Definition:
The Grandfather Clause in functional safety allows systems designed before the adoption of ANSI/ISA S84.01 or IEC 61511 to continue operating without full retrofitting to the new standard, provided they are well maintained and can demonstrate safe operation. It originally surfaced in ISA S84-01-1996 and was later carried into IEC 61511-1.
It reflects both the transition from ad hoc or earlier standards to formalized, risk-based safety lifecycle standards and the regulatory realism that massive retrofits are impractical.
Key Points:
- Important under ISA 84 and IEC 61511 transition guidance.
- Requires documented proof of safe operation.
- There is a level of being locked into the original design. Meaning, if major changes to the facility occur, it would trigger the newer requirements.
Example:
A refinery’s SIS installed in 1993, operated and maintained according to its original design and tested per established requirements. No major upgrades or modifications have occurred. During a 2025 safety audit, the facility demonstrated proof of safe performance, so it retains “grandfathered” status. However, if the facility installs a new control system, they must upgrade their SIS to fully meet IEC 61511.
Cited Source:
- ISA 84.01:1996.
- Emerson – Can You Safely Grandfather your SIS (2005)