What is the defining characteristic of a "protected" circuit?

The defining characteristic of a "protected" circuit is that it has a fuse or circuit breaker designed to interrupt current flow in the event of a fault. This safety feature is fundamental in preventing overloads and electrical fires by breaking the circuit when an unsafe condition arises, such as a short circuit or an overload situation.

Fuses and circuit breakers serve as critical components in maintaining electrical safety, as they ensure that excessive current does not continue to flow through the wiring or connected devices. This protective mechanism helps to safeguard not only the electrical infrastructure but also the users and property from potential hazards associated with electrical faults.

While certain other options may involve aspects of electrical circuits, they do not embody the essential function of providing protection against electrical faults as effectively as a fuse or circuit breaker does. Thus, the presence of these devices is what fundamentally characterizes a "protected" circuit.