This paper proposes a new algorithm to compute the residual risk of failure of an explosion protection system on an industrial process plant. A graph theoretic framework is used to model the process. Both the main reasons of failure are accounted for, viz hardware failure and inadequate protection even when the protection hardware functions according to specifications. The algorithm is shown to be both intuitive and Simple to implement in practice Its application is demonstrated with a realistic example of a protection system installation oil a spray drier. (C) 2009 Elsevier Ltd. All rights reserved