Human response-time criteria

The human response time is the time from the alarm/process indication to the completion of the actions neoessary to place the process into a safe state. The human response can be broken down into four functions (1) Recognize the unsafe condition, (2) Analyze the condition properly, (3) Perform the required safety aotion, and (4) Continue to monitor the process to determine whether safety actions were 

Sample times for human action in response to alarms are shown in Table B.1. These IPL time limits are based on research and are confirmed by industry experience. If the owner/operator wishes to use shorter response times or lower PFDavg values, the owner/operator is cautioned to do a detailed human reliability assessment to confirm the assumed PFDavg. The values in the table below represent the PFD of the entire IPL. They are not to be interpreted as PFDavg of the human action only. 

There are a number of methods for evaluating the probability of human error. Two of the better-known methods are the Technique for Human Error Rate Prediction (THERP) (Reference NUREG/CR-1278) and the Accident Sequence Evaluation Program Human Reliability Analysis Procedure (Reference NUREG/CR-4772). Error rates are usually established on a per-demand basis. 

The nominal human error rates can be reduced or increased based on operator-related environmental factors (quality of displays, control layout and clarity, control area environment, procedures, access), personnel faotors (training, experience), and stress factors (personal, shift schedules, response time pressure, severity or magnitude-of-safety condition). The best source for determining the human error rate would be company/facility-specific historical data, but in most organizations, this is not available. Therefore, an owner/operator often uses other published, acknowledged sources and adjusts the human error rate for their application and circumstances accordingly. 

In addition to the initial evaluation of the credited operator action, a program should be established to ensure that all assumptions about the reliability of the operator response are maintained and improved. This would include, but is not necessarily limited to initial training, refresher training, procedures engineered to decrease the likelihood of human error, important human factors that have been identified. 

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Due to the second criterion, time-to-tumor models were eliminated from consideration. These models require more detailed experimental data than is generally available. Moreover, it is difficult and unproductive to interpret the distribution of time-to-tumor in the context of human exposures. In most cases, the time-to-tumor variable would be integrated over a human lifetime, thus reducing the model to a purely dose-dependent one. Therefore we restrict our attention to quantal response models that estimate lifetime risks. 

A finite element computer program is used to solve the equations. Using the data of Stoll and Chianta [39], the copper test sensor temperatures are used to estimate the time it would take human skin to sustain second-degree bums. The authors show that estimates of the gross thermal response of the materials tested in common bench top tests, and times to exceed the Stoll second-degree bum criterion can be modeled using relatively simple models. 

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