Definition:
Continuous Mode describes a system that will have a safety function at all times. Assuming it is a SIF, this means the SIF continuously is acting to maintaining safety. It is “always on” and is not passive.
This and high demand mode both use probability of failure per hour (PFH) as their metric, and not PFDavg.
Continuous mode can be somewhat scary in the sense that if the safety system fails, the process immediately goes into a dangerous state. Two common scenarios of this are burner management systems and overspeed protection on rotating equipment.
Key Points:
- Allows use of PFH rather than PFDavg for SIL calculations.
- Less common but still used in certain applications.
- Wrongly called continuous demand mode. There is no “demand” in that sense.
Example:
A turbine generator in a natural gas plant has an overspeed system. That overspeed system is a SIF with various components and operates in continuous mode.
See also: mode of operation, demand mode, PFH
Cited Source:
- IEC 61511-1:2016, Clause 3.2.39.(c).