# Control Questions

## Control Questions

# Control Questions

On this page you will find a number of questions. When you have read a chapter of the book, you may use these questions to control that you remember the main topics of the chapter.

### Chapter 1 - Introduction

- What is the main difference between the two concepts "reliability" and "quality"?
- What is the main difference between the concepts "safety" and "security"? - Give some examples that illustrate the difference.
- What is the main difference between a "primary failure" and "secondary failure"?
- What is an "evident failure"? - and what is a "hidden failure"?
- What is the main difference between the concepts "failure mode" and "failure mechanism"?

### Chapter 2 - Failure Models

- List some possible time concepts that may be used to measure the time to failure of a unit - give examples
- Explain verbally what we mean by the concept "failure rate function"
- What is the main difference between the two concepts "failure rate function" and "probability density function"?
- Why is the failure rate function sometimes called the "force of mortality" (FOM)?
- How can the failure rate function, z(t), be expressed as a function of the survivor function, R(t)?
- Mention some practical implications of assuming an exponential time to failure distribution.
- What do we mean when we say that a component with a constant failure rate function is "as good as new" as long as it is functioning?
- Mention some items that might realistically be modeled by an exponentially distributed time to failure.
- Describe verbally what we mean by the concept "mean residual life" (MRL)
- What is the relation between the MRL and the MTTF of the exponential distribution? Give a brief explanation.
- What assumptions do we have to make to use a homogeneous Poisson process (HPP)?
- Describe verbally the meaning of the intensity of a homogeneous Poisson process (HPP)
- What is the distribution of the times between events in a homogeneous Poisson process (HPP)?
- Describe verbally the main difference between the two concepts "failure rate in an exponential life distribution" and "intensity of a homogeneous Poisson process".
- Why is the parameter "lambda" in the Weibull distribution called a scale parameter?
- Why is the parameter "alpha" in the Weibull distribution called a shape parameter?
- What is the characteristic lifetime in a Weibull distribution?
- What do we mean when we say that the Weibull distribution is discontinuous as a function of the shape parameter "alpha" for "alpha"=1? Why is this important?
- Describe the relation between the normal and the lognormal distribution.
- Why is the median less than the mean of the lognormal distribution?
- In what type of applications do we assume the lognormal distribution to be realistic?
- What do we mean by the concept "error factor" when using the lognormal distribution?

### Chapter 3 - Qualitative System Analysis

- What is the main difference between FMEA and FMECA?
- An FMECA can be carried out as a bottom-up analysis and as a top-down analysis. Describe the main difference between these two approaches.
- Mention some drawbacks (problems/limitations) related to FMECA.
- Which questions should be answered in the description of the TOP event of a fault tree?
- What is the main difference between a "basic event" and an "undeveloped event" in a fault tree?
- Is fault tree analysis a suitable technique for analysing a dynamic system? (if not, why?)
- What do we mean when we say that fault tree analysis is a deductive technique?
- What do we mean when we say that a valve is "fail safe"?
- What do we mean by saying that a component is "irrelevant"?
- What is MOCUS?
- What is a "minimal path series structure"?
- How can we qualitatively evaluate a fault tree?
- What is 2-out-of-4 system?
- What is a "coherent" structure?
- Draw a reliability block diagram illustrating a non-coherent structure.
- What is a "critical path set for component no. i"?
- What is the difference between a "cut set" and a "minimal cut set"?
- What do we mean by the "order" of a minimal cut set?
- Mention at least three different approaches to establish the structure function of a system.
- What is "pivotal decomposition"?

### Chapter 4 - Systems of Independent Components

- What do we mean when we say that two components are "independent"?
- Explain why the "reliability" of a component can be written as the mean value of its "state variable".
- In what cases can we use the binomial distribution to determine the reliability of a k-out-of-n structure?
- What do we mean by a "non-repairable" system?
- Will all systems of independent components with constant failure rates have a constant failure rate?
- Give a physical explanation of the form of the failure rate function in Figure 4.3.
- Can you give a physical explanation of why a 2-out-of-3 system of identical and independent components with constant failure rate will have a shorter MTTF than a single component?
- What do we mean by the "upper bound approximation" formula?
- Describe and explain the various elements of the "upper bound approximation" formula.
- What do we mean when we say that the "upper bound approximation" formula gives a conservative estimate?
- In what cases will the "upper bound approximation" be a good approximation?
- What is CARA?
- What do we mean by "cold standby"?
- What do we mean by "warm standby"?

### Chapter 5 - Component Importance

- Explain briefly why Birnbaum's measure of component importance can be considered as a "sensitivity".
- What do we mean by saying that a component is critical for the system?
- Compare and explain the difference between the two definitions of Birnbaum's measure
- How can we use formula (5.5) to calculate Birnbaum's measure of component i?
- How can we use Birnbaum's measure to find the measure Criticality importance?
- What is the main difference between the measure Criticality importance and Fussell-Vesely's measure?
- Can you suggest an alternative to the measure Improvement potential?
- List some possible applications of the verious measures of component importance.

### Chapter 6 - Dependent Failures

- What is a common cause failure?
- Give an example of positive dependency.
- Give an example of negative dependency.
- What are cascading failures? Do know another name of such failures?
- Describe briefly the common cause failure that happened in Browns Ferry.
- How can we classify the reasons for common cause failures?
- Mention some defensive tactics to avoid common cause failures.
- Describe why diversity can be a means to avoid common cause failures.
- Describe the main idea behind the square-root method.
- Describe the main idea behind the beta-factor method.
- How can we interpret the parameter beta in the beta-factor model?
- What are the strengths and weaknesses of the beta-factor method?

### Chapter 7 - Counting Processes

Nothing yet

### Chapter 8 - Markov Processes

- What is a Markov diagram? Do you know alternative names for the same diagram?
- What is a state space? Give an example.
- What is a stochastic process?
- What do we mean when we say that a stochastic process has the Markov property?
- What is a Markov process?
- What are the main assumptions we have to make to use a markov process to model the transitions between the possible states of a system?
- What do we mean when we say that a Markov process has no memory?
- What do we mean by the concept transition probability?
- Describe with words what the Chapman-Kolmogorov equation tells us?
- What is a transition rate?
- Why are the transition rates constant (independent of time)?
- What do we mean by state equations?
- What is a transition rate matrix?
- How can we write the state equations on matrix form?
- What do the diagonal elements of the transition rate matrix tell us?
- Why does not the transition rate matrix have full rank? What is the practical implication of this with respect to solution of the state equations?
- What extra information do we need to find a unique solution of the state equations?
- How can we determine the mean duration of a visit to state j from the transition rate matrix?
- What do we mean by a departure rate? How can we find the departure rates from state j from the transition rate matrix?
- What is the formula for the time dependent availability A(t) of a component with constant failure rate and constant repair rate?
- What do we mean by limiting availability?
- What do we mean by steady state probabilities?
- Write the state equations we need to find the steady state probabilities.
- What do we mean when we say that a Markov process is irreducible?
- What do we mean by visit frequency?
- What formula can we use to find the visit frequency to state j?
- How can we determine the mean time between system failures?
- How can we find the frequency of system failures?
- What is an absorbing state?
- How can we find the survivor function of a repairable system?
- How can we find the mean time to the firs system failure?

### Chapter 9 - Reliability of Maintained Systems

- What do we mean by the concept "availability"?
- What is "average availability"?
- In what cases can we use formula (9.10) to find the "average availability"?

### Chapter 10 - Reliability of Safety Systems

- What do we mean by a "hidden failure"?
- What is the main difference between the failure modes "fail to function" (FTF) and "false alarm" (FA)?
- Why do we carry out "periodic testing"?
- For what type of equipment will it be relevant to carry out periodic testing? (Give some examples)
- What is PFD?
- Give at least two interpretations of the concept PFD?
- In what cases can we use formula (10.3) to calculate the MFDT?
- Will the approximation formula (10.8) produce a conservative or a non-conservative approximation to PFD?
- What do we mean by "staggered testing"?
- What is "staggered testing"? Mention some pros and cons related to "staggered testing".
- What is the main difference between a "random hardware failure" and a "systematic failure"? Give some examples of failures of each category.
- Explain the main difference between a "dangerous undetected" (DU) failure and a "dangerous detected" (DD) failure. Give some examples of failures of the two categories.
- What do we mean by the concept safety instrumented function (SIF)? What is the difference between a SIF and a SIS?
- What is the meaning of the parameter "beta" in the beta-factor model? Do you consider this mode to be adequate for analyzing common cause failures of an SIS? Mention some pros and cons.
- What do we mean by "diagnostic testing"? What type of impacts can such testing have on the occurrence of system common cause failures?
- What is a spurious trip (ST) failure? Give some examples. When designing a safety system, why do we sometimes have a conflict between ST failures and the system availability?
- What is "imperfect testing"? Give some examples of testing that is not perfect. How will this type of testing affect the PFD formula (10.3)? - Illustrate by drawing a sketch of the unavailability as a function of time.
- What is (according to IEC 61508) a "safety lifecycle"?
- What are the main characteristics of the OLF guideline? Do you consider the approach adopted in the OLF guideline to be in accordance with IEC 61508/11?

### Chapter 11 - Life Data Analysis

Nothing yet

### Chapter 12 - Accelerated Life Testing

Nothing yet

### Chapter 13 - Bayesian Reliability Analysis

Nothing yet