In an autotransformer, overcurrent devices shall not be installed in which location?

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

Answer: (A)

The essential idea is that an autotransformer uses a shunt (common) winding that is shared between the input and output sides to transfer part of the current. Because this winding sits between the source and the load, a fault can involve currents flowing through multiple paths due to the transformer action. Placing an overcurrent protection device in series with the shunt winding places the device in a part of the circuit that can remain energized through the other windings even when the device opens, making it difficult or impossible to interrupt the fault properly. This could leave the circuit partially energized or subject the protective device to currents it wasn’t designed to interrupt. For that reason, you don’t install overcurrent devices in series with the shunt winding. The protection should be arranged so that it can interrupt the main current paths on the line and load sides, rather than through the shared winding.