Plasma breakdown using second-harmonic electron cyclotron (EC) waves has been studied experimentally in three helical devices, Heliotron J, TJ-II and CHS. Recent technological progress on electron cyclotron heating (ECH) systems enables us to determine what the important factors for second-harmonic plasma breakdown are. Comparison of the experimental results among three devices shows a common feature that the plasma starts up from the good confinement region and strongly depends on the nonlinear interactions with the electric field of the local beam rather than the multi-reflected field. This suggests that the confinement of high energy electrons generated by ECH has a dominant role in second-harmonic plasma breakdown.