Making complex protection and control systems easy to maintain

Process bus, the concept of distributed I/O for protection and control, has long been promoted as more cost-effective than traditional systems. I/O devices can potentially be manufactured as part of primary equipment, effectively eliminating the need for custom copper wiring design, installation, and terminations. A large part of the business case is that the cost of the additional devices (I/O devices and communications devices) will be more than offset by time and labor savings. However, this is only the capital cost of process bus. There is also an operating and maintenance cost to consider: will the increased number of devices that make up a process bus protection and control system lead to increased costs due to device reliability. Simple reliability analysis shows that process bus has the same level of availability as conventional protection systems. However, the overall reliability, in terms of mean time between failures (MTBF) will be greatly reduced, due to the large number of devices that comprise the system. Based on this simple reliability analysis, a key requirement for process bus must be to limit process down time. The best way to do this is to use simple to replace devices and components. The goal is exactly that of the commercial aviation industry, where uptime and availability is critical: modular components, with simple physical interfaces, and limited configuration requirements drive process down time towards 0 through simple swap out of failed components. Process bus systems, properly designed, permit this reduced process downtime, offsetting the smaller MTBF of the system through simple component replacements.