9 Relative Reliability
The contribution of redundancy to improving reliability varies greatly between applications, but some basic guidelines are
useful. Field devices and wiring are the major cause of forced outages. Therefore, adding redundant electronics to a system
with historic field device problems accomplishes little. Improving the quality of the devices and adding redundancy will help.
Two surveys of GE turbine control installations indicated that field devices and wiring contributed to 57% and 69% of forced
outages. Redundancy was applied to both the field devices and the electronics at these sites to mitigate the overall control
system forced outages.
Relative Contributions to Reliability and MTBFO
Adding redundancy to the electronics can improve the MTBFO of the electronics by approximately 15:1 from a completely
non-redundant control to a fully triple redundant control. Between these extremes are redundancy options that can be added to
simplex and dual redundant controls that add to their fault tolerance, as previously discussed. The option with the biggest
impact on the reliability of the electronics is redundant I/O. Control systems with large I/O quantities have a significant
amount of electronics (I/O processors, A/D converters, relays, and so on) that are in the critical path for running and starting
reliability. Selective addition of redundancy to these systems can have a dramatic impact on the reliability of the electronics.
Mark VIe Controls System Redundancy Options