Electrical penetrations (EPs) in nuclear power plants play important roles in keeping a primary containment vessel airtight and in connecting electrical cables from the inside to the outside. Safety-related EPs, which are categorized as “severe-accident (SA) equipment,” are required to maintain their intended functions under various SA environmental conditions after having been aged under the normal operating conditions throughout their service lives. To investigate the insulation performance of EPs in a SA environment, we tested two EPs that are equivalent to those used in pressurized water reactors in Japan. One EP specimen was pre-aged in a concurrent thermal and radiation environment at 100 °C and 100 Gy/h for several thousand hours to simulate the degradation induced during the designated service period. The other EP specimen was used without pre-aging. To simulate the degradation induced by radiation during a SA, the EPs were irradiated with gamma rays to a total dose of 500 kGy at a dose rate of 7 kGy/h. Subsequently, the EP specimens were exposed to saturated steam at 155 °C and 0.45 MPaG for 168 h. The electrical insulation resistance between the insulated cores of cables in each EP specimen were monitored during the steam exposure. The minimum insulation resistances that we measured during the steam exposure for EP specimens with and without the pre-aging are about 109 and 108 £1, respectively, confirming that they retain sufficiently good insulation performance.