4.1.6 Instrumentation to provide post-incident records
The detectors required to provide these records are derived from other categories of instrumentation, mainly from the supervisory instrumentation. These records provide critical plant data for designated periods before and after an incident to enable the cause to be investigated.
The recording is initiated by a unit or plant trip (for example, the trip of a feed pump), or directly by the operator on demand. These records are produced by data processing computers and, since continuous scanning of all the input parameters is necessary, some compromises have to be made in the scanning speed. Thus, assuming a minimum scanning interval of 3 s and depending on their relative positions in the scanning cycle, a parameter might respond up to 3 s before or 3 s after the recording shows the trip to have operated. In specifying the requirements of the data logging system, the turbine plant designer considers these limitations to provide a compromise which is both cost effective and valuable to operators.