Root cause fault tree

The disciplines of engineering and quality control have long recognized the principles of root cause analysis. Some process safety tools for root cause analysis have been borrowed from these disciplines. For example, fault tree analysis was developed as an engineering tool, but its logic tree structure has been adapted to meet process safety requirements. 

Another type of logic tree, the event tree, is an inductive technique. Event Tree Analysis (ETA) also provides a structured method to aid in understanding and determining the causes of an incident.(i) While the fault tree starts at the undesired event and works backward to identify root causes, the event tree looks forward to display the progression of various combinations of equipment failures and human errors that result in the incident graphically. 

This step is always performed. Using analysis tools and methods such as fault trees, causal factor charting, checklists, predeveloped trees, or alternative methodologies will help to identify the root causes of the failures. 

Method A involves a deductive search for all credible ways an occurrence could arise using timeline construction and a simplified fault tree approach. It can be viewed as an integrated method for systematically searching for all underlying root causes. The structured framework helps the investigator to keep on track, reach sufficient depth, and not stop prematurely at the symptoms or apparent causes. 

Figure 9-1, the two flowcharts describing root cause determinations using Methods A and B, presents general frameworks for root cause determination. Method A focuses on the logic tree method using a simplified fault tree approach. Method B focuses on the predefined tree method. 

Causal factor identification is relatively easy to learn and apply to simple incidents. For more complex incidents with complicated timelines, one or more causal factors can easily be overlooked, however, which inevitably will result in failure to identify their root causes. There are a number of tools, such as Barrier Analysis, Change Analysis, and Fault Tree Analysis, that can assist with bridging gaps in data and the identification of causal factors. Each of these tools has merits that can assist the investigator in understanding what happened and how it happened. 

Root (or primary) causes, immediate (or secondary) causes, and contributory factors are identified, analyzed, and discussed in this section of the report. As described in Chapter 9, process safety incidents are the result of many factors, and therefore singling out one cause is rarely the proper approach. Some experts indicate that if a fault tree or causal factor chart was developed as part of the investigation it should be incorporated to facilitate understanding. 

The fault tree cited in literature for this process is shown in Fig. 26 (Battelle, 1985). Notice the similarity between Figs. 26 and 25, particularly in the structure of the two trees, and recognize that as a result of quantitative analysis. Fig. 26 has an and-gate as its top-level gate. More importantly, recognize that without complete quantification of the root causes, the fault tree given in Figure 26 may be incomplete. 

The fourth type of root cause analysis discussed at the start of this chapter is Systems Analysis. Two methods are described here to illustrate this approach Why Tree Analysis and Fault Tree Analysis. 

Chapter 3 presents introductory aspects of safety and human factors. Chapter 4 is devoted to methods considered useful to perform patient safety analysis. These methods include failure modes and effect analysis (FMEA), fault tree analysis (FTA), root cause analysis (RCA), hazard and operability analysis (HAZOP), six sigma methodology, preliminary hazard analysis (PFfA), interface safety analysis (ISA), and job safety analysis (JSA). Patient safety basics are presented in Chapter 5. This chapter covers such topics as patient safety goals, causes of patient injuries, patient safety culture, factors contributing to pahent safety culture, safe practices for better health care, and patient safety indicators and their selection. 

Techniques used in systems safety frequently have specific goals and areas that they can address. For example, some techniques are used to analyze the hardware and equipment aspects of the system while other techniques are used to assess the human aspect. From a safety metrics standpoint, systems safety techniques can be used to identify areas for improvement in the organization. While there are hundreds of system safety techniques available, some of the more commonly used are Fault Tree Analysis (FTA), Procedure Analysis, Failure Modes and Effects Analysis, and Root Cause Analysis. 

The constraint variable suggested in our previous study of fault-effect tree model (Lee et al., 1999) are used to increase the resolution. It represents the quantitative governing equation such as the balance equation and valve relation as variables. The previous study used a mass balance and two control valve equations for CSTR. As the control valve equations are expressed by DELS model, this study uses only mass balance equation. Reactor leaking (RX-LK) is a root cause of the positive deviation of the constraint variable, DF that is defined as Fq - Fp... 

The creation of a fault tree begins with the identification of the top event. This event can be as broad and general as Total System Failure or as narrow and specific as Component X Malfunction. This top event will be placed at the top of the tree and all subsequent events that lead to the main event will be placed as branches on the tree. Figure 12.1 illustrates the beginning of a simple fault tree, with the location of the top event, the placement of contributing events, and undeveloped events, down to the basic (or root) events. As the user moves from the top event downward, each level of the tree will materialize. In order to proceed from one level to the next, the analyst must continually ask the fundamental question What could cause this event to occur As causal events are identified, they are placed in position on the fault tree (Figure 12.1). 

The Circle Used to depict a basic event in the FTA process. It can be a primary fault event (i.e., the first in the process to have occurred) and, therefore will require no further development. Use of the circle symbol offers the analyst some flexibility. A causal chain could conceivably become quite extensive. Many times, the analyst will obtain sufficient casual information from analysis of higher level events in the chain. Therefore, in order not to waste valuable time and resources analyzing a single event to its lowest possible level, the analyst can label a particular event as basic, using the circle symbol indicating that no further development is required. For this reason, the symbols of the fault tree places the circle at the base of the tree (i.e., a basic event). The basic event is also often referred to as a root event or root cause, for obvious reasons. 

Fault tree analysis is also very effective in determining the root cause of accidents and near miss incidents. It helps evaluate equipment failure and predict potential hazards. Fault tree analysis is a useful safety audit tool that uses very simple statements of fact and is very objective and realistic. 

Adopting a Safety-II perspective does not mean that everything must be done differently or that currently used methods and techniques must be replaced wholesale. The practical consequence is rather a recommendation to look at what is being done in a different way. It is still necessary to investigate things that go wrong, and it is still necessary to consider possible risks. But even a root cause analysis can be done with another mindset, and even a fault tree can be used to think about variability rather than probability. 

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