Which device is classified as an overcurrent protective device?

An overcurrent protective device is designed to interrupt the flow of electricity when a current exceeds safe levels, thus helping to prevent damage to electrical circuits and equipment. A circuit breaker fulfills this function by automatically disconnecting the circuit when it detects an overcurrent condition, which can occur due to overloads or short circuits.

The key role of a circuit breaker is to protect both the wiring and the equipment connected to the circuit by stopping the current flow in hazardous situations. This characteristic is essential for ensuring safety and preventing electrical fires or equipment failure.

Other devices in the list, such as insulation resistance testers and power factor meters, serve different purposes related to electrical testing and measurement rather than overcurrent protection. Transformers, while crucial in power distribution, do not provide any overcurrent protection by themselves; they are used to change voltage levels and cannot interrupt current flow in the event of an overcurrent situation. Thus, the circuit breaker is the only correct choice as an overcurrent protective device.