Module 9 - Redundancy

Question 1

What is redundancy?

Answer 1

More than a single means of performing a safety function

Question 2

Explain hardware fault tolerance as applied to a MooN architecture

Answer 2

If N is the total number of channels available to perform a safety function, and M is the minimum number of channels, then HFT = N-M

Question 3

In a 1oo2 architecture, how many devices must trip to trip the safety system?

Answer 3

1

Question 4

In a 2oo2 architecture, how many devices must trip to trip the SIF?

Answer 4

Both devices must trip

Question 5

How is SIF architecture typically documented?

Answer 5

Voting block diagrams

Question 6

How is the probability of an AND gate calculated?

Answer 6

P = P(A)*P(B)

Question 7

How is the probability of an OR gate calculated?

Answer 7

P = P(A) + P(B)

Question 8

Where can the calculations for PFDavg with HFT be found?

Answer 8

The formula sheet

Question 9

Is a higher HFT safer or more unsafe?

Answer 9

More safe

Question 10

Does a higher HFT lead to a higher or lower spurious trip rate?

Answer 10

Higher spurious trip

Question 11

What is a common cause failure?

Answer 11

Concurrent failure from a single event where the failures are not consequences of each other

Question 12

Provide an example of common cause failure

Answer 12

Corrosion of a terminal strip

Question 13

What are common mode failures?

Answer 13

Concurrent failures of devices via the same failure mode

Question 14

What is a single point of failure?

Answer 14

A common mode failure due to a common device

Question 15

What is the beta factor?

Answer 15

The probability that if one channel fails the others will fail the same way

Question 16

What is the most important factor in a PFDavg calculator with HFT?

Answer 16

The beta factor

Question 17

What is the default beta value for field devices?

Answer 17

2% for independent measurement devices, 10% for shared tapping point devices

Question 18

What is a typical beta factor for final elements?

Answer 18

5%

Question 19

What is a typical beta for electronics in equipment rooms?

Answer 19

1%

Question 20

When should you determine beta more precisely rather than using a coarse value?

Answer 20

when PFD is close to the SIL boundary

Question 21

How can you improve common mode failures?

Answer 21

Diversity

Question 22

Does beta model systematic failures?

Answer 22

No, only common mode failures

Question 23

Using IEC 61511 how can you achieve systematic capability?

Answer 23

1. Independent certification

2. Prior use

Question 24

When can a device be accepted on the basis of prior use under IEC 61511?

Answer 24

When the device has been supplied with sufficient evidence of low systematic faults, and the device must be used in similar operating environments. Evidence includes the manufacturers quality systems, and volume of operating experience.