Reasons for installation and implementation of on-line condition monitoring system in medium voltage switchgear have been described in this thesis. On-line condition monitoring system is the basis for transition from time-defined maintenance of apparatus to maintenance according to the condition of apparatus, which can lead to significant cost savings and prolong lifetime of apparatus. On-line condition monitoring system is based on using numerical feeder terminals and joined sensors for monitoring apparatus features which are very important for reliable functioning of apparatus. Principles of maintenance of apparatus have been described after the introduction. Advantages and disadvantages of each principle have been stated as well as law regulations which define the maintenance. All the advantages for using on-line condition monitoring system of apparatus and switchgear have been stated according to cost savings and technical criteria for life management of switchgear. On-line condition monitoring system of switchgear and apparatus has been planned with using feeder terminal REF 543 and on-line monitoring circuit breaker device. Features which can be controlled by feeder terminal have been stated, as well as further improvement of on-line monitoring circuit breaker device. Selected monitoring apparatus and reference criteria for monitoring features have been described in the recommendation of configuration of on-line condition monitoring system. Economic effect of implementation of on-line condition monitoring system has been analyzed at the end of thesis, by comparison of costs made due to non-delivered electric energy in a defined time period and costs of equipping circuit breaker/switchgear with on-line condition monitoring system equipment.