This paper describes and analyzes a cascading fault in an actual power grid. During the temporal evolution of the complicated contingency, not only several fault patterns occurred successively on a 110 kV line, but also a substation main transformer and another 110 kV line, which was powered by the faulted line, tripped due to the operation of some protection devices. This paper uses simulation and theoretical analysis to discuss the operation behavior and reasons of corresponding protection devices, and derives the mathematical formulae of related electric quantities in case of open-phase fault of phase C and lap joint of phase B and phase C. Simulation and theoretical calculation results show that the proposed analysis is effective and correct.