Definition:
The main interlock list (MIL), which can be called various other things, defines how Safety Instrumented Functions (SIFs) and other critical protective functions relate to process events and system responses. This document can and will be called many different things. It can be confusing. This is usually done with a spreadsheet.
Other names: cause and effect diagram, cause and effect matrix, the C&E, or an interlock list.
In documenting how controls work a MIL can never get to the same degree of complexity as a logic diagram. Having only an MIL for a large DCS would not typically be possible. You would need a logic diagram for pump control, all sorts of different start-up and shut-down states, etc. But an MIL often does make sense for a SIS because an SIS is relatively simple.
The functional narrative, main interlock list (MIL), and the logic diagrams together comprise the primary design inputs to the programming effort.
Key Points:
- The MIL is often the master document coordinating shutdown system design.
- Interlock lists catalog all automatic trips and shutdown actions.
- Not technically required in a SIS design as other documents such as functional narratives or logic diagrams may be sufficient.
Example:
A high pressure trip causes an automatic closure of the inlet valve and shutdown of the feed pump as shown in the Cause and Effect diagram.
See Also: functional narrative, logic diagram, C&E
Cited Source:
- ISA TR84.00.03-2002.
- Excellent article on LinkedIn – Cornerstone of Successful Automation