Read these short articles:
• IEEE: What We Can Learn about Reliability and Safety Analyses from Different Industries (Links to an external site.)
• IEEE: Foreword: Reliability and Safety in Real-Time Systems (Links to an external site.)
Reliability analysis often begins with Failure Modes and Effects Analysis (FMEA). The FMEA is performed by considering each component and asking what the component’s failure modes are and the effects of the failures on the system. This is a “bottom-up” approach. Fault trees begin with a top event, and the analyst asks what could cause the top event. This is a “top-down” approach. Thus, one says that FMEAs are bottom-up, while fault trees are top-down.
Fault trees can be used in reliability analysis, but the FMEA approach followed by constructing a reliability block diagram is more typical. Fault trees are often used for hazard analyses. The top-down approach leads to creative thinking about what can go wrong and is helpful for generating a list of hazards.
The system safety hazard analysis and the reliability analysis are complementary ways of ensuring system safety and reliability. The reliability analysis helps ensure hardware robustness, while the system safety approach looks more globally at hazards that can affect people, hardware, and facilities. Often the hazard analysis will find hazards that can cause system failures that the reliability analysis misses, due to ground rules and assumptions in the reliability analysis.
Figure 4.1 shows a simple warning system circuit. Whenever there is fault the system is designed to turn the indicator light on. Based on the circuit in Figure 4.1, one could prepare the fault tree in Figure 4.2.

Figure 4.1 – Warning System Circuit

Figure 4.2 – Fault Tree

Review Chapter 15, from, System Safety For the 21st Century: The Updated and Revised Edition of System Safety 2012 / Richard A. Stephans, page 175. For example, see Figure 15-3 in the textbook.
Assignment:
• Use Figures 4.1 and 4.2 to create your own warning system circuit and fault tree. The final alert might be a light, buzzer, or another sensory queue.
• Choose a system from the list below or one of similar complexity.
o Fuel level indicator
o Water temperature indicator
o Aircraft Altimeter  I choose this one
o Stall warning
o Alarm clock audio alert
o Toaster pop-up mechanism
o Child danger alert sensor
o Door lock release system
o Obstruction proximity alert
o Swimming pool gate switch

• Write a paper describing the system, how it works and listing possible failure modes.