With expiration of equipment's lifetime electronic switchyards and development of new tehnologies it comes to replacing old equipment with new one. In that purpose, replacement of old air blast circuit breaker with new SF6 circuit breaker is taking place in 220 kV switchyard thermal power plant Rijeka. For better understanding purpose of circuit breaker replacement, there is comparation of air blast and SF6 circuit breakers, their principles, advantages and shortcomings. It's also given description of switchyard and it single line diagram and dispositional draft. For circuit breaker in all conditions to be able to stop interrupting current failure it's needed to conduct calculations that proves lowest possible condutions which can show up during short circuit. In that purpose it's given calculation 220 kV SF6 circuit breakers. It's been made calculation for breaking and thermal current circuit breaker, calculation of influencing forces due to own weight of conductor, due to wind and example calculation of tensile force of circuit breaker during the time of short circuit. At the end it's been given description of switchyard that will be put in operation after refurbishement.