Lanny R. Berke, P.E., C.S.P.
A few key design guidelines can help keep efforts on the right track. Several of these guidelines sound obvious, but it is surprising how often they are overlooked.
Easy to see. Locate controls, displays, and indicators that relate to safety or process events so they are easy to see and use. We have seen instances where information pertaining to process safety and quality limits were out of view or had to be manually brought up on a video screen.
Operators shouldn't have to search for such information. At Three Mile Island, a red warning light to indicate the failure of a crucial pressurerelease valve sat some distance from the main control-panel area. And the absence of the red light indicated trouble! The operator's failure to notice that the light went off was reported as an operator error. It should have been reported as a design error. The obvious fix was to relocate the light and alarm it.
Easy to find. In general, the most frequently used indicators, displays, and controls should sit where they are easy to see and use. Their placement should also correspond to the sequence in which they are used. And those that have related functions should be grouped together.
Locate and space controls or indications to prevent confusion or unintentional operation. It is particularly important to allow for operators who will wear protective equipment when at the control panel.
Easy to use. Keep the delay between activation of the control and the response in the display to under 1.5 sec. There should be as little delay as possible introduced by interruptive procedures. In the same vein, the direction of movement and labeling should be what users expect. For example, people generally anticipate that moving a control to the right or clockwise will produce an increase in the process.
Use some color and visual patterns in panel design and provide a schematic model of the process. Pictures and diagrams get more attention and are easier to remember than verbal or other serially coded information. Always organize a control panel along pictorial lines to represent the process being controlled. Studies have shown that operators who have a model of the process they control are safer and more efficient.
Easy to understand. Place written labels for controls so the operator's hand doesn't cover the label. The usual placement is above or to the left of the control. Avoid abbreviations in labels and use complete words whenever possible to prevent confusion. We've seen one instance where the word "pump" was represented four different ways on the same control panel — pump, PP, PMP, and PU — even when there was no lack of space on the label.
Finally, safety procedures and process controls should not force operators to rely on their memory for the right sequence of steps. It was just such recall-dependent links in safety procedures that were among the "human-error" elements of the Three Mile Island disaster. And always check for applicable regulatory standards such as ANSI, NEC, UL, and FM for system safety requirements.
is a registered professional engineer and a Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].