During a routine inspection of an HMI screen, an operator reports that a critical alarm message ('High Temperature Alarm') is appearing, but no audible alarm is sounding and no flashing light is active. The HMI clearly shows the alarm state. What is the most likely cause of this discrepancy?

Question: During a routine inspection of an HMI screen, an operator reports that a critical alarm message ('High Temperature Alarm') is appearing, but no audible alarm is sounding and no flashing light is active. The HMI clearly shows the alarm state. What is the most likely cause of this discrepancy?

Answer options:

• The PLC program is not generating the alarm signal.
• The HMI is displaying a 'stale' or outdated alarm message.
• The HMI's alarm acknowledgement button is stuck. ✅ The physical alarm annunciator (horn/strobe) is disconnected or faulty, or its output from the PLC is not active.

Correct answer: The physical alarm annunciator (horn/strobe) is disconnected or faulty, or its output from the PLC is not active.

Explanation: If the HMI clearly shows the alarm and it's active, the issue isn't with the PLC generating the alarm or the HMI displaying it correctly. The most likely cause for the lack of audible/visual indication is a problem with the physical annunciator itself (e.g., disconnected, faulty horn/strobe) or the PLC output controlling it is not active when the alarm condition is met.