Am currently attending an Indo US workshop on Science and technology for preventing terrorism. A speaker from the US mentioned an incident involving human factors - a highly secure facility which had three layers of security was penetrated by a group of activists, who managed to penetrate all three layers without being noticed by guards. When the activists reached the secure building, they tried throwing some stones to attract attention, but the security guards thought that carpenters were working late and ignored the noises. Finally they were detected and apprehended. The incident was captured by video. The speaker highlighted that the guards never anticipated a breach as they thought that the three layer security was foolproof. The guards let their guard down! When people do routine monotonous jobs, there is a tendency to ignore warning signs as they are not alert enough.
The same applies to operators who run complex chemical plants with layers of automation. It is a human tendency to think that automation will always do its job. Make sure your operators are trained in the hazards of over dependence of automation. (automation addiction).
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