This paper presents a technique for the diagnosis of short circuits in power transmission networks. A three-layers diagnostic engine is formally defined, including local interpretation, global interpretation, and heuristic interpretation. The local interpretation focuses on the behavior of single protective components that are distributed over the power network and operate when a short circuit occur. The global interpretation provides a global behavior of the protection system by combining consistent local behaviors. The heuristic interpretation is meant to localize the short circuit and possibly faulty protection components within the transmission network.