The article briefl y describes the algorithm of searching decision over the time zone for analysis of linear and nonlinear electric circuits, used by the Ngspice circuit simulation software. The article also proposes the measures for improving the performance of the algorithm for the purpose of reducing the CPU time for simulating the complex circuit objects, such as railway automation and remote control systems. The CPU time, needed to perform the experiment, is proposed to reduce on the expense of «inbuilt» features of simulating environment (improvement of convergence conditions for current and voltage of the circuit branches, that include open contacts), as well as modifi cations of the algorithm of the program (increasing the convergence of the Newton-Raphson method, using of additional matrix of conductivities of linear circuit elements and parallel computing).