Module 11 - Logic Solvers

Question 1

What are the 4 types of logic solvers?

Answer 1

1. Relays
2. Solid state logic
3. PLCs
4. Safety PLCs

Question 2

What is a safety configured PLC?

Answer 2

IEC 61511 permits safety configured PLCs up to SIL2 if they can satisfy requirements for prior use

Question 3

Is it worth using an ordinary PLC in a safety configured application?

Answer 3

Not usually, high effort not warranted

Question 4

What are the two common safety PLC architectures?

Answer 4

1. TMR 2oo3D

2. 1oo2D/2oo4D

Question 5

What is one advantage of 2oo4D vs TMR safety PLC architectures? What is one advantage of of TMR over 2oo4D?

Answer 5

2oo4D has a lower spurious trip rate but TMR has a faster scan time

Question 6

When is a communication link safety rated?

Answer 6

When at least one SIF fails to function because of the failure of the comms link

Question 7

What is the difference between white and black channel communication?

Answer 7

White channel communication has the entire link compliant with 61508/61784. Black channel only has the interfaces compliant

Question 8

What constitutes separation of the BPCS and the SIS?

Answer 8

A BPCS failure shall not prevent the SIS from successfully executing any SIFs

Question 9

What are three issues with integrating the BPCS with the SIS?

Answer 9

1. Systematic failures
2. Security breaches
3. Common mode failures