Lecture 9 - Safety and Fail-safe Design

Question 1

What does safety mean as engineers?

Answer 1

It is a property of a system and it is the freedom from accident or loss; the system does not endanger life or the environment

Question 2

What are secondary costs that may follow after an unsafe product?

Answer 2

1) Loss of customer goodwill and/or customers
2) Warranty expenses
3) Litigation
4) Reputation

Question 3

What formula represents product safety from an economic standpoint?

Answer 3

T = P + S where P (Primary cost of a product; including safety measured) and S (Secondary costs)

Question 4

Define the term ‘hazard’

Answer 4

(uh oh); A situation with potential danger to people, environment, and/or material

Question 5

Define the term ‘incident’

Answer 5

(uh oh becomes real); A situation with actual danger to people, environment, or material

Question 6

Define the term ‘accident’ or ‘mishap’

Answer 6

(uh oh affects someone); Events that cause death, injury, environmental, and/or material damage

Question 7

What is a formula used to calculate risk?

Answer 7

Risk = Penalty x Likelihood
(Likelihood of that penalty happening)

Question 8

Define the term ‘risk’

Answer 8

it is the potential that something unwanted and harmful may occur

Question 9

What are three types (manner) of failure handling?

Answer 9

1) Fail operational
2) Fail-over to reduced capability system
3) Fail safe

Question 10

What is ‘fail operational’?

Answer 10

Systems’ functionality dosent dependent on component failure; accomplished through redundancy

Question 11

What is ‘fail-over to reduced capability system’?

Answer 11

1) Simple algorithms
2) Mechanical backup
3) Manual backup

Question 12

What is ‘fail safe’?

Answer 12

System no longer delivers the original function but transitions to a safe mode; may used a reduced capability system as a ‘limp home’ until it reaches a safe state

Question 13

What are two ways to avoid faults from occuring?

Answer 13

1) Careful design to avoid defects
2) Using more robust software and hardware

Question 14

What are two ways to detect and contain faults?

Answer 14

1) Error correction in hardware; redundant CPUs
2) Watchdog timers for failed tasks; raises flags

Question 15

What does FTA stand for?

Answer 15

Fault Tree Analysis

Question 16

What is FTA?

Answer 16

it is a failure analysis in which an undesired state or system is analyzed through the use of boolean logic to link and relate to prior events

Question 17

What can FTA be used for? Name three of its uses

Answer 17

1) To identify the best ways to reduce a risk
2) To determine the probability of an accident
3) To determine a particular system’s level’s failure (isolation)

Question 18

From an FTA standpoint, what would correspond to “AND”?

Answer 18

If two causes are independant and both must occur to lead to the end event

Question 19

From an FTA standpoint, what would correspond to “OR”?

Answer 19

If either event can be a cause/can lead to the final event

Question 20

What is the safe-life design philosophy?

Answer 20

Says that the component or system is designed to not fail within a specific period; also says that testing and analysis provide resonable estimates for the expected lifetime

Question 21

What is the fail-safe design philosophy?

Answer 21

Sort of design that incorporates a plethora of techniques to mitigate losses due to component or system failures; failure is inevitable so prepares for a ‘safe failure’

Question 22

What is damage tolerance design philosophy?

Answer 22

The ability of an asset to sustain an already anticipated damage until the damage is picked up in one of the inspections; safety-by-inspection

Question 23

How can safe-life design be implemented?

Answer 23

Involves testing and analysis to estimate how long the component can be in service before it will be most likely to fail

Question 24

How can fail-safe design be integrated?

Answer 24

Use of redundancy, intentional weak links, physical laws and early detection

Question 25

When should fail-safe design be implemented?

Answer 25

1) Whenever the cost and likelihood of failure outweighs the cost of implementing either fail-safe/safe-life designs

Question 26

What can costs of failure refer to?

Answer 26

1) Physical harm to people/environment
2) Loss/destruction of property/equipment
3) Loss of productivity/use of the failed product
4) Damaged reputation

Question 27

What are elements which cost of implementation may include?

Answer 27

1) Increased expense and time for design and testing
2) Increased production costs
3) Decrease in product performance

Question 28

How to implement safety into design?

Answer 28

1) Try to eliminate hazards from the design
2) Identify causes of hazards and try to reduce their likelihood of occuring through design

Question 29

What are the ways of eliminating hazards from a design?

Answer 29

1) Substitution
2) Simplification
3) Decoupling

Question 30

What are ways to control hazards once they occured?

Answer 30

1) Reduce exposure
2) Isolation
3) Containment
4) Fail-safe design