We propose a segment delay fault model to represent any general delay defect ranging from a spot defect to a distributed defect. The segment length, L, is a parameter that can be chosen based on available statistics about the types of manufacturing defects. Once L is chosen, the fault list contains all segments of length L and paths whose entire lengths are less than L. Both rising and falling transitions at the origin of segments are considered. Choosing segments of a small length can prevent an explosion of the number of faults considered. At the same time, a defect over a segment may be large enough to affect any path passing through it. We present an efficient algorithm to compute the number of segments of any possible length in a circuit. We define various classes of segment delay fault tests-robust, transition, and non-robust-that offer a trade-off between fault coverage and quality.