Page "Three Mile Island accident" Paragraph 12

Critical human factors and user interface engineering problems were revealed in the investigation of the reactor control system's user interface.

Despite the valve being stuck open, a light on the control panel indicated that the valve was closed.

In fact the light did not indicate the position of the valve, only the status of the solenoid, thus giving false evidence of a closed valve.

As a result the operators did not correctly diagnose the problem for several hours.