2018, In article laws of change of resistance of contact connections and probability of no failure of low-voltage switching devices depending on the modes of operation are revealed and the method of complex assessment of effectiveness of functioning of low-voltage devices is described. For assessment of technical condition and reliability of switching devices of low tension is offered to use resistance of contact connections. Parameter of probability of no failure is offered to be estimated dependence of resistance of contact connections of devices on number of switching. During operation of low-voltage devices there are total failures of contacts and short-term during one switching, for refusal of contact take excess by resistance of contacts of threshold value. For switching devices by results of the conducted researches the critical value of resistance of the device is accepted at triple increase in initial resistance. At the same time definition of the quantitative characteristics of eff