1). Metering CT Classes
Metering CTs are utilized to properly measure current under typical conditions. They are essential for billing, energy management, and load analysis.
- According to IEC standards, common classes include 0.1, 0.2, 0.5 & 1.0.
- Lower class numbers indicate greater accuracy in the current measurement.
- Designed to work accurately between 10% and 120% of rated current.
- They are unreliable at large fault currents.
- These CTs may saturate under fault or overload conditions.
- Saturation causes inaccurate readings and relay breakdown.
- Used in energy meters to bill residential, commercial, and industrial customers.
Ex: CT of class 0.5 is utilized in an HT metering panel.
2). Protection CTs (Current Transformers)
Protection CTs (Current Transformers) drive relays during fault conditions. They ensure quick and precise fault detection and resolution.
- Common classes include 5P10, 10P20 & Class PS.
- “5P10” indicates a 5% inaccuracy up to ten times the rated current.
- Performs well at fault current levels ranging from 10 to 20 times normal. Maintains output accuracy to ensure the relay trips correctly.
- These CTs can withstand strong fault currents without becoming saturated. This ensures that the relay operates reliably even during short circuits.
Ex: Utilized in overcurrent, differential & REF protection methods.
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