Interval representation of logic signals suggested to estimation of the test sequences length for faults in digital circuits with memory. Developed pseudo-Boolean algebra over a set of intervals, which defines the operations that are analogous to logical disjunction, conjunction, and inversion, and neutral elements. On the basis of interval representations of signals, a new method for estimating the length of test sequences for the given fault presented, which has a higher accuracy compared with the metric SCOAP.