Definition:
The Rule of 20 or the “20, 20, 20 rule” is an informal adage describing the typical lifecycle of a SIS.
- 20 weeks to design the SIS
- 20 months to implement and install the SIS
- 20 years in operation
This rule highlights that while the design and implementation phases are relatively short, the vast majority of a SIS lifecycle is spent in operation and maintenance, over 90%. Thus do not skimp on functional safety management in the 40 weeks of design and implementation. It is very impactful.
Key Points:
- Encourages early lifecycle planning for long-term maintainability and safety compliance.
- Highlights that most SIS failures and risk exposures occur during the decades-long operation, not the brief installation/design phases.
Example:
A new SIS is designed over five months, installed in just over a year, and then operates for decades—so the majority of proof testing, audits, maintenance, and modifications happen after initial commissioning.
See Also: FSM