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Polarity of Instrument Transformers

Instrument transformers are marked to indicate the instantaneous direction of primary and secondary currents. Usually, one primary and one secondary terminal are marked with a cross (×) or a dot (•) or a square (■) to indicate the polarity.

Operation, Maintenance and Testing Of Electrical Power Equipment (Theory and Practice)
Operation, Maintenance and Testing Of Electrical Power Equipment – Theory and Practice (photo credit: TUYEN LAM TECHNOLOGY ELECTROMECHANICS CO., LTD)

The following conventions apply to either current or VTs with subtractive or additive polarity:

The current flowing out at the polarity marked terminal on the secondary side is nearly in phase with the current flowing in at the polarity marked terminal on the primary. This is shown in Figure 1.

The voltage drop from the polarity to the non-polarity marked terminals on the primary side is nearly in phase with the voltage drop from the polarity to the non-polarity marked terminals on the secondary side.

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