In which case might an isolated line be energized unexpectedly?

An isolated line can be unexpectedly energized due to operator error because this scenario typically involves human actions that deviate from established safety protocols or procedures. In this case, an operator might mistakenly believe that a line is de-energized when it is not, or might fail to follow the necessary verification processes. This can result from miscommunication, lack of adherence to safety checks, or not properly using protective equipment and devices that help confirm whether lines are active or not.

In contrast, scenarios such as after maintenance would generally involve stringent safety checks and lockout/tagout procedures to prevent unexpected energization. Similarly, during regular operation, systems are usually in a controlled state where energization is expected and monitored. The concept of "over time" might suggest gradual wear or environmental effects leading to issues, but it typically does not directly indicate a sudden, unexpected energization as a result of immediate operator actions.