Connection of power lines in transmission system is activated after defined conditions are met in the setting of synchro-check relays; namely Delta V - difference in voltage amplitudes, Delta f - frequency difference, and Delta alpha - difference in phase angles. The aim of this contribution was experimental verification of the connection condition focused on transient phenomena course on connected line and operation of distance protections in power system, the transient stability of generators, and the analysis of their influence on power system. As the past critical situations (state of an imminent blackout threat) in transmission system has shown, inability to connect line into the power system because the connection conditions are not met (blocking of a power switch) could have severe negative influence on the continual operation of the power network. Setting of synchro-check relays as connection conditions can influence also the process of electric power system restoration after collapse. Thus, this paper discusses also the current state of synchro-check relays setting in transmission system.