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2. Situation-Oriented Errors
3. System-Oriented Errors
Human Errors in System Design
Human errors enter the instrument design process in many ways. Some of the ways are mistakes, specification errors, failure to communicate, lack of competency, and functional errors.
Mistakes--Probably the most common human errors in instrument designs are mistakes. A common form of a mistake is slips (lapses or execution) errors where the intentions are correct but the execution is not. Mistakes can also be due to lack of competency or they can be facilitated by the design system itself. Some of the errors that can be facilitated by the design system methodology are data errors, drawing errors, informational errors, and change errors.
Since there are many tasks and details involved in an instrument system design, there are many opportunities for mistakes. The "devil is in the details" is very applicable. The design document review processes as well as self-checking methods can help. People tend to make the same errors when dealing with details or small matters (particularly if there is no large negative result), e.g., misspelling the same word, entering data incorrectly, etc. Knowing your own pet errors can improve your self-checking methods. Some of these errors result from a short-term memory error process--you think the entry is correct because you "know" you entered the "correct" data (a check at a later time typically reveals this type of error). Larger errors or errors of significant negative impact should to be treated as learning experiences and analyzed to prevent the error from happening again (rather than just being rationalized).
Data errors are mistakes that result from improperly entering data or errors due to the propagation of data on design documents. Instrument designs contain a tremendous amount of data, however, much of it is duplicated. The more you enter the same data, the more the opportunity for error. The common means to reduce this error typically includes a time delayed checking process and multiple reviews. The design of the engineering process to provide minimum duplicate data entry is also a method to reduce this type of error. Data must flow from document to document in a design. If the data paths are torturous or complex, data may not get to where it should correctly. The design of efficient data flow to minimize errors is a means to reduce this type error.
Drawing errors can come from errors on existing drawings that are used in a design. These spring from uncaught errors in previous designs, as-builts that have not been picked up, and changes by field forces that are not relayed to engineering. Field verification is the primary method of minimizing these errors. Drawing errors also can come from using computerized functions such as cut and paste where the pasted function is not updated with the new drawing's data. With CAD comes increased speed in doing drawings, but with increased speed also comes more potential for errors.
The normal review process for detecting mistakes for an operating company may have included reviews by the designer, a drafting checker, the instrument engineer, the instrument engineering supervisor, instrument maintenance, and operations. Unfortunately, many companies have reduced their personnel and re-structured their organizations. This has in some cases resulted in fewer reviewers and sometimes less qualified reviewers. An engineering and construction (E&C) firm will have a equivalent review process but the increasingly competitive E&C market can lead to tighter bids with shorter, less extensive review processes.
Specification Errors--One of the major ways errors enter the design process is in the specification phase. This error occurs many times because the initial scope of a project was not done properly. A well known Health and Safety Executive (HSE-UK) study that investigated accidents involving computerized systems concluded that 44% of accident were caused by a failure in specification. In a process plant this can occur due to failure of engineering to scope the project or failure of the requesting party to properly scope the project.
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