What is the primary purpose of marking polarity on instrument transformers?

• A. To ensure correct phasing for metering and protection
• B. To indicate coil diameter
• C. To show insulation grade
• D. To indicate color coding

Answer: (A)

Polarity marking shows which end of the winding has the same instantaneous polarity as the primary, so the secondary signal preserves the same direction in time. An instrument transformer (current or voltage) provides a scaled, isolated representation of the primary, and the polarity marks tell you which terminals to connect so that the secondary voltage or current has the correct sign and phase relative to the primary. When connected with proper polarity, meters read true values and protective relays respond correctly to faults; reversing polarity can make readings invert or relays trip incorrectly because the phasor relationship is now wrong. These marks are not about coil size, insulation grade, or color coding.