Electric, Hydraulic & Electronic Control Systems

It is usual to have three installation wide control systems on a process plant:

- The Process Control System (PCS). This system takes care of running the process and utility systems within normal parameters. Some operators call this system a SCADA system: Supervisory, Control And Data Acquisition.

- The Emergency Shut Down (ESD). This system supervises important /critical parameters in the process and utility system and will shut down the plant if predetermined values are superseded. The ESD system normally has graduated shutdowns with each shutdown level given a name e.g. ESD1, ESD2, ESD3 etc. The levels correspond to severity with an ESD 1 shutting down the complete plant, an ESD 2 perhaps closing down a complete system, etc. See our page on Levels of Shutdown for more details.

- Process Shut Down System (PSD). The lower levels of the ESD system are known as the process shutdown system

- The Fire and Gas System (F&G). This system, unlike a PCS or ESD system, is not process related but facility related. While the first two supervise and control the process, the F&G system supervises and controls hazardous situations i.e. fire or gas release, on the facility.


In addition to the installation wide control systems there are often local control systems:

- High Integrity Process Protection System (HIPPS). Typically a local and normally small control system. In principle this is an ESD system but totally independent to ensure a (high enough) Safety Integrity Level (SIL) according to IEC 61508: Functional safety of electrical/electronic/programmable electronic safety-related systems and IEC 61511: Functional safety - Safety instrumented systems for the process industry sector.

- Local Control System (LCP). Usually housed in a panel close to the piece of machinery that the LCP controls e.g. compressed air unit, water maker unit, HVAC unit. A Local Control System has typically a limited interface to the PCS.

- Unit Control System (UCP). In principle a local control system but has a lot more two way communication with the PSD and ESD. Unit systems are typically turbine control.

- Specialist systems that may feed into LCPs, UCPs or the PCS e.g. anti surge systems on a compressor, or vibration monitoring systems on rotating equipment.


Technical Library

The following pages on Control and Instrumentation.com give more detail on other topics associated with Electric and Electronic Control Systems:




Further Reading - external links

For those who want to delve further into the complex, and sometimes confusing world of control system engineering, or just want to broaden their knowledge of the subject, then the following books will be of interest: