Which device is typically donut-shaped and located around the bushings to measure current?

• A. current transformer
• B. voltage transformer
• C. power transformer
• D. potential transformer

Answer: (A)

The device being described is a current transformer. Its donut or toroidal core is placed around the conductor (often through a bushing) so the primary path is the single wire itself. When current flows, it creates a magnetic field in the core, and the transformer action induces a proportional current in the secondary winding. This setup provides electrical isolation and delivers a scaled current to meters or protection relays, making it ideal for measuring line current without connecting directly to the high-current circuit.

A voltage transformer, by contrast, couples to the line to sense voltage and is not designed to encircle a conductor to measure current. A power transformer is a larger device used to transfer energy and step voltages between circuits, not for direct current measurement on a line. A potential transformer is another term for a device that measures voltage, not current.