The insulation between the current transformer windings and windings to ground should be checked for dielectric strength while performing a comprehensive CT test. Three tests are performed to determine the condition of the insulation of the CT under test:

1. Primary to secondary: Checks the condition of the insulation between high to low.

2. Primary to ground: Checks the condition of the insulation between high to ground.

3. Secondary to ground: Checks the condition of the insulation between low to ground.

Insulation resistance readings should remain fairly constant over a period of time. A sharp dip in trending of insulation resistance values point towards insulation degradation and further investigation is required to diagnose the problem.

Insulation tests on current transformers rated 600V or less are usually performed at 1000VDC. Prior to testing, short the primary winding of the CT under test by connecting H1 and H2, then short the secondary winding of the CT under test by connecting X1 and X2-X5.

Remove the neutral ground and isolate the CT from any associated burden. After the windings are shorted, the CT will be a three terminal specimen.

Insulation resistance test values for CT's should be compared with similar readings obtained with previous tests. Any large deviation in historical readings should call for further investigation.

ANSI/NETA MTS-2019 Table 100.5 Specifies a minimum insulation resistance of 500 Megohms at 1000VDC for transformer coils rated 600V or less. Reference Section 7.10.1 for more information.

The minimum insulation resistance that is generally accepted is 1 Megohm. Any reading in Megohms is considered to be a good insulation, however, it's the trending of insulation test results that gives the true condition of CT insulation.

Insulation readings are greatly affected by the specimen temperature. Should a reading be compared to previously taken readings, proper correction factors need to be applied, if taken under different temperature conditions before drawing any conclusion.