First of all we will find out what is polarity and then we will see why it is so important in CT.
What is Polarity ?
It is the Polarity of Voltage present between High Voltage Terminal and the Low Voltage Terminal. It can also be referred as, the direction of the current flow.
In the Ac System, the terminals are changing their polarity periodically, and the direction of the current also changes accordingly.
The Polarity of a CT is determined, by the direction in which the coils are wound, around the CT Core and by how the leads are brought out of the CT Case.
What are the different types of Polarity?
The Polarity marks on a CT designate, the relative instantaneous directions of the current I.e for a given direction of Primary current (entering), what should be the direction of the secondary current (leaving).
There are two types of Polarity, Additive & Subtractive Polarity.
Primary and secondary side voltage is in phase. Primary Current (Ip) will be in Phase with the secondary current (Is). And so they will have the same direction of the flow. It is indicated by the Dot markings at the top of the Primary and secondary windings. The Dot at the Primary winding , indicates, that the current is entering the Primary winding, while the dot at the Secondary winding indicates, that the current is leaving the secondary winding.
Substractive Polarity, Primary and Secondary side Voltage are 180 deg out of phase.
Why is it so Important?
Wrong Polarity test in Protection core, may result in Mal Operation of the Protection Relay. I.e the Relay may operate in the Normal Condition or may operate for External Fault.
Wrong Polarity in Metering Core, may result in wrong metering.