Which statement is incorrect regarding an auto transformer?

In the context of auto transformers, the correct answer highlights a key characteristic of their design and operation. An auto transformer does not have complete electrical isolation between the primary and secondary windings, which is a critical distinction from traditional transformers.

Unlike traditional transformers where the primary and secondary coils are magnetically coupled but electrically isolated, an auto transformer utilizes a single winding that serves as both the primary and secondary circuit. This means that the secondary, or load side, is not insulated from the primary power source. Therefore, the assertion that the primary is insulated from the secondary is incorrect, making this statement align with the true nature of auto transformers.

While the other statements can accurately describe certain aspects of auto transformers, such as the primary being connected to the power source, they do not challenge the fundamental operation of the device as the isolated nature suggested in the incorrect statement does. Auto transformers can function without needing a circuit breaker; however, safety practices may still incorporate circuit breakers for protection against overloads or faults in the system.