AADvance Safety Manual - Tuv-fs.com

Chapter 4 AADvance System ArchitecturesThe internal diagnostics, therefore, detect and reveal both covert and overt failures. Adual module arrangement, for example, with diagnostics can address covert failuresand help redress the balance between failure to respond and spurious responses. Adual system could therefore be 2oo2D reverting to 1oo1D on the first detected faultand reverting to fail-safe when both modules have a fault.Self-test facilities used to diagnose faults within the remainder of the system are definedto provide optimum system and functional safety availability. These self-test facilitiesmay require short periods of off-line operation, or introduce conditions such as alarmor fault test conditions, which effectively result in the channel (point) being off-linewithin that redundant channel. Within fault tolerant configurations, this period of offlineoperation only affects the system's ability to respond under multiple faultconditions.Document number 553630 Issue 7: February 2010 4-13