The article describes the mathematical and software module of computer-aided design of vehicle electric systems: selection and verification of protective devices. The following problems are solved: calculation of currents in the circuit sections of scheme for normal and directive modes taking into account the changes in the current distribution as to the modes of consumption, calculation of the currents in the circuit sections of design scheme for short-circuit modes, checking the correct choice of protection devices as to the rated current, checking the correct selection of protection device in overloads in case of electric motor loads, determining the sequence of triggering of protection devices in the modes of short-circuit and checking the selectivity of protection, checking the protection devices for resistance to short-circuit currents