Human Machine Interface (HMI) for SCADA systems is providing the functions of status indication, operator intervention in control action. Alarm reporting, and data storage and programming. Several layers or levels of operator interfaces are required to provide a maintainable and reliable system: controller level, equipment level, and supervisory level. At the supervisory and controller level, HMI may also provide capability to modify the controller program.
Equipment level HMI should consist at minimum of the control indicators and switches necessary to permit an operator to control manually the equipment in the absence of communications from the controller or for maintenance purposes. The capability of manual control to be provided for mechanical and electrical system components:
1. Manual control substitutes the operator facility for the automatic control system in the feedback loop, and leads to the risk system or equipment mis-operation due to human error. Safety interlocks, such as high pressure switches, motor overload, fire detection etc. switchgear protective relaying required for fault protection should always be hard-wired in the circuit breaker.
2. Hard-wired manual controls for entire facilities have been located in a bench board or control panel at the control room. Although this simplify the operator intervention upon complete failure of the automatic control system.