Implicit rules

Operators are expected to follow operational rules, even when they are implicit.

Definition

An implicit rule is one missing from the formal system documents, and thereof may or may not be implemented.

The risks of implicit rules

  Implicit rules are error-prone.

• They could be taught in training sessions, which operators might forget later
• they could be documented in the operational manual, which the operators might miss when reading them
• Operators might fail to trace the system mode, or might fail to obey the rules.

Examples

• The design of the TMI nuclear power plant enabled the operators to close the backup cooling system ( Perrow ).
• The design of the navigation handle of the Torrey Canyon supertanker enabled the operators to change it to the Control position, which disconnected the wheel from the rudder (Casey ).

  If the rules are implicit, then the operators might execute maintenance activities in functional operation. Sometimes, these activities are risky, especially, when the maintenance activities involve safety add-ons.

Related topic

Updated on 29 Mar 2016.