What Is an Event Tree?

Event Tree Analysis (ETA) is an inductive reasoning analysis method commonly used in security system engineering. It originates from decision tree analysis (DTA) and it starts from the initial event in the time sequence of the accident development. A method of inferring possible consequences to identify a hazard. This method uses a tree diagram called an event tree to represent the logical relationship between a certain accident that may occur in the system and the various causes that lead to the accident. Through qualitative and quantitative analysis of the event tree, find out the accident The main reason for the accident is to provide a reliable basis for determining the safety countermeasures to achieve the purpose of guessing and preventing accidents. The event tree analysis method has entered the aerospace and nuclear industries into the general power, chemical, mechanical, and transportation fields. It can diagnose faults, analyze weak points in the system, guide the safe operation of the system, and optimize the system. Event tree analysis ^[1]

Event tree analysis

1. ETA can predict accidents and unsafe factors in advance, estimate the possible consequences of accidents, and seek the most economical prevention methods and methods. 2. It is very convenient and clear to analyze the cause of the accident with ETA afterwards.

3. The analysis data of ETA can not only be used as intuitive safety education materials, but also help to speculate on preventive measures for similar accidents.

4. When a large amount of accident data is accumulated, computer simulation can be used to make ETA's accident prediction more effective.

5. Using ETA in safety management to make decisions on major issues has advantages not available in other methods.

1.Determine the initial event

Event tree analysis is a method to systematically study how the initial event as a hazard source forms a temporal and logical relationship with subsequent events and eventually lead to an accident. It is important to choose the right initial event. The initial event is a hazardous event or dangerous event during the development of the accident when it does not occur, such as machine failure, equipment damage, energy escape or loss of control, and human misoperation. There are two ways to determine the initial event:

According to system design, system

The qualitative analysis of the event tree has been performed during the process of drawing the event tree. The drawing of the event tree must be based on the objective conditions of the event and the characteristics of the event. In the process of drawing the event tree, a possibility analysis has been made for each development process and the path of event development.

The work after the event tree is drawn is to find out the ways and types of accidents and the countermeasures for preventing accidents.

1.Find out the chain of accidents

The branches of the event tree represent the possible development path of the initial event once it occurs. Among them, the ultimate cause of the accident is the chain of accidents. In general, there are many ways to cause system accidents, that is, many accident chains. There is a "logical AND" relationship between the initial events included in the accident chain and the subsequent events of the safety function failure. Obviously, the more accident chains, the more dangerous the system is;

2. Find out how to prevent accidents

The eventual approach to safety in the event tree guides how we take steps to prevent accidents. In the way to achieve security, events that exert security functions constitute a successful chain of event trees. If these safety functions are guaranteed to work, accidents can be prevented. In general, there may be multiple successful chains in the event tree, that is, there are several ways to prevent accidents. Obviously, the more successful chains, the more secure the system is, and the fewer event trees in successful chains, the more secure the system is.

Because the event tree reflects the chronological order between events, you should start with the safety features that function first as much as possible.

Event tree quantitative analysis refers to calculating the probability of accidents in various ways according to the probability of occurrence of each event, comparing the magnitude of the probability values of each path, making a sequence of accident probability, and determining the path most prone to accidents. Generally, when each event is statistically independent, its quantitative analysis is relatively simple. When the statistics are not independent of each other (such as common cause failure, sequential operation, etc.), the quantitative analysis becomes very complicated. Only the former case is discussed here.

1. Probability of each development pathway

The probability of each development pathway is equal to the product of the occurrence probability of each event from the initial event.

2. Probability of accident

In the quantitative analysis of the event tree, the probability of an accident is equal to the sum of the probabilities of the various development paths leading to the accident.

Event tree analysis of fire accidents caused by flammables on trains

In railway passenger transportation, it is strictly forbidden for passengers to bring inflammables on board to ensure the safety of passenger transportation. However, some passengers carry flammable materials in violation of regulations and did not find them when they entered the station. They were brought to the train, which may cause fire accidents, resulting in personal injury and property damage; but proper handling can also prevent fire accidents. The specific analysis is shown in Figure 1:

The motor vehicle was driving at an unguarded level crossing, the engine suddenly stopped, and the vehicle was staying on the track. An event tree analysis of the accidents that may result from this event is shown in Figure 2: