4.1.2-6 First-Out Processing

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).