## 3 Fault Tree Analysis (FTA) Analysis
It represents a combination of independent events. That is, the probability of any input event to an AND gate is unaffected by any other input event to the same gate. It is \"favorable\"--an adverse event at the output P(G) occurs only if all events at the input (Ai) occur at the same time. The probability of an output event at this type of gate is calculated using Equation (3). \[\text{P(G)}=\prod_{i=1}^{n}\text{P(A_{i})} \tag{3}\]  
\[\text{P(G)}=1-\prod_{i=1}^{n}(1-\text{ P(A_{i})}) \tag{4}\]  
The \"OR\" gate is \"unfavorable\"--an adverse event at the output P(G) occurs when either of event occurs at an input (Ai). The probability of an event at the output of this type of gate is calculated using Equation (4). From the knowledge of the probability of occurrence of a base event or the intensity of its occurrence, it is possible to estimate the probability or intensity of the occurrence of a top event. The \"tree\" creation process and subsequent calculation are documented in STN EN 61025: 2007 [21]. The fault tree is in a shape that can be understood, analyzed and modified if necessary to simplify the identification of the failure. From the highest level of the failure--the top event--it is possible to proceed at tree level to a base event. In this way, it is possible to investigate any dependencies in the system as well as its subsystems [22; 23]. In analyzing this method, a systematic approach is necessary, as it is necessary to capture the functional linkages between the elements of the monitored system. The FTA method going from the top down through the tree makes it easy to recognize the causal dependence of the top event. FTA is particularly suited for the analysis of complex systems that consist of functionally dependent subsystems serving to fulfil a defined function. It is particularly suited to cases where the process of analysis requires many different specialists. FTA method was first used in 1962 by Bell Telephone Laboratories in the development of intercontinental ballistic missiles (Minuteman Intercontinental Ballistic Missile Launch Control System) and was improved by Boeing in 1966. It started to be used in the nuclear power industry since 1975. Its use in this industry experienced a significant increase after an accident in Three Mile Island in 1979. It is an important part of reliability engineering. It is most used in nuclear technology, in complex systems such as aircraft, chemical operations, and complex information systems [24]. It allows easy understanding of the  
Figure 1: Ishikawa diagram. system and its functional links, as well as its impact factors. If these trees are extensive, it is advisable to use the computing technique, since the process of verifying the tree's faults in the usual way would be difficult. Evolved from the works of [PERSON] et al. and [PERSON] the aim of qualitative or logical analysis is to find all reasonably possible combinations of factors operating conditions, environmental conditions, human errors and faults of elements of which could lead to the top event [25; 26]. A manual solution in a qualitative analysis of the fault tree is currently done only in the case of simple trees (usually a few tens of basic events), otherwise special software products must be used. If the parameters of confidence (probability of developing) of basic events are known, it is possible to make a quantitative fault tree analysis. The goal of this procedure is:  
* The systematic identification of all possible combinations of (basic) causes leading to the top event;
* Easy definition of the top event probability in the system or its arbitrary part;
* Creating a model (causal dependence) for examining the security of the monitored system to know the input/output interactions;
* Modelling for simulation of safety, reliability, etc.