The test voltage should be a minimum of 500 V (dc)

The minimum acceptable insulation resistance is: R in MΩ = (rated voltage in kilovolts + 1) × 304.8/length in meters (1000/length in feet).

Note 1: In the absence of consensus standards dealing with insulation-resistance tests, the NETA Standards Review Council suggests the above representative values.

Note 2: Test results are dependent on the temperature of the insulating material and the humidity of the surrounding environment at the time of the test. See Table 100.14 for temperature correction factors.

Note 3: Insulation-resistance test data may be used to establish a trending pattern. Deviations from the baseline information permit evaluation of the insulation.

Note 4: For rotating machinery insulation resistance test values, refer to Table 100.11.

Note 5: For transformer insulation resistance test values, refer to Table 100.5.

Note 6: For cables, the acceptability for continued energization should not be determined by the recommended minimum insulation resistance values shown above. These values may be used as guidance of typical magnitudes; however, conductor and insulation geometry, temperature, and overall cable length need to be taken into account per the manufacturer’s published data for definitive minimum insulation resistance criteria.

Note 7: The minimum insulation resistance values do not apply to low-voltage metal-enclosed busways. For minimum insulation resistance values of low-voltage metal-enclosed busways refer to Section 7.4.