Which protective device provides a time delay and must be replaced when actuated?

The device that provides a time delay and must be replaced when actuated is the fuse. A fuse functions as a safety component in electrical systems, designed to protect circuits from overcurrent conditions. When the current flowing through the fuse exceeds its rated value, the conductive element within the fuse melts, breaking the circuit.

This melting reaction serves as a time delay, allowing short, temporary surges in current (such as those caused by power ups or down fluctuations) to pass without blowing the fuse, while still providing protection during longer-term overcurrent situations. Once a fuse has operated and has blown, it cannot reset and must be physically replaced to restore functionality to the circuit, which characterizes it as a one-time-use device.

In contrast, thermal circuit breakers and thermal-magnetic circuit breakers can reset once the overcurrent condition is resolved, allowing them to be reused without replacement. Meanwhile, magnetic circuit breakers are primarily designed to act on short circuits, using electromagnetic forces but without a time-delay feature comparable to fuses.