This paper deals with bubble behavior and electrical insulation at quenching state in superconducting coils. Bubble behavior in the simulated electrode system which consists of concentric coil layer-to-cylindrical electrode system with an insulation barrier and spacers immersed in liquid nitrogen was categorized into two types according to the cooling channel width (lc). In this study, bubble behavior and partial breakdown (PBD) characteristics are investigated with lc being narrower than the diameter of induced bubbles. Experimental results show that the bubbles are trapped in the groove between coil turns in the tested electrode system even at no applied voltage. Behavior of trapped bubbles depends on the applied voltage and shape of spacer which was installed for positioning of electrodes. The spacer having no aperture produces the most severe condition for electrical insulation: tight trap of bubble, excitation of electrohydrodynamic (EHD) instability on the surface of trapped bubble and lower inception voltage of partial breakdown in cooling channel than that with the spacer having an aperture. The pressurizing of liquid nitrogen is effective for suppression of active boiling and improvement of electrical insulation performance in the tested electrode system.