Question -
Silversands Lifts (SL) are building a new design of computer controlled lift. The company and customer between them decide that the target probability for the event 'Passenger Injury" should be lx101 per average operating hour. The system has hydro-mechanical components and will be computer controlled. SL will need to interface with a number of organisations who are producing sub-systems, such as the control unit. SL have created an initial fault tree for the event Passenger Injury" based on their initial design, see Figure 3.1.
A basic event means that the design has progressed to component failure events and that SL have access to historical data. An undeveloped event either means that more design activity is required or that this event is an interface event to subcontractor equipment. Where an undeveloped event occurs in the fault tree SL place a target probability on this event.
If the same failure mode of multiple components of the same type appears in the tree, the instances are labeled U1a, U1b, etc. If the same event relating to the same component/system appears multiple times it is given the same label each time it occurs (e.g. all instances of B5 relate to the same event).
(i) Produce the minimal cut sets for this tree.
SL has accumulated the following failure rate data and target rate data. Calculate the probability of passenger injury using this data.
|
Component
|
Failure rate per hour
|
Component
|
Failure rate per hour
|
|
U1
|
1x10-3
|
B1
|
1 x 10-4
|
|
U2
|
5x10-3
|
B2
|
2 x 10-4
|
|
U3
|
1 x 10-4
|
B3
|
3 x 10-4
|
|
U4
|
2 x 10-5
|
B4
|
4 x 10-4
|
|
U5
|
5 x 10-4
|
B5
|
5 x 10-4
|
|
B6
|
6 x 10-4
|
|
B7
|
7 x 10-4
|
|
B8
|
8 x 10-4
|
(ii) The control systems sub-contractor have used event U4 as their top level event. They have produced the fault tree presented in Figure 32 and provided the following reliability data They did not have access to the tree produced by SL
|
Component
|
Failure rate per hour
|
Component
|
Failure rate per hour
|
|
CB1
|
5 x 10-4
|
CB4
|
6 x 10-4
|
|
CB2
|
2 x 10-5
|
CB5
|
5 x 10-4
|
|
CB3
|
7 x 10-3
|
CB6
|
4 x 10-4
|
Produce the minimum cut sets and calculate the probability of this event
(iii) SL wish to incorporate the results of the control system contractor's work into their analysis. Generate the cut sets for the combined tree and calculate the probabilities. What are the implications of these results and how would you go about addressing them?
(iv) Explain how you would use fault trees to address analysis of systematic failures. Illustrate using the fault trees presented in this question.