Do many people take the Hardware Fault Tolerance Credit?
Not many. Although taking the fault tolerance credit decreases some cost, one less sensor, installation, start-up, and maintenance cost, it could decrease availability. It is difficult to tell a transmitter failure from a process failure using only the analog output of the transmitter. Using a single sensor can trip the safety system for a sensor fault. A voting system allows the process to continue and increases the Mean Time To Repair for a sensor failure which increases up-time. This will change with the introduction of Smart SIS logic solvers. A Smart logic solver will be able to distinguish a sensor failure from a process failure. The Smart logic solver is smart enough to not trip the system for as sensor failure.
