Current Transformer (CT)

An instrument transformer that produces a secondary current precisely proportional to the primary (line) current, stepped down to a safe, measurable level (typically 0–5 A secondary) for use by protective relays, revenue meters, and energy management systems. CTs are essential for measuring the hundreds to thousands of amperes flowing in transmission lines without exposing instruments to high voltages. CT accuracy class determines suitability for revenue metering (Class 0.3 or 0.6) versus protection (Class C). CRITICAL SAFETY NOTE: the secondary of an energized CT must never be open-circuited, as this creates dangerous high voltages in the secondary winding.

Ready to test yourself?

Practice this term with interactive flashcards

Explore more

All games · Transmission Planner · Docket Defender · Energy Persona quiz