In an autotransformer, overcurrent devices shall not be installed in series with which winding?

• A. In parallel with the output conductors
• B. In parallel with the shunt winding
• C. In series with output conductors
• D. In series with the shunt winding

Answer: (D)

In an autotransformer, the shunt (common) winding carries only a portion of the total current, while the main current path includes the output conductors and the series winding. Overcurrent protection must interrupt the actual fault current in the circuit. If a protective device is placed in series with the shunt winding, it will only see the shunt winding current, which does not equal the full fault current and can vary with load. That means a fault on the line or load side could occur without the protection device tripping, or the device could trip improperly. The protective device needs to be in series with the main current path so it can reliably interrupt all fault current. Placing protection in parallel with the output or in parallel with the shunt winding would not reliably protect the circuit, and inserting it in series with the shunt winding fails to provide proper protection.