|A frequently used method for monitoring and fault diagnosis of the electric equipment is the one based on the analytic model. The paper applies the diagnosis method based on the analytic model by investigating the waveforms of the currents and voltages specific to a transport and dis-tribution line when interruption fault occurs. This method can be used when the system behavior is compared with the results of the mathematical model which reproduces the system in normal operation conditions. For identifying the fault, the waveforms corresponding to different faults are stored in an information database and compared with the ones corresponding to the normal operation. The paper presents a microcontroller based system used to control switching of interruption fault for transport and distribu-tion lines. In order to reduce the breaker stress due to the commutation overvoltage, the paper presents an analysis of the optimum instant when a long line must be disconnected. Similarly, an analysis is performed on the instants of the long lines reconnection. The both analyses are performed taking into account the specific parameters and the length of the lines. The focused magnitudes during the transients are both the maximum values and the slopes of the overvoltage and the maximum values of the currents.