A control display appears normal after a shutdown
A chilled-water plant has stopped following a low-temperature alarm. The alarm has been acknowledged, and the operator sees a normal-looking temperature value on the building-management display. A recent test, however, showed that the local temperature sensor reads higher than a verified reference. The water-flow permissive is not currently satisfied, and one maintenance isolation remains recorded as open work. A coworker suggests bypassing the flow input and restarting to see whether the chiller runs. The plant uses a drive that may retain electrical energy after supply isolation. The incoming operator must separate display status, actual process condition, completed maintenance, safe isolation, and the requirements for a controlled restart rather than treating alarm acknowledgment as proof of readiness.
1. What should be resolved before a restart?
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Investigate the alarm cause, confirm credible temperature and flow information, reconcile the outstanding isolation and maintenance status, and restore required protections. The flow condition cannot be bypassed merely to test operation. Follow the approved return-to-service sequence and required authorization.
2. How does the sensor bias affect interpretation, and what does stopping prove about electrical safety?
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A positive temperature bias means the actual value may be lower than displayed under the stated model, which matters to freeze-risk assessment. A stopped drive or dark screen is not verified electrical isolation; use the specified isolation, discharge, and absence-of-energy checks before exposed work.