Operations of opening or closing specific breakers/switches in a substation should be blocked in some circumstances for human and power network safety requirements when they are performed. Whether an operation on a breaker/switch should be blocked relies on the working status of the substation at that time， i.e.， relies on the connection status of the power network at that time. By using the concept of adjacency matrix in graph theory with modifying the meaning of elements in matrix and the meaning of matrix operations properly， the following problem is solved: for one substation， based on primary wiring diagram， static expressions（block formula） may be obtained with normative steps and methods. Those expressions indicate whether operations on breakers/switches should be blocked according to the working status of the substation， or in other words， indicate that the logical relations between whether operation on a specific breaker/switch should be blocked and the status of other breakers/switches.