Insulation Value on 480V Cable 2

[sjmgd997]

Hello, I am looking to confirm what is a good insulation value (in mega-ohms) for 480V power cable.In my situation, I have remote sites that each have its own 480V power distribution system and our specifications require megger testing of the cable.

Here are my questions:

1) Since this is brand new cable, is it better to run tests with 1000V or 500V?

2) In looking over the manufacturer (Southwire in this case) data, they state that "good" insulation should not have more than 50 mega ohms of resistance on it. From speaking with an engineer from an electrical testing firm, he has told me that 100 mega-ohms should be a "good" benchmark indicating that electrical insulation is acceptable. Does any one out there have any information as to the value to be used?

3) From looking over the formula given from Biddle, it appears that we have to account for the following variables:
- Length of cable
- The temperature correction factor

I was wondering approximately how much influence these values will have on the megger test results. As an example, length is affected by factors such as the cable's capacitive charging current. (The longer the cable, the longer it will take for the reading from a megger to 'settle out' as I understand it)

The temperature correction factor is a factor that is based on the megger reading you get at the time of testing and to some extent is influenced by the type of cable.

I thank you in advance for any hints you may offer on this matter.

 

[rbulsara]

Mfr.'s recommendation is the best source. You can also look up LV cable's acceptable IR values in NETA-ATS (Acceptance Test Specifications), available at

http://www.neta.org.

The IR values do vary by ambient conditions, but if you have several cables, comparing their values would give a good idea if one the cables is misbehaving.

You can also rely on expertize and opinion of a good electrical testing company (NETA certified). You, of course, would have to hire them for the test.

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

Minimum test voltage on 600V cable is 1000V. Minimum acceptable IR is 100M at 20 degrees C.