Hell generators used to be generators that ran on little amounts of the energy discovered in Hell. But when the UAC tried to open a dimensional "window" to hell, a wave of demonic energy surged through the Mars base and supercharged the generators, turning them into miniature portals to Hell which allowed the demons to begin their invasion. The only way to close these portals is with the manual shutdown button. As a side effect to the abrupt shutdown, the overflowed generator will combust.