Module 3 - IEC 61508 and IEC 61511 Flashcards
When should you use IEC 61508 instead of IEC 61511?
IEC 61508 should be used by device manufacturers, IEC 61511 should be used by system integrators and end users in the process industry
For a SIL 4 application, which functional safety standard should be used?
IEC 61508
When is it a legal requirement to comply with IEC 61508?
When specified by government regulation or through a commercial contract
When is it a legal requirement to comply with IEC 61511?
Compliance with IEC 61511 is the same as for IEC 61508 but only as far as IEC 61511 is applicable. This means it is only required by government regulation or through a commercial contract
What is the difference between a SIF, SIL, and SIS?
A SIF is a an automatic function to protect process system. A SIL is a rating applied to a SIF to identify its target failure measures. A SIS is the combination of all the SIFs in a system.
What are the two components of total risk reduction?
Partial risk reduction through a SIS and partial risk reduction through all other means
What is a protection layer?
A protection layer is any independent mechanism that reduces risk by control, prevention, or mitigation
What can be done to optimise the cost and level of protection when designing a SIS?
Setting appropriate acceptable risks, and selecting appropriate SILs
What is the difference between high demand and low demand modes?
Low demand mode is where the SIF is only performed on demand with a frequency of less than once per year. High demand mode is also performed on demand but with a frequency greater than once per year.
What is continuous mode?
Where the SIF retains the process state as part of its normal operation
Why is it important to identify If a system is high or low demand mode?
Because IEC 61508 stipulates different formulas for each mode of operation
Why is it important to identify If a system is high or low demand mode?
Because IEC 61508 stipulates different formulas for each mode of operation
When can low demand be used even if the SIF frequency is greater than once per year?
When the test interval can be guaranteed to be at minimum twice the demand frequency
List the PFDavg and RRF values for each SIL rating
SIL 1 : RRF 10-100 PFD 10^-2 – 10^-1
SIL 2 : RRF 100-1000 PFD 10^-3 – 10^-2
SIL 3 : RRF 1000-10000 PFD 10^-4 – 10^-3
SIL 4 : RRF 10000-100000 PFD 10^-5 – 10^-4
What are the two main types of failures?
Random hardware failure and systematic failure
