What feature must be provided when flame sensing detectors can fail in the flame-proven mode?

The requirement for a self-checking feature when flame sensing detectors can fail in the flame-proven mode is critical because it ensures that any failure in the flame detection system is identified immediately. This preventive measure contributes to maintaining the safety and reliability of combustion systems by automatically assessing the functionality of the detectors.

In flame-proven systems, the expectation is that the detection mechanisms will reliably signal the presence or absence of flame. If the detectors can fail, the self-checking feature serves as a proactive solution to verify that the flame detection is operational without the need for manual intervention. It reduces the risk of catastrophic failures that could arise from undetected flames or improper operations, thereby enhancing overall system safety.

The other choices do address important system characteristics, but they do not specifically emphasize the necessity for continuous operational verification that the self-checking feature provides.