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Current Transformer Subtractive Polarity


Staff member
This thread is in response to the Introduction to Current Transformers Part 2: CT Polarity. A comment made by MegaRonin777 who said:

"My apologies, my comment should have been framed in a more positive, constructive manner. Rule one, all protective relay ct’s are wound subtractive. I have run into an occasional additive auxiliary ct but it is not common. Understanding positive and subtractive is important and in the case of protective relaying it is imperative. With understanding the subtractive rule you can assess the orientation of the ct and how to connect it to your equipment correctly. Of course as an installer, you will thoroughly test the ct to prove its orientation (polarity), saturation and ratio."

MegaRonin1777 is absolutely correct. Current Transformers are found to be subtractive polarity. However, I couldn't find any IEEE standards that confirmed this assessment. There wasn't any relevant material in IEEE Standard C37.110 nor C57.13 . The presentation by Steve Laslo at BPA (slide 17) explains that "Instrument Transformers are almost universally ‘Subtractive’ polarity"

The CT circuit drawn at @2:41 of the video is assumed to be subtractive polarity. However, the CT circuit drawn in @3:30 is NOT substractive polarity. It's actually addative polarity.

It should be noted that CTs are almost wound subtractive as MegaRonin777 pointed out. The circuit given in 3:30 should only be used as an example to understand how polarity works.

Perhaps a video tutorial on addatative vs substractive polarity will be useful?