It is interesting to note that these group commands need not be fully automatic, but can be semiautomatic also.
All subgroups, subloops, and drive interfaces (as applicable) under each of these groups shall be started one by one under this single group command. For group starting or shut down, each group needs separate command. When in the unit level, all groups are started one by one from a single start/stop command. At this group level, independent automatic start up and shutdown is possible. GROUP LEVEL CONTROL: Such controls are quite common in most of the systems such as BMS, ATRS, ATT, etc., as discussed in Sections 2.3 and 2.4 of this chapter. While designing a control system of this kind, the safety aspect is given the highest priority at every stage so that the sequence may proceed properly. Such start up or shut down is done at the unit level. UNIT LEVEL START UP AND SHUT DOWN: It is possible nowadays to start up/shut down a complete plant from a single keystroke. It is purely the design philosophy of the manufacturer. However, some system drive controls are implemented in I/O cards. A hierarchical structure may be helpful for sequential control implementation. OLCS HIERARCHY: As discussed above, the OLCS system can be conceived to be arranged in a hierarchical structure, such as unit level, group level (such as boilers, turbines, feed water systems, etc.), subgroup level such as the evacuation subgroup, and drive level. For OLCS, a few features shall include but are not limited to the following: 1. OLCS functions shall include sequential controls (ATRS), I nterlock, and protection for various plant auxiliaries, valves, dampers, etc.