Doughnut-shaped transformers are often found around the bushings of circuit breakers and power transformers. What type are these transformers?

• A. current transformers
• B. voltage transformers
• C. power transformers
• D. isolation transformers

Answer: (A)

Donut-shaped devices around circuit-breaker bushings are current transformers. The toroidal core surrounds a single conductor, which acts as the primary (one turn); the winding on the core is the secondary that provides a scaled current to meters or protective relays. This arrangement lets you measure high currents directly from the line with electrical isolation from the measurement circuitry, and it’s especially convenient around a bushing where the conductor can pass straight through the hole. The purpose here is current sensing for protection and metering, not stepping down voltage or providing isolation in the general sense. If the CT secondary is not properly burdened, or is left open, it can cause unsafe conditions or saturation, so it must be connected to the appropriate load on the secondary side.