In previous work Petri net models were developed by the authors (1977) for detecting fault locations in general. In the current paper, the models have been modified to include backup protection as well as primary protection. The modification overcomes many limitations of the original models. Hence, the advanced models are more capable of analysing fault in a highly meshed network or network with more than one layer of protective devices, such as those in substations. The developed methodology is tested using an actual substation network. Fast and accurate results are obtained, and the proposed method has great potential for on-line applications.