During primary injection testing of a circuit breaker with zone selective interlocking, what should occur?

During primary injection testing of a circuit breaker with zone selective interlocking, the expected outcome is for the breaker to trip instantaneously. This behavior is crucial because zone selective interlocking is designed to enhance the coordination and responsiveness of circuit protection systems.

In an electrical system where zone selective interlocking is implemented, when a fault occurs, the upstream breaker is designed not to trip if a downstream breaker can clear the fault quickly. If the primary injection testing simulates a fault condition, the system's ability to function as intended relies on the downstream breaker tripping immediately to clear the fault and mitigate potential damage to the equipment and maintain system stability.

The instantaneous trip feature during the primary test validates the effectiveness of the interlocking functionality, ensuring that any faults downstream are quickly isolated. This prompt action is essential for safety and prevents further complications in the electrical system.