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Human-Computer Interaction - Need for FTA

Need for FTA

FTA (Fault Tree Analysis) is a reverse engineering technique used for analyzing the probability of occurrence of an undesired state. This concept was first proposed by Bell Telephone Laboratories in 1962 for the US Air Force. It employs a logic block diagram of pathways that may lead to an undesirable event (or failure) in a system. It is mostly used in high-risk industries where a small error could lead to catastrophic effects. It is a top to bottom approach because it finds possible failures first and then moves on to what might cause the failure.

Basic Layout

Each block in the diagram refers to an event and it may be connected to another event by AND, OR, XOR gates with the top block denoting failure. For instance, consider two blocks connected to each other by an AND gate while the resulting failure event is described by a block above the two blocks. It means that the occurrence of both events causes a particular failure. Similarly, if they are connected by an XOR gate, then the occurrence of only one of the two blocks triggers the failure. Similar to flow charts, FTA diagrams also designated symbols for different events.

Effect Of An Incomplete FTA

The case study of Therac 25 is inevitable in a Safety Engineering course. The Therac 25 was a radiation therapy machine primarily intended for treating cancer patients. The general scheme of the machine is that it accelerates electrons to high energy. This energetic beam when irradiated destroys the tumour tissues without greatly affecting the surrounding tissues. It was produced by AECL and was subjected to Fault Tree Analysis.

Between 1985 and 1987, serious cases of patients being treated with massive overdoses of radiation by the machine were reported. The reason was found that the computer had chosen the wrong mode. There have been two modes – normal mode (allowable level of radiation) and high mode (really high level of radiation). The computer had mistakenly been operated in the high mode instead of the normal mode. What went wrong with the FTA and why it couldn’t predict the failure were the two questions put forth before the specialists. It was figured out that the software was mostly taken from the previous versions of the machine - Therac 6 and Therac 20. They hadn’t concentrated much on design problems in the software. “Computer selects wrong mode" failure was assigned a value 4x10^-9 (whose reason could not be found). More importance was given to hardware failures like wear, fatigue, etc.

This shows that software needs to be of equal importance as hardware.


*Image courtesy Pixabay.com

More about: HCI, FTA, success, failure

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