If the two main generators failed the back up would automatically start up. The back up was much smaller capacity than the main gens and was only for maintaining critical systems, no AC, no power in the guest cabins, no stoves in the galley. If the gens went down but the main engines were still up the ship could maintain navigation. I can't remember if the fresh water system was on backup or not. A fire in the engine room would really suck, we had a fire in the laundry once which was bad enough.