Which of the following is NOT a common operating function of a circuit breaker?

The correct answer is based on the understanding of how circuit breakers operate in comparison to fuses. Circuit breakers are designed to interrupt current flow and protect circuits from overloads and short circuits. They achieve this through various operating mechanisms, but not through a melting fusible element, which is a characteristic of fuses.

Current interruption in circuit breakers typically relies on mechanisms such as thermal, magnetic, or electronic sensing to detect fault conditions. When a fault occurs, the mechanism triggers the breaker to open, stopping current flow. This process does not involve a fusible element melting, which is specific to fuses that rely on the element's physical properties to achieve current interruption.

The other options reflect actual operating functions of circuit breakers. The current-sensing means (thermal, magnetic, electronic) are essential for detecting overcurrent conditions, while the current-voltage interrupting means cover the methods used to separate contacts and quench the arc formed when contacts are opened. The unlatching mechanism refers to the mechanical components that allow the breaker to reset and close again after it has been tripped.

Thus, the inclusion of a melting fusible element is consistent with fuses, not circuit breakers, validating why this choice is correct as the function described is not found in circuit