For a time constant versus input rate curve, as shown in Figure 6-16, there is a specific probability of obtaining a false safety signal for any particular trip point setting, the number of ‘ occurrences of the false signal being related to the time constant curve of the circuitry. * Table 2-3a shows the interrelationship between the point of operation (i. e., the particular input rate), a trip point count rate which has been set, and the resultant average number of spurious trips per unit time for this subsystem. Note that the functional variation is very rapid and therefore relatively small changes in time constant can greatly increase the average number of spurious trips. For this subsystem, operation up to an input rate of 3 x 10^ cps with a trip setting of 5 x 10^ will probably have no spurious trips from this cause during the life of a plant.

♦See Section II for a derivation of these’ relationships.