the only thing that the megger voltage possibly effects is the rate at which capacitive charging current decays. Is my thought on this correct?

Thanks in advance.
Helpful Member!  rasevskii (Electrical)
25 Aug 10 11:01
You do not say what are the conditions: Is it a brand new installation where everything is dry, clean, dustfree, etc-are the cables actually terminated in panels or not, what about that?  22gigaohm is practically open circuit, what is the ground reference, is there a shield or screen on the cable, is that grounded, did you megger the screen to ground...Are the cables in conduit, direct burial, in racks, etc...Are you meggering conductor to conductor or to ground.

You have to, as it were, view the readings with suspicion if that is really what you are getting. Try another megger...recheck everything again.

Sorry, it is a bit of black art, this...

rasevskii  
Helpful Member!  dapotiti18 (Electrical)
25 Aug 10 13:07
I want to let you know that their is thumb rule for insulation test.state as follow: insulation resistance test voltage=1kv, minimum expected=1megger ohm, 2kv = 2megger ohm, 5kv = 5 megger ohm etc. So your result is okay.
Helpful Member!  sjmgd997 (Electrical)
25 Aug 10 13:13
In this case, to clarify:

1) The cable used is unshielded cable
2) The cable is routed in conduit
3) The cable is partially run underground and that it transitions to above ground.

The values given were measured in phase to phase.

I thank you in advance for your responses.
 
Helpful Member!  Zogzog (Electrical)
25 Aug 10 13:13
Please ignore that thumbrule.

Minimum test voltage for 600V cable is 1000VDC, it should be at least 100M at 20 degrees C (ANSI Spec).

22G is not unheard of for new cable, but as rasevskii mentioned, it depends on the conditions of the test.  
sjmgd997 (Electrical)
25 Aug 10 13:14
I forgot to mention that this is a NEW installation.

 
rasevskii (Electrical)
25 Aug 10 14:17
Obviously you are OK with these cables. 22Gohm is practically Infinity as seen by older type Meggers. If you want to be sure, have a helper put an artificial ground on each conductor at the far end, and see that the Megger shows the fault. That should convince any client or inspector about the good quality of the job.

rasevskii
Helpful Member!  CanadianKid (Electrical)
25 Aug 10 15:29
When one determines a test voltage for a this type of test, one should know that the test you performed is an insulation resistance test. This is a quality determination test, which should not change dramatically with voltage, unless you approach the maximum voltage spec of the cable. In your case, 500V DC is typically used, but many use the 1KV DC with equally good results.

As you performed an "insulation resistance" test (Megger is a brand name which is trade mark protected), then the value of resistance of 22Gohms is at question. For a 480V cable, this sounds like a very good result, but one can be fooled with a high resistance reading if there exists a "nick" or partial "fault" in the cable and the nick is clean. A cable which is newly installed typically yields good results even when nicks or small faults exist. Only after the partial damage is exposed to dirt and moisture, does it show the problem via a lower resistance, and/or failure.

This is why new installations are tested at much higher voltages, and the cable is subjected to this voltage for a longer time period. The test is considered a "destructive" test because weak cable will fail. In your testing, you are looking for a quality number such as ">50Mohms" for pass/fail.

Sorry to be so long winded. Hope this helps. Much of this information is available in various publications (some of which are free) on the internet.
dpmac (Electrical)
26 Aug 10 10:42
How does this one reading compare with the other cables associated with this installation?  If all other phase to phase readings are in the same range (assuming test equipment and set up is good) then your all set.

I tend to follow the teachings of the late great Jim Henderson (Sesame Street):

"Which one of these are not like the other ones?"
Helpful Member!  mmt019 (Electrical)
26 Aug 10 12:39
Rule of thumb that 1 Megohm for every 1kVDC of applied voltage is a MINIMUM only for maintenance testing (cable that was installed 15+ years or more).  If it is an acceptance test (new cable), then you absolutely have a problem.  ANSI/NETA MTS-2007 (maintenance standards for testing) and NETA ATS-2007, Table 100.1 requires 100 Megohm as a recommended minimum for 600V rated cable.  Anything higher than that number obviously is better.  Those standards that I listed are good references     
mmt019 (Electrical)
26 Aug 10 14:05
I forgot to state that Table 100.1 at 100 Megohms is for a DC test voltage of 1000VDC for 1 minute.  You should record each Megohm value every minute for 10 minutes.  A polarization index is the (Megohm value @ 10 minutes)/(Megohm value @ 1 minute).  This should never be less than 1.  You should ALWAYS file this data in your records.  If this is an acceptance test and found to be acceptable values, then this will be your baseline for future tests on that cable run.  If this is maintenance testing, use Table 100.1 and if the results are found to be less than 2 Megohm, then further investigation is recommended.  This is where you compare results from previous tests conducted on the cable.     

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