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Understanding CT Polarity: A Critical Factor for Accurate Protection and Metering

Posted by lyz

What Is CT Polarity?

Current Transformer (CT) polarity refers to the directional relationship between the primary winding and secondary winding. It determines how induced current flows in the secondary circuit when current passes through the primary conductor.

According to international standards:

Primary winding: Beginning marked as P1, end marked as P2
Secondary winding: Beginning marked as S1, end marked as S2

In standard wiring, P1 corresponds to S1, and P2 corresponds to S2 — these are called “same polarity ends.”

Why Polarity Matters？

Subtractive vs. Additive Polarity

Subtractive Polarity (Standard)

Current flows into P1 and out of P2 on primary side
Current flows out of S1 and into S2 on secondary side
Used in 99% of modern protection and metering applications

Additive Polarity

Current flows in same direction at corresponding terminals
Rarely used; only for specific legacy applications

How to Verify CT Polarity？

Battery Test Method:

Connect a DC battery momentarily to primary terminals (P1 positive, P2 negative)
Connect a DC milliammeter to secondary terminals (S1 positive, S2 negative)
Observe deflection direction when battery is connected and disconnected
Correct polarity: Positive deflection when connecting, negative when disconnecting

CNMIBO CT Solutions

All CNMIBO current transformers feature:

✓ Clear, permanent P1/P2 and S1/S2 terminal markings
✓ Standard subtractive polarity configuration
✓ Multiple accuracy classes (0.2, 0.2S, 0.5, 0.5S, 1.0) for protection and metering
✓ Comprehensive installation documentation

Need support? Our technical team provides polarity verification guidance for every installation scenario.

Understanding CT Polarity: A Critical Factor for Accurate Protection and Metering

What Is CT Polarity?

Why Polarity Matters？

Subtractive vs. Additive Polarity

How to Verify CT Polarity？

CNMIBO CT Solutions

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