ABSTRACT
Application of HEART involves the identification of a generic task type to which is assigned a range of human unreliability values as well as a suggested proposed nominal value. There is a set of error producing conditions (EPCs) each to which is assigned a weighting by the analyst. The Human Error Probability (HEP) is then computed from:
HEP = GTTIl ((E j - 1) Pi + 1), and smaller than 1 1
Where GTT = General Task Type nominal value for human unreliability n
Il is the product over all EPC factors, 1 to n assessed 1
E j = assigned EPC factor
Pi = assigned weighting factor applied to individual EPC factor
The methodology is highly questionable as regards determining accurate human error probabilities but can be valuable as regards comparing situations. An example is presented here that involves comparing an operation of a batch reactor, involving 5 tasks and comparing the HEPs for both an inexperienced operator with a highly experienced operator. The 5 tasks are described as follows:
• Task # 1 consists of emptying contents of last batch (simple task) • Task # 2 requires checking of inside of reactor to detennine there is no residual
material (difficult to see inside reactor) • Task # 3 introduces measured quantities of feed and chemicals (more complex
task) • Task # 4 involves controlling reaction by providing sufficient agitation and
cooling to prevent a runaway reaction (very complex task) • Task # 5 requires assessment of when reaction is complete by sampling and
testing (very difficult task). The calculation for HEPs is shown, in part, below:
Values assigned for inexperienced operator:
Each task is then considered for EPCs and, only where relevant, are values assigned. For each EPe assigned a weighting factor, judged and estimated by the analyst and always less than 1, is applied.
