Megger Test Results for 480V Cable
Megger Test Results for 480V Cable
(OP)
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.
- 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.






RE: Megger Test Results for 480V Cable
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
RE: Megger Test Results for 480V Cable
RE: Megger Test Results for 480V Cable
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.
RE: Megger Test Results for 480V Cable
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.
RE: Megger Test Results for 480V Cable
RE: Megger Test Results for 480V Cable
rasevskii
RE: Megger Test Results for 480V Cable
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.
RE: Megger Test Results for 480V Cable
I tend to follow the teachings of the late great Jim Henderson (Sesame Street):
"Which one of these are not like the other ones?"
RE: Megger Test Results for 480V Cable
RE: Megger Test Results for 480V Cable