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current jumps up for a small increase of voltage; is very important for comparison of curves with
published curves or similar CT curves.
The MVCT plots the saturation curve on the display screen. Once a knee point is determined by the test
set, it will also display the corresponding saturation voltage and saturation current on the display
screen. Multiple CT saturation curves can be plotted on the same screen either for multi taps or for
comparison of similar type CT’s. The excitation test results should be compared with published
manufacturer’s data or previous recordings to determine any deviations from previously obtained
curves.
The unit automatically demagnetizes the CT after performing the saturation test by slowing ramping
down the secondary voltage all the way back to Zero. It is advisable to demagnetize the CT before
performing any test that requires accurate measurements. Please refer to the section of CT
Demagnetization for details.
3.2.4 Insulation Resistance Test
The insulation between the windings and winding to ground should be checked while performing a
comprehensive CT test. The test set has inbuilt 1kV DC insulation tester that is used for measurement of
insulation resistance.
The following three tests are recommended by the ANSI standard.
Primary winding of the CT to Ground.
Primary winding of the CT to the secondary winding of the CT
Secondary winding of the CT to Ground
These three tests are performed to determine the condition of the insulation of the CT under test.
The DC test voltage is automatically ramped up slowly and the insulation reading in (M) ohms is
displayed on the LCD display screen in digital format. The measured values should be compared with
similar readings obtained previously and a large deviation should call for further investigation. Any
reading in M ohms is considered to be a good insulation. The minimum insulation resistance that is
accepted is 1 M ohms. More than a number, it’s the trending of insulation test results that gives the true
condition of CT insulation.
Insulation readings are greatly affected by the 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. Insulation resistance readings should remain fairly constant
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