In a fault tolerant system, there may not be a back-up component to take over in the event of a component failure. Redundant systems will have back-up components that take over in the primary component fails.
For instance, a DCS system may have redundant processors that take over control if the primary processor fails for some reason. Typically, these systems have redundant power supplies as well. You may also see redundant field devices on critical systems. Boiler drums will usually have several level transmitters set up so that any one failure would not impact the operation of the boiler.
In a fault tolerant system, you may not have any back-up components. The system is designed to continue operations in the event of a failure of one or more components. In thinking about this topic, a newer model automobile engine came to my mind. The emissions controls on these newer engines use inputs from several sensors to control the engine and limit emissions. Typically, these systems would run on what is called a closed loop control, where the sensor inputs are used by the computer to control parameters to the engine. If one of the components fails in the emissions control system, the engine will continue to run, but not as efficiently. The emissions control system will go into open loop mode where a set of fixed parameters is used to let the engine continue to operate at some reduced efficiency level.