Failures in control systems

There is an increasing demand for detailed safety and reliability analyses of new and existing plant designs. Quite often this will involve fault-tree construction. There are well-known methods for constructing trees for ordinary items of plant. Many attempts have been made to provide computer-based aids. There are some quite difficult problems when control loops are encountered. The major contribution to advance in this area has been by Lapp and Powers. Their algorithm has been the subject of much debate and theoretical argument. This paper reports some very simple laboratory experiments to test the algorithm with a real control system. The algorithm is found to be generally good (with minor discrepancies) for proportional controllers, but somewhat lacking if integral control action is used, as is nearly always the case in practice.