In instrument transformers, which type is used to measure current without interrupting the circuit?

In instrument transformers, which type is used to measure current without interrupting the circuit?

• A. Current transformer (CT)
• B. Voltage transformer (VT)
• C. Power transformer
• D. Isolation transformer

Answer: (A)

Measuring current without interrupting the circuit is done with a current transformer. It works by clamping around the conductor carrying the load current, so the primary current flows through the transformer without opening or altering the circuit. The secondary winding then supplies a small, proportional current (scaled by the turns ratio) to a meter or relay, allowing accurate current measurement at a safe, low level while the circuit keeps operating. This setup also provides galvanic isolation between the high‑current circuit and the measurement equipment, enhancing safety. The other devices aren’t used for this purpose: a voltage transformer is built to measure voltage, not current; a power transformer is designed for transferring electrical power at different voltages and is not intended for current measurement; an isolation transformer isolates but doesn’t provide a proportional current signal for measurement.

Measuring current without interrupting the circuit is done with a current transformer. It works by clamping around the conductor carrying the load current, so the primary current flows through the transformer without opening or altering the circuit. The secondary winding then supplies a small, proportional current (scaled by the turns ratio) to a meter or relay, allowing accurate current measurement at a safe, low level while the circuit keeps operating. This setup also provides galvanic isolation between the high‑current circuit and the measurement equipment, enhancing safety.

The other devices aren’t used for this purpose: a voltage transformer is built to measure voltage, not current; a power transformer is designed for transferring electrical power at different voltages and is not intended for current measurement; an isolation transformer isolates but doesn’t provide a proportional current signal for measurement.