As an aid to diagnostic procedures and root cause analysis, provision should be made for identifying the initiating event associated with automatic plant trips through the use of first-out alarms.
Additional Information:
In most conventional alarm systems used in nuclear power plants, first-out alarms, which identified the parameter within an interrelated group that first exceeded its setpoint, were provided to support operators in determining the initiating cause of a reactor or turbine trip. Advanced alarm systems should include this first-out capability along with the results of any additional processing that could improve the identification of the initiating event. First-out alarms work well where all signals respond equally quickly (e.g. electrical 'sequence of events' monitoring), but are not necessarily as useful to operators where response characteristics can be time-variable. This situation arises in process systems because of differential lags in some measurements (e.g. temperature, level) compared to others (e.g. pressure, electrical parameters).